Rift zones are very extended (many hundreds and thousands of kilometers long) planetary scale strip-like tectonic zones, distributed within continents and oceans, in which the rise of deep (mantle) material occurs, accompanied by its spread to the sides, which leads to more or less significant transverse stretching in the upper levels of the earth's crust. The most important structural expression of the extension process on the Earth’s surface is usually the formation of a deep and relatively narrow (from several kilometers to several tens of kilometers), often stepped graben (symmetrical or asymmetrical), limited by normal faults of great depth (the rift itself or “rift valley”), or several (sometimes a whole series) similar grabens. The bottom of grabens is also cut by faults and tension cracks. The subsidence of the bottom of grabens relative to their sides, as a rule, precedes the accumulation of sedimentary material in them, although the latter in many cases is supplemented by filling them with volcanic products, and therefore rifts usually have a clear direct expression in the relief in the form of linear depressions. For the most part, rifts are framed on both sides, or at least on one side, by asymmetrical uplifts (sloping semi-arches, one-sided horsts and, less commonly, horsts), broken to one degree or another, like grabens, by longitudinal, diagonal and transverse cracks, faults and often complicated by secondary narrow grabens. In some cases, uplift also occurs within the rift, splitting it into two branches. The ratio of the volumes of these uplifts and rift depressions reflects the ratio of the scales of uplift and extension in a particular rift zone. Some of them, especially oceanic ones, are characterized by a significant role of transverse shear displacements, in particular, along the zones of the so-called transforming faults.
Rift zones in general and primarily axial grabens (rifts) have increased or even very high seismicity, with earthquake foci lying at depths from a few kilometers to 40-50 km, and the stress pattern in the foci is characterized by the predominance of maximum subhorizontally directed tensions, approximately perpendicular to to the rift zone axis. Rift zones, with rare exceptions, are characterized by increased heat flow, the magnitude of which generally increases as they approach their axis, often reaching 2-3, and sometimes even 4-5 units of heat flow. The development of most rift zones is accompanied by manifestations of hydrothermal activity and magmatism and, in particular, by volcanic eruptions fed from subcrustal, and in some continental rift zones, perhaps from intracrustal magma chambers. However, the scale of the magmatic process, the volume of its products, their composition, and their association with certain stages of rifting and certain areas of the rift zone vary within extremely wide limits. Along with rift zones, in which magmatic activity accompanied all stages of their development, and its products cover almost their entire area and reach volumes of hundreds of thousands of cubic kilometers, there are rift zones where it manifested itself locally, sporadically or completely absent.
Rift zones of the oceans are characterized by a contrasting stripe-shaped bilaterally symmetrical magnetic field, which, according to prevailing ideas, is created during the process of rifting and, as it were, imprints its individual stages. However, the magnetic field of continental rift zones largely reflects the structural features of their basement and underwent only some restructuring during the process of rifting. Rift zones are usually, although not always, characterized by gravitational minima in the Bouguer anomaly field, but the axial parts of some of them have narrow maxima caused by the rise of mafic and ultramafic material. However, the shapes, sizes of gravity anomalies and the nature of the factors causing disturbances can vary significantly. As a rule, rift zones are close to a state of isostatic equilibrium.
The Earth's crust in modern rift zones is somewhat thinner compared to adjacent areas, and the upper part of the mantle, at least immediately below the M surface, in many of them is characterized by an anomalously low velocity of longitudinal seismic waves (7.2-7.8 km/s ) and somewhat reduced density and viscosity, which is apparently due to increased thermal conditions and, in some cases, the emergence of selective melting centers in the upper mantle. These lenses or “pillows” of decompressed mantle material probably represent projections of the roof of the asthenosphere, reaching the base of the Earth's crust beneath modern rift zones. Rift zones rarely exist in isolation; as a rule, they form more or less complex combinations. The methods of “joining” neighboring rift zones and the general plan of their grouping can be very diverse and at the same time differ significantly between continental and oceanic zones. We call combinations of a number of closely interconnected spatially approximately coeval rift zones of similar or different types rift systems. This term can be applied to any combination of rift zones, regardless of their size, complexity and pattern, but is mainly used in relation to those combinations that are characterized by the presence of differently oriented rift zones, a tree-like pattern, or the presence of several semi-isolated branches, not band-like, but similar to an isometric general outline. In cases where rift zones (or their systems), combined with each other, form together linearly elongated structures with a length of several or even many thousands of kilometers, we call them rift belts (by analogy with geosynclial and orogenic belts). The term rift system is also used to refer to all the interconnected rift belts of the Earth, which together form a complex meandering and branching network on the surface of our planet. In the latter case, we are talking about a global rift system. The latter, with its main branches, unites most of the Earth's rift belts (and systems). Its main part crosses the oceans, and its fading ends and branches in several regions of the Earth penetrate deep into the continents. However, within the continents (and possibly in the oceans) there are also separate, isolated rift belts and even separate rift zones that are not associated with the global rift system.
1) oceanic, or intraoceanic, in which both the axial “rift valley” and its framing have a crust close to oceanic, which is underlain by a protrusion of mantle material with anomalously reduced seismic wave velocities and density compared to those typical for the upper part of the mantle;
2) intercontinental, in which the axial part of the rift has a crust close to that of the intraoceanic rift zones, its peripheral parts have somewhat thinned and reworked continental crust, and the “shoulders” have typical continental crust. Intercontinental rift zones, like intracontinental ones, can be formed either on platforms (Adensky and Krasnomorsky rifts) or within a young folded area (Gulf of California rift);
3) continental or intracontinental, in which both the rift and its “shoulders” have continental-type crust, but usually somewhat thinned, especially under the rift (from 20 to 30-35 km), fragmented, abnormally heated and underlain by a lens of somewhat decompressed mantle material.
Mutual transitions and close structural connections of intercontinental rifts observed in nature as a result of the far-advanced process of development of intracontinental rifts. At least some of the width of intercontinental rift zones (on the order of several tens of kilometers) is apparently due to thrust or thrust-shear deformations of blocks of the continental crust and the protrusion of material of mantle origin between them, while in intracontinental rifts we are mainly dealing with graben-like subsidence of blocks of the continental crust with an extension amplitude of the order of several kilometers and not always with filling of opening cracks with dike-like intrusions. In turn, intercontinental rift zones are structurally closely related to the rift belts of the Indian and Pacific Oceans, in which the process of uplift of deep material and horizontal expansion occurs even more intensely. However, it would be imprudent to assume by analogy that all rift zones and ocean belts represent a further stage in the development of intercontinental rifts and, therefore, arose as a result of even greater separation of blocks of the continental crust. For example, with regard to the East Pacific Rift Belt, we can say with reasonable confidence that it is younger than the Pacific Ocean and arose on the oceanic crust. The fact that the continuation of this rift belt almost completely passes to the North American continent and is superimposed on the Cordilleran Mesozoic folded region obviously suggests that the driving mechanism of rifting is associated with such great depths, at which differences between oceans and continents are no longer affected, but The specific manifestations of this process on the Earth's surface differ significantly depending on whether it affects the crust of the oceans, young folded areas, platforms, etc.
