Collision Of Continental Plates Research Articles

Geological and Morphometric Characteristics of Quaternary Pyroclastic Aquifers in Salak and Pangrango Stratovolcano.

Indonesia is located in a subduction zone formed by the collision of continental and oceanic plates. Many volcanoes form as a result of these conditions along the arc. Springs on the volcano's slopes are widely used for domestic, irrigation, and industry water use. Investigating the characteristics of aquifers and springs is essential to ensure groundwater sustainability by providing a robust geological framework. Meanwhile, groundwater in a volcanic geological setting has good quality characteristics because of its occurrence process, which interacts with many minerals in pyroclastic rocks that act as aquifers. The study area is located in the Lido Catchment Area (LCA), which is situated between two distinct volcanic slopes. Geological and morphometric analysis at LCA is the basis of important information relating to its hydrogeological systems. The analysis of thin rock sections and parameters of the physical properties of water in groundwater springs supports our research. We applied a comprehensive geological and morphometric analysis to obtain detailed information about the aquifer. This study aimed to determine the characteristics of aquifers in pyroclastic rocks and their relationship to the formation of springs. From the research conducted, the characteristic of water can be distinguished based on the geological conditions of its constituents. There are 6 different lithological characteristics in the study area: polymictic breccia, monomictic breccia, lapilli, lapilli tuff, coarse tuff, and fine tuff. From the lithology variations obtained, breccia, lapilli, and coarse tuff can play a good role as aquifers. Geological correlations and groundwater systems in the study area show groups of superficial, mixed, and alteration springs. The system of water flows in aquifers through inter-grains or rock fractures. The types of springs in LCA are dominated by depression and fracture types. These results are fundamental information for understanding hydrogeological systems in future volcanic geological settings.

Ongoing formation of felsic lower crustal channel by relamination in Zagros collision zone revealed from regional tomography

Complex interaction of rheologically contrasting layers within the lithosphere during the collision of continental plates leads to active faulting, which represents a serious hazard to the population and infrastructure. One of the collision scenarios presumes the existence of a middle-lower crustal channel composed of subducted silicic upper crustal rocks, which is thought to exist in the Tibetan-Himalayan system. Based on the results of seismic tomography, we argue that a similar mechanism of crustal channeling takes place beneath the Zagros mountain system in southwestern Iran. The 3D seismic velocity model reveals an inverted crustal architecture of the collision zone, in which the low-velocity felsic (granitic and sedimentary) upper crustal rocks of the Arabian plate form a seismically inactive lower crustal channel below the higher-velocity mafic (basaltic) middle-upper crustal layer of the Iranian crust. Based on existing numerical models, we suggest that the formation of the felsic channel is likely governed by separation (delamination) of the weak felsic upper crust of the subducting Arabian lithosphere and its ductile underplating under rheologically stronger upper-middle crust of the Iranian plate.

Autunite-Group Minerals and Their Paragenesis from the Sheared Granite of Gabal El Sela, South Eastern Desert, Egypt

G. El Sela is located in the Southern Eastern Desert of Egypt cropping as two parts, occupied by monzogranites that were categorized as biotite granite, muscovite granite and two mica granites. The northern part is more significant according its high concentrations of uranium that controlled by complicated structure regime; faulting, infrastructures and shearing are the most common structural criteria of this part. The Egyptian Nuclear Materials Authority (NMA) mined this part to produce the uranium ore. The previous mineralogical studies indicated that this granite was dominated by primary uranium minerals (pitchblende and coffinite) and secondary minerals belong to the autunite group (autunite, metautunite, phurcalite) in addition to uranophane. In the present work, petrographic and mineralogical studies are applied for the granites using the polarized and stereo microscopes and followed by electron microscope and XRD. The result of the microscopic examinations revealed the tectonic regime controlling the radioactivity and recognized the sodic autunite (meta-natroautunite) beside the pre-mentioned autunite group minerals completing the paragenetic sequrnce of these minerals. In this study, it is concluded that the sheared biotite granite is monzogranite originated during the episode of the continental plate collision (syncollision). The study finished to presence of two main types of the alteration corresponding to the two high levels of radioactivity (moderate and anomalous). The first is the thermal alteration (saussiritization, sericitization, kaolinization, silicification and hematization) and the second is the chemical transformation (oxidation, dehydration, ion substitutions and confusion) responsible for formation of the secondary uranium minerals. The temperature needed for the thermal alteration is sourced by the hydrothermal solutions, while the temperature needed for the uranium minerals transformation may be generated during the episode of the continental plate collision (syncollision). Paragenesis of these minerals indicates that they represent a series of uranyl phosphate minerals (autunite group) with paragenetic sequence starting by autunite (calcic uranyl phosphate) and ends by meta-natroautunite (sodic uranyl phosphate). An advanced process of dehydration accompanies the process of mineral transformation from autunite to meta-natroautunite leading to formation of the anhydrous uranyl mineral (phurcalite) formed by oxidation and dehydration of autunite. Meta-autunite is recorded as a transitional mineral composed of sodic-calcic uranyl phosphate. Uranophane is created by thermal confusion of autunite with the silica.

Research on geological features of Huaniushan Gold Deposit in Beishan of Gansu province

The Beishan orogenic belt is located in the south rim of central Asia orogenic belt, the intersection of east Tianshan and Tarim plates. Thus, the geological structure is extremely complex. The gold deposits are widely distributed in various types and the met allogenic mechanism is complex. Overall, the gold deposits in Beishan orogenic belt are mostly developed in Palaeozoic continental plate collision and butting belt, which has a certain spatial and temporal distribution law. Huaniushan gold deposit is a typical gold deposit in Beishan Orogenic belt. This paper carried out a detailed study of the geology of Huaniushan gold deposit, to provide some basis for the classification of met allogenic stage.

The Caucasian-Arabian segment of the Alpine-Himalayan collisional belt: Geology, volcanism and neotectonics

The Caucasian-Arabian belt is part of the huge late Cenozoic Alpine-Himalayan orogenic belt formed by collision of continental plates. The belt consists of two domains: the Caucasian-Arabian Syntaxis (CAS) in the south and the EW-striking Greater Caucasus in the north. The CAS marks a zone of the indentation of the Arabian plate into the southern East European Craton. The Greater Caucasus Range is located in the south of the Eurasian plate; it was tectonically uplifted along the Main Caucasian Fault (MCF), which is, in turn, a part of a megafault extended over a great distance from the Kopetdag Mts. to the Tornquist-Teisseyre Trans-European Suture Zone. The Caucasus Mts. are bounded by the Black Sea from the west and by the Caspian Sea from the east. The SN-striking CAS is characterized by a large geophysical isostatic anomaly suggesting presence of mantle plume head. A 500 km long belt of late Cenozoic volcanism in the CAS extends from the eastern Anatolia to the Lesser and Greater Caucasus ranges. This belt hosts two different types of volcanic rocks: (1) plume-type intraplate basaltic plateaus and (2) suprasubduction-type calc-alkaline and shoshonite-latite volcanic rocks. As the CAS lacks signatures of subduction zones and is characterized by relatively shallow earthquakes (50–60 km), we suggest that the “suprasubduction-type” magmas were derived by interaction between mantle plume head and crustal material. Those hybrid melts were originated under conditions of collision-related deformation. During the late Cenozoic, the width of the CAS reduced to ca. 400 km due to tectonic “diffluence” of crustal material provided by the continuing Arabia-Eurasia collision.

A brief account of the continuum from Karakoram and Himalayan ranges to foreland and ocean basins