Rift zones and belts belonging to the three identified categories differ significantly in their size, morphology of structural forms, the scale of volcanism (the largest in the rift zones of the oceans), the chemistry of its products (tholeiitic basalts in rift zones, rocks very diverse in acidity and alkalinity in rift zones). zones of continents), the magnitude of heat flow (the highest in oceanic rift zones), the structure of the magnetic field, the pattern of stresses in earthquake sources (in continental rift zones the vector of compressive stresses is oriented subvertically, and in oceanic ones - usually subhorizontal and subparallel to the strike of the rift zone), etc. e. Continental rift belts are characterized by such spatial combinations of adjacent rift zones as their clear-cut, en echelon arrangement, elbow articulation, fan-shaped splitting, the junction of three zones converging at different angles, mutual parallelism, the bending of two adjacent zones around a relatively “rigid” block separating them , playing the role of a kind of middle massif in the structure of the rift belt. On the contrary, the rift belts of the oceans are characterized by their intersection by numerous transverse or diagonal so-called transforming faults, dividing these belts into separate transverse segments (rift zones), the axes of which seem to be displaced relative to each other.
Types of continental rift zones. When identifying types among modern continental rift zones, the following main criteria should be taken into account: a) features of the tectonic position, basement structure and previous geological history of the area that became the arena of rifting, b) the nature of tectonic structures created in the process of rifting and the patterns of their formation, c) the role, scale and characteristics of magmatic processes accompanying rifting, and sometimes preceding it.
Based on the first criterion, rift zones and continental belts can be divided into two main groups: 1) rift belts and platform zones (epiplatform rift belts and zones), in which reef formation began after a very long period (200-500 million years or more). ) stage of platform development or close to it; 2) rift belts and zones of young folded structures (epiorogenic rift belts and zones), where a similar process directly followed the completion of their geosynclinal development, i.e., the orogenic stage, or was even combined with phenomena characteristic of epigeosiclinal orogenesis. Epiplatform rift belts are characterized by rift zones with large single axial grabens and the subalkaline or alkaline nature of the products of accompanying volcanism, often with the participation of carbonatites. On the contrary, combinations of many narrow grabens, horsts and one-sided blocks are typical for epiorogenic rift belts and zones, and the volcanic formations of them belong to the calc-alkaline series.
Most modern continental epiplatform rift zones are confined mainly to the protrusions of the folded base of platforms, i.e., to areas that experienced long-term stable uplift, and much less often - to areas of development of the platform cover (Levantine, North Sea, and partially Ethiopian rift zones). In most cases, rift zones are superimposed on areas of Late Proterozoic (Grenville, Baikal) folding or tectono-magmatic regeneration, “avoiding” areas of more ancient - Archean or Early Proterozoic consolidation, which serve as the outer “frame” of these rift belts or form inside them peculiar “hard » middle massifs (Victoria massif in the southern part of the African-Arabian belt). Much less often, rift zones arise on the EpiPaleozoic platform foundation (Rhine-Rhone section of the Rhine-Libyan rift belt). In most cases, young rift structures inherit the strikes of ancient folded and fault structures of the basement or “adapt” to them, forming elbow, zigzag, and en echelon combinations. Thus, during the process of rifting, the ancient anisotropic basement splits along the weakest directions, just as a log of firewood splits according to the fibrous texture of the wood. Weakened zones of the basement, used by Cenozoic rift structures, during long platform development from time to time (in the Paleozoic or Mesozoic) became more active and served either as zones of increased permeability for magmatic melts and the introduction of intrusions, in particular ring-type alkaline massifs, or as zones of faults and grabens.
Among epiplatform rift zones, two types are clearly distinguished, differing significantly in the nature of the structures, the relative role of volcanism and the history of formation. The author called them crevice and dome-volcanic (Milanovsky, 1970):
a) rift zones of the arch-volcanic type (Ethiopian and Kenyan zones of East Africa) are characterized by exceptionally powerful and prolonged ground volcanic activity. It begins over a wide area even before the initiation of the rift, and subsequently continues within the axial graben and associated secondary grabens and fault zones. The main role is played by eruptions of basic and intermediate lavas and pyroclastolites of the strongly alkaline and weakly alkaline series. In the Ethiopian Rift Zone, acidic (high alkalinity) volcanics also play a significant role. The emergence of a rift is preceded by the long-term growth of an extensive gentle oval arched rise, accompanied by powerful eruptions, then a relatively shallow graben is formed in its axial weakened zone, as well as additional grabens and faults associated with it - transverse and diagonal on the wings of the arch and fan-shaped diverging on its periclines. The amplitude of horizontal extension in domed-volcanic rift zones is minimal. They are characterized by moderate seismicity. The formation of a dome characterized by a large gravitational minimum is apparently associated with the emergence of a lens of decompressed, abnormally heated material and with individual magmatic chambers in the upper mantle, and the formation of grabens is partly due to the subsidence of crustal blocks during the unloading of these chambers during eruptions;
b) rift zones of the slot type are characterized by a greater depth of grabens, which can reach 3-4 (Upper Rhine graben) and even 5-7 km (South Baikal graben). Large gravity minima are associated with the large thickness of loose sediments in grabens. Grabens often set each other up in a cowardly manner. The marginal uplifts are much narrower than in arch-volcanic rifts, they are not traced everywhere, often only on one side of the graben, and sometimes are completely absent, and in some cases (rift zone of the North Sea) the development of rifts occurs against the background of general subsidence. In some places, arch- and horst-shaped uplifts arise within the rift zone, reaching in some cases enormous heights (up to 4-5 km in the Rwenzori block in the Tanganyika zone). Gravity maxima are associated with internal uplifts, and their protrusion is anti-isostatic in nature. Slot rift zones are characterized by relatively weak, local and episodic manifestations of volcanism or their complete absence. Based on this feature, weakly volcanic (Tanganyika, Upper Rhine) and non-volcanic zones (middle segment of the Baikal rift belt) can be distinguished among them. The centers of eruptions are confined to saddles between clearly located grabens, their edge steps, marginal rises and other elevated areas. Petrochemically, volcanism is close to dome-volcanic zones, but extremely alkaline series (sodium or potassium) and carbonatites are more often present here. Volcanic activity can occur at different stages of rifting.
The process of formation of crevice zones begins with the establishment of narrow linearly elongated grabens (usually confined to ancient weakened zones), filled initially with fine-clastic (“molasseoid”), as well as carbonate and chemogenic sediments, which are subsequently replaced by coarser continental molasse. This formation series, as well as geomorphological data, show that the intensive growth of marginal and internal uplifts began later than the initiation of grabens, and in some places has not yet manifested itself. The concept of a rift arising as a result of arch collapse is not applicable to slot rift zones. These zones are more seismic than dome-volcanic zones. The amplitude of horizontal extension in them may be greater than in the latter, but, apparently, usually does not exceed 5-10 km. In the grabens of slot rift zones, there is obviously a significant “leakage” of thermal energy. In some gap zones, in addition to the sliding component, there is a shear component. In the Levantine zone, the latter apparently significantly exceeds the transverse extension, and in some of its sections the horizontal deformation approaches pure shear.
In rift belts and zones of young folded structures, rifting follows the geosynclinal development cycle, being a direct continuation of its final, orogenic stage. During the process of rifting in these zones, a system of narrow but very extended (up to many hundreds of kilometers) mutually parallel grabens, separated by comparable narrow horsts or one-sided horsts (the Cordillera rift system), often arises. The amplitudes of the relative movement of blocks along the normal inclined faults separating them reach 2-5 km. Along with general significant horizontal stretching, significant shear deformations can occur (for example, the San Andreas shift in California). The formation of rift structures is preceded and accompanied by exceptionally powerful eruptions of calc-alkaline magma, both acidic and basic. The volcanoes were fed from sources of different depths, located both in the upper mantle (foci of basaltic volcanism) and in the crust (foci of liparitic-dacite volcanism). The dispersion of extension and accompanying volcanism within a very wide strip with numerous grabens in some epiorogenic rift zones is obviously due to the fact that rifting develops under conditions of a more “warmed up” and “plastic”, and in the upper part - fragmented lithosphere compared to the relatively “hard” and “cold” lithosphere of epiplatform rift zones.