Himalayan orogeny in relation to Global Tectonics is a hotly debated subject. Orogeny can result either from collision of continental plates or by an endogenic process of mantle upwelling along pre-existing fracture zones in the crust. This paper describes a new technique of frequency analysis of radiometric ages of crystalline rocks in the mountain ranges straddling the Indus-Tsangpo Suture Zone (ITSZ), the supposed line of collision between India and Asia, and shows that all the ranges from Karakoram to Lesser Himalaya across the ITSZ got uplifted simultaneously in a particular sequence. This contradicts the concept of suturing, places the ITSZ in the category of one of the intracrustal thrusts of the Himalayan orogenic system and establishes the family affinity between the Karakoram and the Himalaya. Analysis of stratigraphic and structural data points out that this family affinity is not confined to the ranges only but extends beyond into the foreland and oceanic basins as well. This paper also explains a number of apparent contradictions in terrain geomorphology, ophiolites in suture zones, thickness anomaly of Himalaya-derived sediments in eastern and western wings of terrestrial and oceanic basins to the south of the ranges, the status of Mid-oceanic Ridges in global tectonics, convergence aspect between Himalaya and India, nature of Foothills Fault and metamorphism of early Tertiary sediments in the Himalayan foreland basin.

Molecular Divergence in AllopatricCeratosolen(Agaonidae) Pollinators of Geographically WidespreadFicus(Moraceae) Species

AbstractSpeciation in pollinating seed predators such as fig wasps (Hymenoptera: Agaonidae) is likely to have been influenced by a combination of ecological and geographical isolating mechanisms, but recent molecular analyses of fig wasps have focused on pollinator specialization as the main factor driving speciation. This study investigates the contribution of geographic modes of speciation such as dispersal, vicariance, and isolation by distance. We sampled haplotypes of mitochondrial cytochrome oxidase I from Ceratosolen pollinators of six geographically widespread Australasian fig (Moraceae: Ficus) species, including four species spanning Wallacea. Phylogenetic analysis investigated the extent of host conservatism and host switching accompanying divergence in Ceratosolen. Geographically widespread Ceratosolen showed deep intraspecific divergence exceeding or comparable to divergence between named sister species. Maximum parsimony and Bayesian analyses supported species monophyly in five of six cases, whereas results for a sixth species were equivocal. Bayesian divergence time estimation suggested dispersal across Wallacea during the Miocene epoch, after the collision of Australian and Asian continental plates. Cryptic species were evident in all six focal taxa. Because the deep mitochondrial divergence within these taxa is regionally distributed, allopatric divergence provides a simple explanation for the existence of these cryptic lineages pollinating widespread fig species. We found little evidence of divergence accompanied by host switching. The ancient origin of cryptic and geographically isolated species suggests that long-distance dispersal may be rare in Ceratosolen and that host associations are generally conserved during range expansion.

Three-dimensional mantle lithosphere deformation at collisional plate boundaries: A subduction scissor across the South Island of New Zealand

The continental plate collision across the South Island of New Zealand is highly oblique (dextral) and bounded by oppositely verging ocean plate subduction zones. As such, the region can be considered as a type of ‘subduction scissor’. Within this tectonic context, we use three-dimensional computational geodynamic models to consider how convergent mantle lithosphere can be modified by scissor and strike–slip effects. Bounding subduction at both ends of the continental collision causes flow of the descending mantle lithosphere in the direction along strike of the model plate boundary, with thinning in the centre and thickening towards the subduction zones that bifurcates the continental mantle lithosphere root. With dipping bounding subduction, the mantle lithosphere root takes on a more complex morphology that folds over from one subduction polarity to the other, but remains as a continuous feature as it folds under the collision zone. In the absence of bounding subduction, the plate convergence causes a linear (along strike) mantle lithosphere root to develop. A rapid strike–slip motion between the converging plates transfers material in the plate boundary-parallel direction and tends to blur out features that develop in this direction—such as descending viscous instabilities. The along-strike variations in the morphology of the mantle lithosphere root that develop in the models—viz., thickening of the root towards the subduction edges, thinning in the centre—are consistent with recent, albeit poorly constrained, geophysical interpretations of the large-scale lithospheric structure of the South Island. We speculate that this reflects the nature of the evolution of the South Island collision as a limited continental segment of the plate boundary that it is dominated and guided by adjacent well-developed/developing ocean plate subduction.

Continental plate collision, P– T– t– z conditions and unstable vs. stable plate dynamics: Insights from thermo-mechanical modelling

There was much debate recently on the mechanisms of continental convergence and related pressure–temperature ( P– T) conditions, both in modeling and petrologic community. Depending on the mechanisms of convergence (subduction, collision, folding or RT instability) one can argue about the possibility of large-scale deviations of pressure and temperature in the accretion prism and below it, from “reference” (i.e. lithostatic) conditions commonly used for petrologic reconstructions of evolution of exhumed metamorphic rocks. These deviations can be caused, for example, by tectonic overpressure that, as suggested in some studies, may be responsible for formation of UHP (Ultra High Pressure) rocks. However, overpressure in exhumation zones can be built only in specific collision scenarios associated with plate coupling (pure shear, folding). In this study, we analyze conditions that define various mechanisms of convergence, and consequently, of exhumation. These mechanisms can be represented as a super-position of (1) simple shear (subduction), (2) pure shear (collision), (3) folding and (4) Rayleigh–Taylor instability. We study these scenarios using a thermo-mechanical model that accounts for brittle–elastic–ductile rheology, surface processes, and metamorphic changes. It appears that stable, “oceanic-type” subduction may occur in the case of cold lithospheres ( T Moho < 550 °C) and relatively high convergence rates (> 3–5 cm/yr). Depending on the lower-crustal rheology (strong or weak), either the whole (upper and lower) crust or only the lower crust can be involved in subduction. In case of weak metamorphic rheologies, phase changes improve chances for stable subduction. Pure shear becomes a dominant mechanism when T Moho > 550 °C or convergence rates are lower than 3 cm/yr. Large-scale folding is favored in case of T Moho = 500–650 °C and is more effective in the case of mechanical coupling between crust and mantle (e.g., strong lower crust). Gravitational (Rayleigh–Taylor) instabilities overcome other mechanisms for very high values of T Moho (> 800 °C) and may lead to development of subvertical “cold spots.” However, it is reasonable to assume that in most cases continental collision is initiated at oceanic subduction rate, which is rarely slower than 5 cm/yr. This rate is sufficient to drive continental subduction during the first several Myr of collision. In this case, the subduction channel is characterized by nearly lithostatic pressure conditions. Large-scale zones of tectonic overpressure may be built outside the channel but do not affect the exhumed rocks. Overpressure may be built inside the channel in the short moment of its closure. We suggest that most continental orogenic belts could have started their formation from continental subduction. This evokes small tectonic overpressures and thus deep origin of the UHP rocks that may be brought to the surface via the suggested multi-level mechanism of exhumation.

Geodynamics of continental plate collision during late tertiary foreland basin evolution in the Timor Sea: constraints from foreland sequences, elastic flexure and normal faulting

Tectonic subsidence of the Australian lithosphere during the Late Tertiary propagates from the southwest to the northeast in the Timor Sea, as a consequence of the oblique collision between the Eurasian and Australian plates. We reconstruct the asynchronous nature of deflection of the Australian plate created during the plate convergence by best-matching the geometry of de-compacted foreland strata against the predictions of simple bending elastic beam models. We infer a maximum subsidence of 3500 m and a maximum width for the basin of ∼470 km. The effective elastic thickness of the Australian lithosphere (∼80 to 100 km) does not change significantly during basin evolution. The low curvature imposed on the plate (∼5.1×10 −8 m −1) during bending is too small to weaken the plate. Yet, abundant but small-slip, normal faulting related to bending implies some degree of inelastic yielding. The polarity of fault propagation supports the oblique nature of the collision. Flexural models indicate that at least 570 km of Australian plate (mostly areas of stretched continental crust) was flexed, primarily by the tectonic loading of the Timor Island and that the total amount of subducted plate was at least 100 km during basin evolution.