RIFT (a. rift; n. Rift; f. rift; i. rift), rift zone, is a large strip-like (in plan) zone of horizontal extension of the earth's crust, expressed in its upper part in the form of one or several close linear grabens and conjugate with them block structures, limited and complicated mainly by longitudinal faults such as inclined faults and thrust faults. The length of the rift is many hundreds or more than a thousand km, the width is usually tens of km. In relief, rifts are usually expressed as narrow and deep elongated basins or ditches with relatively steep slopes.
Rifts during periods of their active development (rifting) are characterized by seismicity (with shallow earthquake foci) and high heat flow. During the development of rifts, they can accumulate thick strata or , which contain large oils, ores of various metals, etc. The anomalously heated and low-viscosity upper part of the mantle under developing rifts usually experiences uplift (the so-called mantle diapir) and some spreading to the sides, and the overlying bark shows some arch-like bulging. Some researchers consider these processes to be the main cause of rift formation, others believe that local uplifting of the upper mantle and crust only favors the emergence of a rift and predetermines its localization (or even is its consequence), while the main cause of rifting is regional (or even global?) stretching bark. With particularly strong horizontal stretching, the ancient continental crust within the rift undergoes complete rupture and between its separated blocks, in this case, due to the igneous material of basic composition coming from the upper mantle, a new thin crust of the oceanic type is formed. This process, characteristic of ocean rifts, is called spreading.
Based on the nature of the deep structure of the crust in rifts and their framing zones, the main categories of rifts are distinguished - intracontinental, intercontinental, pericontinental and intraoceanic (Fig.).
Intracontinental rifts have continental-type crust that is thinner compared to the surrounding areas. Among them, according to the characteristics of the tectonic position, rifts of ancient platforms (epiplatform or intracratonic) of the dome-volcanic type (for example, Kenyan, Ethiopian, Fig. 1) and weakly or non-volcanic crevice type (for example, Baikal, Tanganyika) (Fig. 2) are distinguished. as well as rifts and rift systems of mobile belts, which periodically arise and then transform during their geosynclinal development and are mainly formed during the post-geosynclinal stages of their evolution (for example, the rift system of the Basins and Ranges in the Cordillera, Fig. 3). The scale of extension in intracontinental rifts is the smallest compared to their other categories (several km to the first tens of km). If the continental crust in the rift zone undergoes complete rupture, intracontinental rifts turn into intercontinental rifts (rifts of the Red Sea, Gulf of Aden, and California; Fig. 4).
Intraoceanic rifts (so-called mid-ocean ridges) have oceanic-type crust both in their axial zones (zones of modern spreading) and on their flanks (Fig. 5). Such rift ridges can arise either as a result of the further development of intercontinental rifts, or within older oceanic areas (for example, in the Pacific Ocean). The scale of horizontal expansion in intraoceanic rifts is the largest (up to a few thousand km). These rifts are characterized by the presence of transverse faults (transform faults) intersecting them, as if displacing neighboring segments of these rift zones relative to each other in plan. All modern intraoceanic, intercontinental, as well as a significant part of intracontinental rifts are directly connected to each other on the surface of the Earth and form the world rift system.
Pericontinental rifts and rift systems, characteristic of the margins of the Indian Ocean, have a highly thinned continental crust, which replaces the oceanic crust towards the interior of the ocean (Fig. 6). Pericontinental rift zones and systems formed in the early stages of the evolution of secondary ocean basins. Intercontinental and intraoceanic rifts arose at least from the middle of the Mesozoic, and possibly in earlier eras. Intracontinental rifts within ancient platforms have been formed since the Proterozoic and subsequently often experienced regeneration (so-called). Rift-like linear zones of extension, which were later subjected to compression, arose already in (greenstone belts).
The origin of Baikal is still a matter of scientific debate. Scientists traditionally estimate the age of the lake at 25–35 million years. This fact also makes Baikal a unique natural object, since most lakes, especially those of glacial origin, live on average 10–15 thousand years, and then fill with silty sediments and become swampy. However, there is also a version about the youth of Baikal, put forward by Doctor of Geological and Mineralogical Sciences Alexander Tatarinov in 2009, which received indirect confirmation during the second stage of the “Worlds” expedition on Baikal. In particular, the activity of mud volcanoes at the bottom of Baikal allows scientists to assume that the modern shoreline of the lake is only 8 thousand years old, and the deep-water part is 150 thousand years old.
Some researchers explain the formation of Baikal by its location in the transform fault zone, others suggest the presence of a mantle plume under Baikal, and others explain the formation of the depression by passive rifting as a result of the collision of Eurasia and Hindustan. Be that as it may, the transformation of Baikal continues to this day - earthquakes constantly occur in the vicinity of the lake. There are suggestions that the subsidence of the depression is associated with the formation of vacuum centers due to the outpouring of basalts onto the surface (Quaternary period).
P.A. Kropotkin (1875) believed that the formation of the depression was associated with splits in the earth's crust. I.D. Chersky, in turn, considered the genesis of Baikal as a trough of the earth's crust (in the Silurian). Currently, the “rift” theory (hypothesis) has become widespread. According to this hypothesis, as a result of compression of the earth's crust, a huge arched uplift is formed, and tension, which subsequently replaces compression, causes the upper part of the arch to subsidence along the axis.
N.A. Florensov considers the Baikal depression as the central, largest and oldest link of the Baikal rift zone, which arose and is developing simultaneously with the world rift system. The “roots” of the depression, cutting through the entire earth’s crust, go into the upper mantle, that is, to a depth of 50-60 km. Under the Baikal basin and, apparently, under the entire rift zone, an anomalous heating of the subsoil is occurring, the cause of which is still unclear.
The light heated substance, floating up, lifted the earth's crust above itself, in some places breaking it through its entire thickness and forming the basis of the modern ridges surrounding Baikal. At the same time, the heated substance spread under the crust to the sides, which created horizontal tensile forces. The stretching of the crust caused the opening of ancient faults and the formation of new ones, the descent of individual blocks along them and the formation of intermountain depressions - rift valleys - led by the giant Baikal depression.
When studying the bottom sediments of Baikal using special piston vacuum tubes, scientists were able to select columns of bottom sediments 10-12 m long in various areas of the lake. The surface layers of bottom sediments in all basins are represented by fine-grained silty silts. But in the lower part of the columns, at a depth of 8-10 m from the bottom surface, in different places there were sand deposits, which usually form in shallow areas of the lake or in river beds, in their deltas and in deltaic areas with intense mixing of bottom sediments. However, there is currently nothing similar in Baikal at depths of 1000-1600 m, where sand deposits are found. Based on this, a hypothesis was born that Baikal with its great depths arose quite recently, and some researchers began to call the sandy deposits under the silt layer pre-Baikal. The rate of sedimentation in open Baikal is currently averaging 4 cm per 1000 years. Consequently, it is not difficult to calculate the time when Baikal was not yet Baikal, but in its place there were shallow reservoirs or watercourses - only 200-250 thousand years ago. On a geological time scale, this is quite recent, almost before human eyes.