Continental plate collision: Unstable vs. stable slab dynamics

Research Article| January 01, 2004 Continental plate collision: Unstable vs. stable slab dynamics Gisèle Toussaint; Gisèle Toussaint 1Lab Tectonique UMR 7072, University of Pierre and Marie Curie, 4 place Jussieu, 75252 Paris, France Search for other works by this author on: GSW Google Scholar Evgene Burov; Evgene Burov 1Lab Tectonique UMR 7072, University of Pierre and Marie Curie, 4 place Jussieu, 75252 Paris, France Search for other works by this author on: GSW Google Scholar Laurent Jolivet Laurent Jolivet 1Lab Tectonique UMR 7072, University of Pierre and Marie Curie, 4 place Jussieu, 75252 Paris, France Search for other works by this author on: GSW Google Scholar Author and Article Information Gisèle Toussaint 1Lab Tectonique UMR 7072, University of Pierre and Marie Curie, 4 place Jussieu, 75252 Paris, France Evgene Burov 1Lab Tectonique UMR 7072, University of Pierre and Marie Curie, 4 place Jussieu, 75252 Paris, France Laurent Jolivet 1Lab Tectonique UMR 7072, University of Pierre and Marie Curie, 4 place Jussieu, 75252 Paris, France Publisher: Geological Society of America Received: 02 Jul 2003 Revision Received: 23 Sep 2003 Accepted: 25 Sep 2003 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (2004) 32 (1): 33–36. https://doi.org/10.1130/G19883.1 Article history Received: 02 Jul 2003 Revision Received: 23 Sep 2003 Accepted: 25 Sep 2003 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Gisèle Toussaint, Evgene Burov, Laurent Jolivet; Continental plate collision: Unstable vs. stable slab dynamics. Geology 2004;; 32 (1): 33–36. doi: https://doi.org/10.1130/G19883.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract We study possible scenarios for the evolution of continental collision zones by using a dynamic thermomechanical model that includes brittle-elastic-ductile rheology, surface erosion, and explicit metamorphic changes. This paper focuses primarily on the influence of four key parameters: (1) geotherm or thermotectonic age (which controls the rheological profile), (2) lower-crustal composition (weak or strong rheology), (3) convergence rate, and (4) metamorphic changes in the downgoing crust. The experiments suggest that, depending on these parameters, plate convergence is accommodated by four distinct mechanisms: stable subduction, shortening by pure-shear thickening or folding, and Rayleigh-Taylor instabilities. It appears that stable, oceanic-type subduction can only occur in the case of cold lithospheres (Moho temperature, TMoho < 550 °C), and basically needs high convergence rates (>4–5 cm/yr). Depending on the lower-crustal rheology (strong or weak), either the whole (upper and lower) crust or only the lower crust can be involved in subduction. It appears that in the case of weak metamorphic rheologies, phase changes only slightly improve chances for stable subduction. Lithospheric shortening becomes a dominant mechanism when TMoho > 550 °C or convergence rates are <4–5 cm/yr. Pure-shear thickening becomes important in all cases of hot lithospheres (TMoho > 650 °C). Large-scale folding is favored in the case of TMoho = 500–650 °C and is more effective in the case of mechanical coupling between crust and mantle (e.g., strong lower crust). Gravitational (Rayleigh-Taylor) instabilities overcome other mechanisms for very high values of TMoho (>800 °C) and may lead to development of subvertical cold spots. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Lithospheric deformation during the early stages of continental collision: Numerical experiments and comparison with South Island, New Zealand

The nature of lithospheric deformation during continental plate collision still remains unresolved. While it has often been proposed that the mantle lithosphere is accommodated by distributed thickening of a viscous root, an alternate hypothesis suggests that significant portions of convergent mantle lithosphere essentially undergo underthrusting or subduction. To further consider this issue, we model the thermochemical evolution of the lithosphere‐mantle system using arbitrary Lagrangian‐Eulerian finite element techniques. We incorporate a mix of viscous (thermally activated power law creep) and plastic (frictional Coulomb with strain softening) rheologies in the numerical experiments to treat disparate composition in the crust and mantle. A range of rheological and mechanical parameters is explored to determine controls on the style of lithospheric deformation. The models suggest that during the initial stages of plate collision the mantle lithosphere is characterized by plate‐like behavior and underthrusting/subduction of the upper region in conjunction with distributed thickening and Rayleigh‐Taylor type viscous instability of the lower portion. Depending on the material rheology, temperature regime, and imposed convergence velocity, the deforming mantle lithosphere demonstrates various combinations of these “end‐member” behavioral modes. The modeling results are interpreted in the context of observed lithospheric deformation across South Island, New Zealand. A combined style of underthrusting and distributed thickening is consistent with the observed crustal structure in the young collisional orogen as well as seismic imaging of the geometry of the underlying lithosphere.

First deep seismic reflection images of the Eastern Alps reveal giant crustal wedges and transcrustal ramps

The Alps are considered as a classical example for an orogen created by continental plate collision. In this study we present new images obtained from deep seismic reflection profiling in the Eastern Alps between Munich and Venice which give rise to examine and revise existing concepts. The seismic sections exhibit a prominent bi‐verging reflection pattern at crustal scale. A major ramp‐like structure, outcropping at the Inn‐Valley fault, can be traced southward over 80 km into the mountain root where relics of the subducted Penninic ocean are expected. New models of the evolution of the Eastern Alps show an upper/lower crustal decoupling along transcrustal thrust faults with opposite thrust directions of both the European and the Adriatic‐African continents.

Microdiamonds in a megacrystic garnet websterite pod from Bardane on the island of Fjørtoft, western Norway: Evidence for diamond formation in mantle rocks during deep continental subduction

We report the startling discovery of in situ microdiamonds in a mantle-derived peridotite lens from Bardane, Fjortoft, western Norway. Diamonds occur within multiphase solid-inclusion assemblages within spinels that are, in turn, inclusions within garnets. Euhedral inclusion-host morphologies of spinel and mineral chemistries all indicate diamond growth from infiltrating fluids at ultrahigh pressure ( P ) and moderate temperature. These results imply that the Bardane peridotite lens was present within a continental subduction zone at depths of ≥130 km. This paper not only documents the first discovery of in situ microdiamonds within the Caledonian ultrahigh-pressure terrane of western Norway, but also represents the first known global occurrence of subduction-related diamond formation within mantle rocks that have been incorporated into a major continental plate collision zone.

3-D flexural modelling of the Silurian Baltic Basin

The Baltic Basin (BB) is situated on the western part of the East European Craton (EEC). It was established initially as a passive margin basin in response to the breaking apart of the Rodinia megacontinent during the Latest Precambrian–Early Cambrian times. Thereafter, it has been suggested that the Late Ordovician–Silurian basin subsidence was generated by foreland bending of the western margin of the EEC as a result of the Caledonian convergence and collision of continental plates, namely Baltica, Laurentia, and Eastern Avalonia. This is based on the regional basin architecture, which indicates accelerating subsidence, rapid deepening of the basin, and significant thickening of the sedimentary succession towards the Tornquist–Teisseyre Zone (TTZ), along the southwestern margin of the EEC, during the Silurian. However, the very broad extent of the Silurian BB is not obviously explicable by a model of simple foreland flexure. This problem has been investigated by quantitative modelling of Caledonian foreland flexure, east of the TTZ, using a 3-D finite difference technique. The results are interpreted in terms of their implications for the rheology of the EEC and the geometry and temporal evolution of Caledonian orogenic loading. Assuming higher values of effective elastic thickness (EET) for the EEC than for Caledonian lithosphere, the model results demonstrate that the evolution of the BB cannot be explained by the effects of orogenic loads lying only on the west of the present Caledonian Deformation Front (CDF). Furthermore, the inferred shape of the BB deflection is not easily reproduced, even allowing lateral changes of EEC lithosphere EET. Rather, additional loads east of the CDF (and TTZ) were necessary, in order to replicate the shape and amplitude of the BB as a flexural basin. These are tentatively interpreted to be dynamic loads related to mantle flow above an eastward dipping Silurian subduction zone.