Research by paleontologists and paleolimnologists shows that on Baikal, in different areas of the coast, lacustrine deposits of the Tertiary period with specific fossil lake fauna - mollusks, remains of plants and other organisms - are quite widespread. The age of these finds and deposits is at least 20-25 million years. Consequently, even then, on the site of modern Baikal, there existed a rather lake-type reservoir with significant depths. Perhaps its outlines did not exactly coincide with the contours of the modern lake - for example, in the southern basin it was somewhat wider. At that time, there was probably a fairly deep lake in the Barguzin Valley and a series of lakes in the Tunka Depression. The modern outlines could have been formed relatively recently, perhaps during the glacial or post-glacial period, because the development of the Baikal basin, as well as the entire Baikal rift, continues - this is evidenced by numerous annual earthquakes.
And sand deposits in the thickness of bottom sediments at great depths could have formed during mudflows, turbidity flows and underwater landslides. For example, the same sandy deposits brought by turbidity currents and underwater landslides were found in the Pacific Ocean at a distance of several hundred kilometers from the coast of California. More thorough research is needed, possibly with drilling of bottom sediments in the area of great depths, in order to trace the history of the development of the basin and the evolution of the animal and plant world of Lake Baikal.
Rifts as global geotectonic elements are a characteristic structure of the extension of the earth's crust. The concept of rifts also includes narrow forms of relief - furrows (“grabens”) that have not yet been compensated by sediments; large and wide depressions with sufficiently spaced sides; dome-shaped, or ridge-like uplift systems complicated by an axial graben (for example, rifts in the central parts of the oceans and in East Africa). It is believed that all this is just different temporary stages of the formation of rift structures that are currently discovered in the oceans and on continents. Age is determined by sediments and sediments.
The first place among planetary rift systems is occupied by the World Rift System (WRS), formed during the Cenozoic and developing to the present day, discovered in 1957, which stretches over a length of over 60 thousand km under the waters of the World Ocean, and with a number of its branches also reaching the continent . MSRs are wide (up to a thousand kilometers or more) uplifts, rising 3.5 - 4 kilometers above the bottom and stretching for thousands of kilometers. Active rift zones are confined to the axial parts of the ridges, consisting of a system of narrow grabens (rift gorges such as Baikal), framed by rift mountain ranges such as the Baikal, Barguzin and other ridges surrounding Baikal.
Other rifts (on a planetary scale) include rifts confined to continents (except for those mentioned above) - for example, the Rhine graben (length about 600 km) or the Baikal rift zone (length more than 2.5 thousand km). Modern continental rift zones have much in common with the rifts of the mid-ocean ridges belonging to the MSR. Their occurrence is also associated with the processes of uplift of deep material, arch uplift, horizontal stretching of the earth's crust under its pressure, thinning of the crust and uplift of the Mohorovic surface. Continental rift systems (CRS) also form branching extended systems (similar to MSRs), but are much less pronounced in relief, so some of their links seem isolated. At first glance, it is difficult to call a rift gorge buried under a layer of water 3-3.5 kilometers thick as an analogue of Baikal. The origin of the Baikal and oceanic rift zones is the same in essence. Most of the KSRs have a Cenozoic age of formation. The Baikal rift formed at the end of the Paleogene. In cross section, the rift zone is a system of blocks sloping at different angles, gradually plunging towards the axial part. The interfaces are usually steeply dipping faults.
The earth's crust of continental rifts is characterized by a noticeable thinning up to 20-30 km, an uplift of the Mohorovic surface and an increase in the thickness of the sedimentary layer, therefore, in section, the earth's crust has the shape of a biconcave lens. In the study of rift structures, much has not yet been clarified and studied. Is rifting a process unique to the Meso-Cenozoic eras? Did this process arise only in the next 100-150 million years of the Earth’s life, or should it be responsible for the transformation of its face in earlier eras? These questions have not yet been clearly answered.
The processes of rifting should be considered as one of the characteristic features of the development of the earth's crust, which took place throughout the history of its life. They are caused by horizontal stretching of the earth's crust, leading to vertical subsidence. Blocks of the earth's crust and the rise of mantle material to the surface. There is a certain stage pattern in the development of rift zones. At the first stage, due to the leakage of decompressed mantle material in the earth's crust, a dome-shaped or linearly extended uplift is formed, then due to stretching, graben troughs are formed in their most elevated parts. At subsequent stages, rift zones can serve as axial parts of larger subsidences, or, in the case of replacement of extension by compression, degenerate into folded elevated structures of the geosynclinal type.
The distribution of rift zones is not strictly linear. Their individual parts (elements) are mutually displaced in the transverse direction along transform faults. The study of modern and ancient rift zones in the ocean and on continents will provide a clear understanding of the structure and geological history of these large geological planetary structures, as well as the petroleum potential of the many kilometers of sedimentary rocks that fill many of the rift basins. Lake Baikal as a relatively young rift zone, with its further study, can provide even more extensive material for a deeper understanding of the essence of geological and magmatic processes in the area of rift zones.
Recently, a new form of existence of the earth's crust has been established - a system of rift zones developed both within the oceanic and continental crust, as well as in their transitional parts and occupying only within the oceans an area equal to the continents. For rift zones, sometimes complex specific relationships between the mantle and crust are revealed, which are often characterized by the absence of the Moho boundary, and the interpretation of their nature has not yet left the realm of discourse, including the issue of their typification. This. It is necessary to keep in mind with regard to the distinguished types of rift systems in accordance with the data of M.I. Kuzmin, who calculated natural geochemical standards for the igneous rocks of these systems in 1982:
oceanic rift zones, confined to mid-ocean ridges, forming a unified system of oceanic rises up to 60 thousand km long with the presence within them, in most cases, of narrow rift valleys 1-2 km deep (in the East Pacific Rise - the central horst rise). Basic rocks are formed from primitive tholeiitic magma of shallow generation depths - 15-35 km;
continental rift zones are grabens genetically associated with faults such as normal faults, being often confined to the axial parts of large arched uplifts, the thickness of the crust under which decreases to 30 km, and the underlying mantle is often decompressed. Tholeiitic basalts appear in the rift valleys, and in the distance - rocks of the alkali-basaltic and bimodal series, as well as alkaline-ultra-basic rocks with carbonatites;
island arcs consisting of four elements: a deep-sea trench, a sedimentary terrace, a volcanic arc and a marginal sea. The thickness of the earth's crust is 20 km or more, magma chambers at a depth of 50-60 km. There is a natural change from low-chromium-nickel tholeiitic series to sodic calc-alkaline series, and in the very rear of the island arcs volcanics of the shoshonite series appear; active continental margins of the Andean type, characterizing the “creep” of the continental crust onto the oceanic one, like island arcs, are accompanied by the Zavaritsky-Benioff seismofocal zone, but with the absence of marginal seas and the development of volcanism within the continental margin with an increase in the thickness of the earth's pores to 60 km, and the lithosphere - up to 200-300 km. Magmatism is caused by both mantle and crustal sources, starting with the formation of rocks of the calc-alkaline (rhyolite) series, giving way to rocks of the andesite formation - the latite series; 5) active continental margins of the Californian type, in contrast to island arcs and active continental margins of the Andean type, are not accompanied by a deep-sea trench, but are characterized by the presence of compression and extension zones that arose as a result of the thrust of the North American continent onto the entire system of the mid-ocean ridge. Therefore, there is a simultaneous manifestation of magmatism, characteristic of both rift structures (oceanic and continental types) and compression zones (deep seismic focal zones).