High-pressure blueschists and eclogites in Hong'an: a framework for addressing the evolution of high- and ultrahigh-pressure rocks in central China

The Hong'an region in the Qinling–Dabie collisional zone in eastern China hosts a series of metamorphic rocks exposing a south-to-north distribution from blueschist/blueschist–greenschist, amphibolite, eclogite (kyanite free) and kyanite–eclogite to coesite–eclogite facies rocks that represent progressively deeper levels of the Mesozoic subduction–collision complex. The Hong'an area is interesting for three reasons: (1) it escaped the thermal and structural overprint imparted on much of the Dabie Mountains during Early Cretaceous intrusion of voluminous granites and granodiorites; (2) the high-pressure (HP) Hong'an eclogites are widely distributed, often preserve prograde crystallization histories and can be directly linked in time and space to the blueschist/blueschist–greenschist rocks exposed to the south; (3) the blueschist/blueschist–greenschist facies rocks are generally better exposed than their equivalents in the southeastern Dabie Mountains and offer some opportunity for simultaneous structural and metamorphic analysis. The Hong'an area HP rocks offer perhaps the closest approximation to a preserved snapshot of Mesozoic pressure–temperature ( P– T) conditions attending early subduction–exhumation in the region, and are thus essential to generating a coherent picture of the dynamics attending both metamorphism and exposure of the coeval ultrahigh-pressure (UHP) rocks. The purpose of this contribution is twofold: (1) to document previously unpublished metamorphic and structural data characterizing these HP sequences and their relative continuity in Hong'an; (2) to incorporate these data with recent geochronologic, structural and paleomagnetic information in the context of protracted, late Paleozoic through Mesozoic subduction, collision and exhumation. Metamorphism and exhumation of some of the southern Hong'an HP sequences appear to have occurred concomitant with oceanic subduction immediately to the west, and thus may have preceded widespread continental subduction/collision. Moreover, all of the HP–UHP sequences in the region were exhumed before the end of collision between the Sino-Korean and Yangtze cratons at ca. 160 Ma. Exhumation of HP–UHP rocks both before and during continental plate collision is neither novel for central China nor for other HP–UHP zones, but is important to take into account when reconstructing the evolution of such orogens.

Petrographic Characteristics and Metamorphic Evolution of Ultrahigh-Pressure Eclogites in Plate-Collision Belts

Ultrahigh-pressure eclogites are metabasic rocks that have equilibrated at pressures within the coesite P-T stability field. This paper reviews their occurrences and petrographic characteristics in exhumed ultrahigh-pressure metamorphic terrenes within certain major Phanerozoic continental plate—collision belts. Unequivocal identification of ultrahigh-pressure eclogites depends on the presence of relict coesite or of polycrystalline quartz pseudomorphs after earlier coesite. However, preservation of such distinctive petrographic evidence is at best sporadic, leading to serious problems over whether or not particular eclogite samples have experienced “peak” pressures consistent with coesite stability. Accordingly, we also review here other petrographic, mineral-assemblage, and mineral-chemistry features that may aid in the identification of ultrahigh-pressure eclogites. In addition, we discuss the quantitative evaluation of P-T conditions for formation based on various reaction equilibria, including the dominant garnet and omphacite phases. Various aspects of the mineral compositions are taken to indicate that the “peak” metamorphic pressures were most likely attained at lower than “peak” temperatures. Hence it is concluded that tight, hairpin-style P-T-time paths for the prograde and retrograde stages in the metamorphic evolution of ultra-high pressure eclogites that assume synchronous achievement of Pmax and Tmax conditions, as deduced in many recently published studies, may be invalid. A rather more open style, clockwise P-T-time path, as deduced for the metamorphic evolution of a typical ultrahigh-pressure phengite-bearing eclogite sample from the Dabie Shan in central China and characterized by a significant further temperature rise during the initial phase of relatively rapid exhumation, is considered to be more realistic.