The petrogeochemical standards (types) of igneous rocks, characteristic of these zones, calculated by M.I. Kuzmin are of great scientific importance, regardless of the playtectonic views of their author, including for typing the nature of Precambrian magmatism. V. M. Kuzmin believes that the features of these geochemical types of igneous rocks are determined not by age, but by the geodynamic conditions of formation, therefore these types can be the basis for the reconstruction in place of mobile belts of past active zones, comparable to modern ones. An example of such reconstructions is the identification of the Mesozoic Mongol-Okhotsk belt with a rift system of active margins of the Californian type. This idea, which denies the existence of geosynclinal systems at least in the Phanerozoic and extends the patterns of rifting rock formation to the distant past of the Earth, is opposed by the idea, also based on the study of geochemical patterns of magmatism, that island arcs do not indicate the presence of a transitional type of crust, much less rift structures, but are typical young geosynclines.
Most modern rift zones are interconnected, forming a global system stretching across continents and oceans (Fig. 5.1). The awareness of the unity of this system, which covered the entire globe, prompted researchers to look for planetary-scale mechanisms of tectogenesis and contributed to the birth of “new global tectonics,” as the concept of lithospheric plate tectonics was called in the late 60s.
In the Earth's rift zone system, most of it (about 60 thousand km) is located in the oceans, where it is expressed by mid-ocean ridges (see Fig. 5.1), their list is given in Chapter. 10. These ridges continue one another, and in several places they are interconnected by “triple junctions”: at the junctions of the Western Chilean and Galapagos ridges with the East Pacific, in the south Atlantic Ocean and in the central part of the Indian Ocean. Crossing the border with passive continental margins, oceanic rifts continue with continental ones. Such a transition was traced south of the triple junction of the Aden and Red Sea ocean rifts with the Afar Valley rift: along it, from north to south, the oceanic crust pinches out and the continental East African zone begins. In the Arctic Basin, the oceanic Gakkel Ridge continues with continental rifts on the Laptev Sea shelf, and then with a complex neotectonic zone including the Momma Rift (see Fig. 5.3).
Where mid-ocean ridges approach an active continental margin, they may be absorbed into a subduction zone. Thus, the Galapagos and Western Chilean ranges end at the Andean outskirts. Other relationships are demonstrated by the East Pacific Rise, over the continuation of which the Rio Grande continental rift formed on the overthrust North American plate. Similarly, the oceanic structures of the Gulf of California (apparently representing an offshoot of the main rift zone) are continued by the continental Basin and Range system.
The extinction of rift zones along strike is characterized by gradual attenuation or is associated with a transform fault, as, for example, at the end of the Juan de Fuca and American-Antarctic ridges. For the Red Sea Rift, the end is the Levantine strike-slip fault.
Covering almost the entire planet, the system of Cenozoic rift zones exhibits geometric regularity and is oriented in a certain way relative to the axis of rotation of the geoid (Fig. 5.2). Rift zones form an almost complete ring around the South Pole at latitudes 40-60° and extend from this ring meridionally at intervals of about 90° by three belts that fade to the north: the East Pacific, Atlantic and Indian Ocean. As shown by E.E. Milanovsky and A.M. Nikishin (1988), perhaps with some convention, also outlined the fourth, Western Pacific belt, which can be traced as a set of back-arc manifestations of rifting. The normal development of the rift belt here was suppressed by intense westerly displacement and subduction of the Pacific Plate.
Under all four belts to depths of the first hundreds of kilometers, tomography reveals negative velocity anomalies and increased attenuation of seismic waves, which is explained by the ascending current of heated mantle material (see Fig. 2.1). The correctness in the placement of rift zones is combined with global asymmetry both between the polar regions and relative to the Pacific hemisphere.
The orientation of the stretching vectors in rift zones is also regular; near-meridional and near-latitudinal ones predominate. The latter are maximum in the equatorial regions, decreasing along the ridges both in the northern and southern directions.
Only a few of the major rifts are located outside the global system. This is the Western European system (including the Rhine graben), as well as the Baikal (Fig. 5.3) and Fengwei (Shanxi) systems, confined to northeast-trending faults, the activity of which is believed to be supported by the collision of the continental plates of Eurasia and Hindustan.
Continental rifting
Active rift zones of continents are characterized by dissected relief, seismicity, and volcanism, which are clearly controlled by large faults, mainly normal faults. The main modern belt of continental rifting, stretching almost meridionally for more than 3 thousand km across all of East Africa, was called the Great African Rift Belt. The zones that form it branch and converge, obeying a complex structural pattern. In the rifts of this belt, lakes Tanganyika, Nyasa (Malawi) and others were formed; among the volcanoes associated with it are such giants as Kilimanjaro and Nyiragongo, famous for its activity. The Baikal rift system is also one of the most representative and well studied.
Relief, structure and sedimentary formations. The central position in the rift zone is usually occupied by a valley up to 40–50 km wide, bounded by faults, often forming stepped systems. Such a valley sometimes stretches along an arched uplift of the earth's crust (for example, the Kenya Rift), but can form without it. Tectonic blocks on the frame of the rift are elevated to levels of 3000-3500 m, and the Rwenzori mountain range in the north of the Tanganyika zone rises to 5000 m. Often rifts are complicated by longitudinal or diagonal horsts. In the Basin and Range region of North America, the extension of the earth's crust was distributed over a vast (almost 1000 km) area, where numerous relatively small grabens were formed, separated by horsts, which creates a complex tectonic relief. Sometimes, as, for example, in the east of the Brazilian shield, systems of asymmetric one-sided grabens are observed. In general, asymmetry of structure and topography is characteristic of many continental rift zones.
In their upper, exposed part, the faults are inclined to the horizon at an angle of up to 60 degrees. However, judging by the seismic profiles, many of them flatten out at depth; they are called listric (Greek: bucket-shaped). When moving along faults, a strike-slip component is often noticeable (on Baikal it is left-sided). For seismically active faults, extension along normal faults and displacements are also determined when solving focal mechanisms. As V.G. showed Kazmin (1987), diagonally oriented faults with strike-slip displacement and their echelon systems in some cases transfer movement from one opening rift to another and in this respect are similar to transform faults of oceanic rifting. In complex rift zones, such as the East African one, faults and strike-slip faults form regular and very expressive parageneses.
Along some relatively gently oriented faults parallel to their displacement, dynamothermal metamorphism develops, which can be judged in cases where, with further extension, metamorphites are exposed or approach the surface.
Sedimentary formations of continental rifts, predominantly molasse, are characterized by a combination with one or another amount of volcanics, up to cases when sedimentary formations are completely replaced by volcanic ones. According to E. E. Milanovsky, the thickness of the Cenozoic filling of rifts can reach 5-7 thousand m (for example, in South Baikal), but usually does not exceed 3-4 thousand m. Lacustrine clastic deposits predominate (including lacustrine turbidites) , alluvial, proluvial, and in the Baikal depressions also of fluvioglacial and glacial origin. As a rule, the roughness of the clastic material increases from bottom to top. Under the climatic conditions of the Afar Rift, the accumulation of evaporites was possible. In the zone of volcanism, the removal of matter by hydrothermal solutions also creates conditions for the deposition of specific chemogenic sediments - carbonate (including soda), siliceous (diatoms, opal), sulfate, chloride.
Magmatism and its products. Continental rifting is accompanied by magmatism and only locally its surface manifestations may be absent. So, in particular, there is no reliably established volcanism in the rift of Lake Baikal, but in the same system in the Tunkinsky and Charsky rifts there are fissure basalt outpourings. Volcanoes are often located asymmetrically - on one side of the rift valley, on its higher side.
Igneous rocks are extremely diverse, among them alkaline varieties are widely represented. Characteristic are contrasting (bimodal) formations, the formation of which involves both mantle basaltic melts (and their derivatives) and anatectic, predominantly acidic melts formed in the continental crust. In contrasting formations of the East African belt, along with alkaline olivine basalts, trachytes and phonolites, V. I. Gerasimovsky and A. I. Polyakov indicate rhyolites, comendites, and pantellerites. In the potassium series there are leucitites and leucite basanites. There are alkaline ultrabasites and accompanying carbonatites.
According to M. Wilson (1989), data on the contents of rare elements and isotopic ratios of neodymium and strontium in different volcanic formations of the East African belt indicate an unequal degree of contamination of mantle magmas with bark matter. It turned out that in some series the entire diversity of rocks was due to fractional crystallization.
Geophysical characteristics. According to geophysical data, the thickness of the crust under continental rifts decreases and a corresponding rise of the Mohorovicic surface occurs, which is there in mirror correspondence with the ground relief. The thickness of the crust under the Baikal rift decreases to 30-35 km, under the Rhine rift - to 22-25 km, under the Kenyan rift - to 20 km, and to the north, along the Afar valley, it reaches 13 km, and then oceanic appears under the axial part of the valley bark.
In the mantle protrusion under the rift, the rocks are decompressed (longitudinal wave velocities vary in the range of 7.2-7.8 km/s), their elastic characteristics are reduced to values characteristic of the mantle asthenosphere. Therefore, they are considered either as an asthenospheric diapir (for the Rio Grande and Kenya rifts) or as a lens-shaped “cushion” extended along the rift zone and to some extent isolated from the main asthenospheric layer. Such a lens with a thickness of 17 km was discovered by seismic sounding near Lake Baikal. It has been noted that in asymmetric rifts the crest of the mantle protrusion most often does not coincide with the axis of the valley, but is shifted towards a higher wing. Volcanic centers are also located there.
The shallow location of the asthenosphere limits the depth of seismic sources. They are located in thinned crust, and depending on its thickness, the maximum depth of the foci varies from 15 to 35-40 km. The solution of the focal mechanism of the sources establishes faults and subordinate strike-slip displacements.
The proximity of the heated asthenosphere, volcanism and increased permeability of the faulted crust are expressed in the geothermal field; the heat flow in the rifts is sharply increased. Magnetotelluric sounding determined the high electrical conductivity of rocks in the asthenospheric ledge.
In the gravitational field, the rift zone corresponds to a negative Bouguer anomaly, which extends in a wide strip and is believed to be caused by the decompression of mantle rocks. Against the background, sharper negative anomalies can be seen above the rift basins with their loose sedimentary filling and positive anomalies marking the intrusion zones of mafic and ultramafic igneous rocks.
Mechanisms of rifting. Physical models of rift formation take into account the observed concentration of extensions in a relatively narrow band, where a corresponding decrease in the thickness of the continental crust occurs. Along the weakened zone, an increasingly thin “neck” is formed, until the continental crust ruptures and moves apart and is filled with oceanic-type crust. In different rifts, such a critical moment apparently occurs at different maximum thickness of the sialic crust (in the Red Sea and Aden rifts it was thinned by approximately half) and signifies the transition from continental to oceanic rifting.
Rice. 5.4. Models of continental rifting. According to R. Allmendinger et al., (1987):
a - classical model of symmetrical horsts and grabens; b - model of Smith and others with subhorizontal failure between the layer of brittle and layer of plastic deformations; c - model of W. Hamilton and others with a lens-shaped deformation; d - B. Wernicke’s model, providing for asymmetric deformation based on a gentle fault
Since stretching at the earth's surface in continental rifts occurs through fault displacements, the original, classical model of rifting took into account only these brittle deformations (Fig. 5.4.a). According to the calculations of J. Angelier and B. Coletta, the total effect of displacement along faults gives a stretch of 10-50% in the Gulf of Suez to 50-100% in the California system and up to 200% in the south of the Basin and Range region. In one section of the Afar Valley, calculations by W. Morton and R. Black gave a threefold stretch. Such high values were satisfactorily explained in later models, which were built taking into account changes in the mechanical properties of rocks with depth, as pressures and temperatures increased. R. Smith's model (Fig. 5.4, b) provides for the existence of a layer of plastic deformations in the lower crust, under the layer of brittle deformations. In this case, as they stretch, the faults bend and flatten in their lower part, becoming listric. The descent of blocks along such faults is accompanied by their rotation (overturning), and the degree of stretching increases from the edges of the rift zone to its center. The same effect can be obtained by assuming that in the middle part of the crust there is another, transitional, layer of deformation, where the displacement is dispersed over many small diagonal shears or subhorizontal slip surfaces.
All of these variants of rifting involve local thinning of the crust under the action of tensile stresses with the formation of a symmetrically constructed rift zone. D. Mackenzie (1978) quantified the consequences of such thinning: isostatic subsidence of the crust and counter-uplift of the asthenospheric ledge, to which this researcher assigns a passive role.
Another model that takes into account new data on the deep structure of continental rifts and the asymmetry inherent in many of them was proposed by B. Wernicke (1981). The leading role is given to a large flat (10-20°) fault, the formation of which may involve the use of intracrustal asthenospheric layers (Fig. 5.4d). As it stretches, the hanging wall becomes complicated by a stepwise system of small listric faults, while the other wall is dominated by a scarp corresponding to the main fault plane. The above-mentioned dynamothermal metamorphism and the release of metamorphites to the surface during further sliding of the hanging wall down the fault plane are also associated with it. B. Wernicke's model successfully explains a number of other features of the structure and development of asymmetric rifts. When the crust is thinned by displacement along a gentle fault, the asthenospheric protrusion should not be under the axial part of the rift, but under the hanging wing, supporting and lifting it, which is observed in many profiles. Volcanism is localized on the same high side of the rift. A similar asymmetry is well expressed in the East African belt, along which rifts with relatively elevated western and eastern wings alternate.
Taking into account new geophysical data, there is no doubt about the diversity of the deep structure of continental rifting zones. Therefore, none of the listed models can claim universality, and the mechanism of rift formation varies depending on conditions such as thickness, structure, crustal temperature and extension rate.
Hydraulic wedging mechanism. All of the above models are based on compensation for the stretching of the cortex by its mechanical deformation (brittle or plastic), a decrease in thickness and the formation of a “neck”. In this case, magmatism is assigned a passive role. Meanwhile, in the presence of pockets of basaltic magma at depth (with its high liquid properties), a fundamentally different mechanism comes into play.
There is every reason to believe that the rapid rise of basaltic magma to the surface is ensured in extension zones: the wedging effect that magma has on lithospheric rocks. Ideas about this process are based on the study of linear dikes and their systems (which are considered as frozen magmatic wedges) and on the application of the theory of hydraulic fracturing of rocks to them. It was based on detailed work on the study of the Tertiary and Paleozoic dikes of Scotland, culminating in generalizations by J. Ritchie and E. Anderson. Already on this material the characteristic features of linear dikes were determined. As a rule, they are introduced along vertical fractures by spreading the wings perpendicular to the fracture without significant compaction or crushing of the rocks hosting the dyke. There is usually no fault or strike-slip displacement during intrusion. The dikes form a subparallel system, within which the thickness of the dikes is maintained uniform.