Exhumation of the Tauern window, Eastern Alps

The exhumation of metamorphic domes within orogenic belts is exemplified by the Tauern window in the Eastern Alps. There, the exhumation is related to partitioning of final orogenic shortening into deep-seated thrusts, near-surface antiformal bending forming brachyanticlines, and almost orogen-parallel strike-slip faults due to oblique continental plate collision. Crustal thickening by formation of an antiformal stack within upper to middle crustal portions of the lower lithosphere is a prerequisite of late-stage orogenic window formation. Low-angle normal faults at releasing steps of crustal-scale strike-slip faults accomodate tectonic unloading of synchronously thickened crust and extension along strike of the orogen, forming pull-apart metamorphic domes. Initiation of low-angle normal faults is largely controlled by rock rheology, especially at the brittle-ductile transitional level within the lithosphere. Several mechanisms may contribute to uplift and exhumation of previously buried crust within such a setting: (1) Shortening along deep-seated blind thrusts results in the formation of brachyanticlines and bending of metamorphic isograds; (2) oversteps of strike-slip faults within the wrench zone control the final geometry of the window; (3) unloading by tectonic unroofing and erosional denudation; and (4) vertical extrusion of crustal scale wedges. Rapid decompression of previously buried crust results in nearly isothermal exhumation paths, and enhanced fluid circulation along subvertical tensile fractures (hydrothermal ore and silicate veins) that formed due to overall coaxial stretching of lower plate crust.

Tectonic and Geodynamic Setting of Oil and Gas Basins of the Soviet Union (1)

Within the territory of the Soviet Union and its offshore economic zone are about 70 sedimentary basins containing oil and gas. The basins include almost all basin types described in present-day plate-tectonic classifications, namely (1) intracontinental and pericontinental rifts, suprarift syneclises, and zones of pericratonic downwarps; (2) ancient passive margins of continents with adjacent overthrust fold system; (3) modern passive margins of continents; (4) zones of convergence of lithospheric plates (i.e., zones of subduction of oceanic plates below continental plates); and (5) zones of collision of continental lithospheric plates. Basins of the first type include the Pripyat'-Dnieper-Donets aulacogen, the Viluy aulacogen of Siberia, the huge West Siberian supra-aulacogen basin, the analogous Yenisei-Khatanga basin, and others. The second type includes the Volga-Urals basin with the Ural foredeep, Azov-Kuban' basin and Terek-Caspian basin with the Greater Caucasus foredeeps, and others. The difference between these two types is rather arbitrary, because the passive margins of continents also undergo the rift stage of development, though their main stage of oil and gas accumulation corresponds to the post-rift stage. The third type is represented by the poorly studied prospective basins of the Arctic margin of the Soviet Union. The fourth includes numerous but relatively small basins of the far eastern and northe stern Soviet Union, of which the East Sakhalin, West Kamchatka, and Anadyr basins have proved to contain oil and gas. Rifting also played a role in the formation of the back-arc group of these basins. Basins in central Asia (Fergana, South Tadjik) and the Transcaucasia region (South Caspian) belong to the fifth basin type. The central Asian basins lie within the area of secondary, epiplatform orogenesis, whereas the Transcaucasian basins are with the area of primary, epigeosynclinal orogenesis. So, far, the only type of basin not identified within the territory of the Soviet Union is the pull-apart basin. The location and distribution of oil and gas deposits in the section of a basin, prevailing types of traps, and scale of potential resources are all features influenced by the geodynami type of the basin.

Precambrian collision tectonics in Africa

Summary Stages of the collisional process are separated. Criteria are proposed by which A- and B-subduction may be distinguished. Late Precambrian structures in NE Africa are attributed to successive gentle collisions of island arcs; the sutures dip E or SE and relative plate motion was NW-SE. In the polycyclic Proterozoic Mozambique Belt of E Africa there are probably three sutures (Sekerr, Baragoi, Moyale-Shakiso) with ophiolites; they appear to dip E. It is argued that despite the lack of geochronological evidence, Archaean basement probably underlies the metasedimentary sequences, right across the Mozambique Belt of E Africa, from the Archaean Tanzania Craton on the W to the Archaean Craton, represented by Madagascar and S India, on the E. The belt is therefore interpreted as having been built by collision of continental plates. Widespread recumbent structures, complex and intense deformation and high-grade metamorphism imply crustal thickening and a multistage collisional history. Relative NW-SE motion was probably followed by N-S post-collisional ductile shear. The Archaean Limpopo Belt in southern Africa shows evidence of drastic crustal thickening and intense deformation in contrast to the much weaker deformation and little crustal thickening in the Kaapvaal and Rhodesian Cratons to the S and N. Collision is implied but in the absence of convincing ophiolites, subduction-type calc-alkaline magmatism and abyssal sediments, B-subduction remains unproven.