E. Anderson showed the active role of magma in the formation of the dike. Intruding along a crack perpendicular to the minimum compressive stress, the magmatic melt has a wedging effect, increasing the length of the crack (see Fig. 5.5,III). A further study of the dependence of the intrusive process on the ratio of the principal stresses near the magma chamber was given by J. Robson and K. Barr. However, a quantitative substantiation of the dike penetration mechanism became possible later, in connection with the development of the theory of hydraulic fracturing of rocks during oil production. M. Hubbert and D. Willis drew an analogy between artificial hydraulic fracturing and the intrusion of magmatic dikes into the earth's crust. In relation to the latter, the issue was specifically considered by A.A. Peck and V.S. Popov.
Hydraulic fracturing (hydraulic fracturing) is the process of formation and propagation of cracks in rocks under fluid pressure, including magmatic melt. The stretching of the earth's crust can be expressed by gaping separation cracks only at very shallow depths - up to 2-3 km. Deeper, with increasing confining pressure and temperatures, brittle separation is replaced, as already noted, by shearing along more and more numerous planes, and then turns into plastic deformation. Since basaltic dike systems originate at great depths, their formation by passive filling of gaping cracks is excluded. The only possible mechanism is active penetration through hydraulic fracturing of rocks with subsequent expansion of the crack walls.
For hydraulic fracturing to develop, it is sufficient that the fluid pressure only slightly exceeds the minimum compressive stress in the rock; Usually in calculations their ratio is taken to be 1.2. A hydraulic wedge is formed, the fluid front comes close to the end of the crack, but never reaches it. The wedging effect is ensured by the concentration of stress at the tip of the crack, where its propulsion pressure increases from the tip in proportion to the cube of the crack opening in accordance with the decrease in hydraulic resistance (see Fig. 5.5,IV). The development of hydraulic fracturing is little affected by real differences in the strength of the host rocks. There is a rapid propagation of the brittle fracture and the magmatic wedge that propels it. As calculations by N.S. showed. Severina, the heat transfer of such an injection is compensated by the release of heat due to friction at the contacts, so there is no significant increase in viscosity, which would slow down the penetration process. According to seismological observations by V.M. Gorelchik and others during the Tolbachik fissure eruption in Kamchatka, the basalt wedge rose there at a speed of 100-150 m/h.
The intrusion of a vertical dike becomes possible when one of the main compressive stresses, directed horizontally, is reduced by tectonic extension. Parallel dikes belonging to the same swarm apparently were injected sequentially: each successive hydraulic wedge created an aureole of compressive stresses, which prevented other injections, and was subsequently gradually removed by tectonic extension.
Thus, if there is liquid magma at the depth of the reservoir, conditions arise for the growth of lithospheric layers under the influence of many parallel hydraulic fractures, in each of which the injection of the melt leads to the spreading of the host rocks. The magmatic underlay of the lithosphere layer injected by dikes provides the necessary freedom of horizontal sliding. It is possible that both hydraulic wedging and mechanical extension may occur alternately or simultaneously (at different levels) in one rift zone.
For continental rifts, the mechanism of hydraulic wedging becomes significant at the final stage of their development, when the thinning of the crust approaches critical values, and a decrease in the load on the asthenospheric ledge contributes to a greater separation of basalt melts. It is under such conditions that longitudinal swarms of parallel dikes, discovered by P. More (1983) and associated with basaltic volcanism, appear on the western side of the Afar Rift. In the Red Sea Rift, a similar phase began about 50 million years ago and intensified 30 million years ago, when powerful swarms of parallel dikes of contrasting composition (from tholeiitic basalts to granophyres) penetrated into the ancient granite crust, which can be traced along the northeastern coast. Only 5 million years ago, magmatic wedges concentrated in a narrow strip, causing the separation of the Arabian Plate. Continental rifting gave way to oceanic rifting, which continues to the present day.
In cases where the development of a continental rift ceases at an earlier stage, it remains as a weakened zone, a groove in the continental plate, as exemplified by aulacogens (see Chapter 13).
5.3. Oceanic rifting (spreading)
Oceanic rifting, which is based on spreading through magmatic wedging, can thus develop as a direct continuation of continental rifting. At the same time, many modern rift zones of the Pacific and Indian Oceans were initially formed on the oceanic lithosphere due to rearrangements in plate movement and the death of earlier rift zones.
The assumption about the formation of the earth's crust in the mid-ocean ridges during their expansion by mantle convection, the rise and crystallization of basaltic magma was expressed by A. Holmes back in the 30s and 40s, likening the ocean crust diverging from the active zone to endless conveyor belts. This idea was further developed after G. Hess (1960) used it as the basis for ideas about the evolution of the oceans. R. Dietz (1961) coined the term seafloor spreading(English, spread - to unfold, spread). Soon G. Bodvarson and J. Walker. (1964) proposed a mechanism for the spreading of oceanic crust through dikes, which was the focus of the Iceland and the Mid-Ocean Ridges symposium and initiated the deciphering of the tectonomagmatic processes that form the crust in the spreading zone. Intensive research over the next decades, including deep-sea drilling and detailed surveying of spreading zones using manned underwater vehicles, provided a wealth of new material for this.
Spreading in Iceland. For understanding ocean rifting, data from Iceland, where the Mid-Atlantic Ridge is elevated above sea level for 350 km, is of particular interest. The history of repeated fissure basalt outpourings has been known there for a millennium, and since the last century, special geological research has been carried out, which was later supplemented by geophysical and high-precision geodetic observations. Modern tectonic and volcanic activity is concentrated in the submeridional neovolcanic zones crossing the island in its central part. The youngest basalts corresponding to the Brunhes era are confined to their axis. They are bordered by basalts with an age of 0.7-4 million years, then from under them there emerges a powerful series of plateau basalts up to the Middle Miocene (16 million years), which lie with a predominant counter slope towards neovolcanic zones. It is characteristic that in the opposite direction (from the axial zones) the basalt covers decrease in thickness and successively wedge out, starting from relatively young ones. As a result, at any point the inclination of the basalts from top to bottom increases: from a horizontal occurrence near the already eroded roof of the plateau basalts to 3-4° at levels of about 1000 m, 7-8° at sea level and approximately 20° at a depth (2000 m (according to drilling data) Each fissure eruption leaves a horizontally lying (and wedging out across the strike of the zone) basalt cover with a thickness of up to 10 m or more, as well as its supply channel - a vertical dolerite dike, most often 10 m wide, oriented perpendicular to the axis of minimum compressive stresses, i.e. along the rift zone. Each subsequent eruption adds one basalt cover and one dike, so down the section of plateau basalts the dikes become thicker. This issue was specially studied by J. Walker in Eastern Iceland. He established a natural decrease in the number of dikes as they rise from sea level to watershed marks. 1000-1100 m and extrapolated their further decrease according to a linear relationship. All such graphs showed the complete pinching of dikes at levels of 1350-1650 m, i.e. exactly where the primary roof of the plateau basalts should have been located. It is assumed that below sea level the number of dikes increases accordingly.
As the plateau basalts layer, they undergo gravitational subsidence, which is largely compensatory in relation to the feeding magma chamber, which was traced by magnetotelluric sounding. At the same time, as more and more parallel dolerite dikes are introduced, they move apart by the value of their total thickness. Based on such observations, G. Bodvarson and J. Walker proposed a mechanism for the expansion of the earth's crust through the intrusion of dikes. In Fig. 5.5.1 from a later publication by G. Palmason (1973) this mechanism is explained by a kinematic diagram. It shows the calculated trajectories and isochrones of the movement of newly formed rocks in the axial zone during their subsequent descent and movement to one side of the axis. The diagram by I. Gibson and A. Gibbs (Fig. 5.5, II) illustrates the ever-increasing slope of plateau basalts at depth and the structure of fan-shaped monoclines that form on both sides of the axial zone as the subsidence of the erupting basalts and wedging of the active zone by dikes. The latter are vertical when intruded, and subsequently tilt together with the host plateau basalts. The end result is the formation of a second layer of ocean crust.
Rice. 5.5. Model of the formation of the second layer of ocean crust in Iceland, Mid-Atlantic spreading zone:
I - kinematic diagram of G. Palmason (1973): trajectories of movement of erupted basalts (dotted line) and isochrones of their movement (solid lines) during the process of spreading and isostatic subsidence. II - diagram by I. Gibson and A. Gibbs (1987), explaining the mechanism of spreading through the introduction of dikes and surface outpourings of basalt: the wedging effect of dikes determines the spreading, subsidence under the load of basalts forms fan-shaped monoclines on both sides of the axial zone (K - a complex of parallel dikes ). III - intrusion of a basalt dike in a plane perpendicular to the minimum compressive stress, according to E. Anderson and M. Habert. IV - basalt dike as a hydraulic wedge: diagram of the stresses (P) propagating the crack, which sharply decrease towards the top of the hydraulic wedge in inverse proportion to the cube of the crack opening, which creates a stress concentration there, a wedging effect and the advancement of the wedge (according to A.A. Peck, 1968) : l
- crack length; d
- crack opening: R k
-
pressure of the injected fluid at the beginning of the crack; R b
-
lateral stresses compressing a crack
The actual implementation of this model in Iceland is complicated by multiple lateral “leaps” of the axis of fissure eruptions within the volcanic zone and even displacement of this entire zone. In addition, some of the extension occurs on faults and open cracks, i.e., pull-aparts. It is believed that such structures compensate at the top for the intrusion of those dikes that did not reach the surface. In particular, screened dikes probably end with dolerite sills, which are numerous among plateau basalts. In addition, during fissure eruptions, part of the basaltic magma spreads from the volcanically active area along the strike of the zone through the longitudinal growth of dikes. According to G. Sigurdson, several such intrusions occurred after the fissure eruption of Krabla in 1975; their advance at a speed of several hundred meters per hour was accompanied by seismic tremors and subsidence of the surface in a strip a few kilometers wide. The total amount of subsidence reached 1.5 m, including the amplitude of displacement along some faults - up to 1 m.
The use of observations from Iceland, despite their detail and reliability, is limited by the anomaly of this segment of the mid-ocean ridge relative to normal submarine spreading zones. The thickness of the ocean crust here is much higher than normal (up to 40 km), which stably maintains the surface of the island above sea level throughout its geological history. Taking into account the characteristic geochemical features of Icelandic basalts, this is explained by the passage of the spreading axis above the mantle jet, lifting material from the deep parts of the mantle and increasing the rate of supply of basaltic melt, which forms an oceanic crust of increased thickness (see Chapters 6 and 7).
Spreading in submarine mid-ocean ridges. With the help of manned underwater vehicles, a number of segments of the rift zones of the ocean have now been studied in detail. This work began with the French-American FAMOUS program, according to which in 1974-1975. sections of the Mid-Atlantic Ridge southwest of the Azores, located in the rift valley, on the transform fault and at their junction, were mapped. The seismically and volcanically active axial part of the rift valley in the studied segment turned out to be built symmetrically (see Fig. 10.1, II). On both sides of the recently erupted pillow lavas, which form mounds stretched along longitudinal cracks, products of increasingly earlier fissure eruptions were traced for a distance of 1.5 km in one direction and the other, which was determined by the thickness of the weathering crusts on the lava pillows.
Subsequently, to the south, in the area of the Kane fault, similar studies under the MARK program covered several segments of the Mid-Atlantic Ridge separated by faults with a total length of about 80 km (see Fig. 10.1, I,IV,V,VII). It was discovered that even such fractional segments have distinct structural differences between themselves and that during spreading, active spreading shifted from one segment to another. Thus, the expansion of the ridge represents the cumulative effect of all these local episodes. The profiles show that even during periods of absence of fissure eruptions, extension continues, expressed by stepwise faults. In some segments, part of the expansion is compensated by the uplift of tectonic blocks of gabbro and serpentinized peridotites, i.e. rocks of layer III of the oceanic crust and lithospheric mantle.
As further deep-sea studies showed, these observations are not accidental. Zones with low spreading rates, such as the Mid-Atlantic, fall into segments, in each of which spreading itself (magmatic, constructive) alternates with phases of structural, deformational rifting, similar to continental, when stretching and thinning of the crust occurs. During these phases, rift valleys limited by faults are formed or renewed, which, as on continents, in some cases are symmetrical, in others, on the contrary, consistent with B. Wernicke’s model of deformations based on a large gentle fault. According to A. Carson (1992), the duration of such alternating phases reaches tens and first hundreds of thousands of years. In this case, neighboring segments of the ridge can be in different phases at the same time.
As each segment undergoes extensional faulting, central rift valleys are observed in low-velocity spreading zones throughout their length. For high-velocity ones, such as the East Pacific, rift valleys are uncharacteristic and their development is clearly dominated by magmatic spreading. At the same time, the stability of the axis of fissure eruptions was noticed in them, in contrast to zones of the Atlantic type, where lateral wandering and small “jumps” of the magmatic axis, similar to those observed in terrestrial conditions in Iceland, are not uncommon.
In the youngest spreading basins, located in a close continental frame, rapid sedimentation is possible, preventing free fissure eruptions and the formation of a normal II layer. Before reaching the surface, the dikes end in the sediment, forming sills, as found in the Guaymas basin of the Gulf of California.
Volcanic zones of mid-ocean ridges are associated with outcrops of high-temperature hydrothermal fluids, which are especially numerous at high spreading rates. Associated with them are copper-zinc sulfide ores, ferromanganese metal-bearing sediments, as well as greenstone alteration of basalts.
Formation of ocean crust in spreading zones. Modern ideas about the mechanisms of formation of the ocean crust are based on observations in active spreading zones in comparison with data from deep-sea drilling, as well as detailed studies of ophiolites - fragments of ancient ocean crust on continents (see Chapter 12). The formation of layer II with a basaltic upper part and a complex of parallel dolerite dikes below has already been discussed above as a result of sequential hydraulic wedging. Sources of basaltic melt feeding magmatic wedges have now been delineated by multichannel seismic profiling, but only in medium- and high-velocity spreading zones. Extending longitudinally, these foci are small in cross section; with a width of about 1 km and a height of only a few hundred meters, they are located at a depth of 1-2 km from the surface. In particular, in the East Pacific belt at 9°30"N, according to R. Detrick et al. (1937), the upper boundary of the magma chamber was traced at a depth of less than 1 km, and the newly formed oceanic crust above it was represented only by a layer II.
In such a roof, in places, stock-shaped bodies of massive gabbro-diabase and microgabbro are intruded, which break through a complex of parallel dikes and, in turn, can be intersected by later dike complexes.
As the newly formed crust moves away from the spreading axis, the corresponding part of the magma reservoir moves away from the feeding system along with it. It is no longer replenished by basaltic melts of the asthenosphere, loses connection with the main source of heat and cools under conditions favorable for crystallization differentiation (see Fig. 2.3, below). Thus, under layer II, layer III of the oceanic crust is formed - a layered complex of gabbroids, which contains gradations from melancocratic varieties at the top to dunite cumulates at the bottom of the section. Small amounts of residual melt are sometimes squeezed out, forming small intrusions of plagiogranites, comagmatic with the rest of the series of rocks.
Later, during the movement of the already two-layer oceanic crust from