Exhumation Processes Research Articles

Multi-stage reaction history in different eclogite types from the Pakistan Himalaya and implications for exhumation processes

Metabasites were sampled from rock series of the subducted margin of the Indian Plate, the so-called Higher Himalayan Crystalline, in the Upper Kaghan Valley, Pakistan. These vary from corona dolerites, cropping out around Saif-ul-Muluk in the south, to coesite–eclogite close to the suture zone against rocks of the Kohistan arc in the north. Bulk rock major- and trace- element chemistry reveals essentially a single protolith as the source for five different eclogite types, which differ in fabric, modal mineralogy as well as in mineral chemistry. The study of newly-collected samples reveals coesite (confirmed by in situ Raman spectroscopy) in both garnet and omphacite. All eclogites show growth of amphiboles during exhumation. Within some coesite-bearing eclogites the presence of glaucophane cores to barroisite is noted whereas in most samples porphyroblastic sodic–calcic amphiboles are rimmed by more aluminous calcic amphibole (pargasite, tschermakite, and edenite). Eclogite facies rutile is replaced by ilmenite which itself is commonly surrounded by titanite. In addition, some eclogite bodies show leucocratic segregations containing phengite, quartz, zoisite and/or kyanite. The important implication is that the complex exhumation path shows stages of initial cooling during decompression (formation of glaucophane) followed by reheating: a very similar situation to that reported for the coesite-bearing eclogite series of the Tso Morari massif, India, 450 km to the south-east.

Fluids in coesite-bearing rocks of the Tso Morari Complex, NW Himalaya: evidence for entrapment during peak metamorphism and subsequent uplift

AbstractFluid inclusions trapped in coesite-bearing rocks provide important information on the fluid phases present during ultrahigh-pressure metamorphism. The subduction-related coesite-bearing eclogites of the Tso Morari Complex, Himalaya, contain five major types of fluids identified by microthermometry and Raman spectroscopy. These are: (1) high-salinity brine, (2) N2, (3) CH4, (4) CO2and (5) low-salinity aqueous fluids. These fluids were trapped during both deep subduction and exhumation processes. The coesite-bearing rocks are inferred to have been buried to a depth of >120 km, where they experienced ultrahigh-pressure metamorphism. The fluid–rock interaction provides direct evidence for fluid derivation during a deep subduction process as demonstrated by silica–carbonate assemblages in eclogite. High salinity brine, N2and CH4inclusions are remnants of prograde and peak metamorphic fluids, whereas CO2and low-salinity aqueous fluids appear to have been trapped late, during uplift. The high-salinity brine was possibly derived from subducted ancient metasedimentary rocks, whereas the N2and CH4fluids were likely generated through chemical breakdown of NH3-bearing K minerals and graphite. Alternatively, CH4might have been formed by a mixed fluid that was released from calcareous sediments during subduction or supplied through subducted oceanic metabasic rocks. High density CO2is associated with matrix minerals formed during granulite-facies overprinting of the ultrahigh-pressure eclogite. During retrogression to amphibolite-facies conditions, low-salinity fluids were introduced from external sources, probably the enclosing gneisses. This source enhances salinity differences as compared to primary saline inclusions. The subducting Indian lithosphere produced brines prior to achieving maximal depths of >120 km, where fluids were instead dominated by gaseous phases. Subsequently, the Indian lithosphere released CO2-rich fluids during fast exhumation and was then infiltrated by the low-salinity aqueous fluids near the surface through external sources. Elemental modelling may improve quantitative understanding of the complexity of fluids and their reactions.

Spinel + quartz assemblage in granulites from the Achankovil Shear Zone, southern India: Implications for ultrahigh-temperature metamorphism

We report the finding of equilibrium spinel + quartz assemblage enclosed within garnet in garnet–orthopyroxene–cordierite granulites from Pakkandom within the Achankovil Shear Zone, a region which is considered as the trace of an accretionary suture in recent tectonic models on southern India. The spinel + quartz bearing granulites are composed of poikiloblastic garnet and subidioblastic orthopyroxene in the matrix of quartz, plagioclase, biotite, cordierite, and Fe–Ti oxides. Garnet contains numerous inclusions of sillimanite and biotite as well as spinel and quartz. The spinel in direct contact with quartz has moderate X Mg (= Mg/(Fe 2+ + Mg) = 0.44–0.47), and is Zn and Fe 3+ poor ( X Zn = Zn/(Fe 2+ + Mg + Zn) = 0.027–0.036, Fe 3+/(Fe 2+ + Fe 3+) = 0.12–0.17). Spinel is also present in the matrix surrounded by magnetite, but the matrix spinel contains more Zn( X Zn = 0.067–0.072) and does not show any contact relationship with quartz. Such Zn- and Fe 3+-poor spinel in direct contact with quartz has been regarded as a diagnostic evidence of ultrahigh-temperature (UHT) metamorphism. The high-temperature stability of the spinel + quartz is also supported by the results of geothermobarometric calculation of garnet–orthopyroxene assemblages that provides robust evidence for peak UHT metamorphism at 920–980 °C and 8–10 kbar, which was further confirmed by Al-in-Opx and magnetite–ilmenite geothermometers (900–950 °C and ∼1000 °C, respectively). The peak UHT event was followed by decompression down to 4.0–4.2 kbar and 640–670 °C toward the stability of cordierite along a clockwise P–T path. Similar spinel + quartz assemblage enclosed in poikiloblastic garnet has also been reported from the Palghat–Cauvery Shear Zone system, the trace of a major suture zone within the Gondwana amalgam with evidence for prograde high-pressure ( P up to 20 kbar) metamorphism followed by UHT event. The fine-grained spinel + quartz may thus indicate decompression from higher pressure conditions, confirming the recent attributes of the Achankovil Shear Zone being a collisional suture with evidence for subduction–exhumation processes which accompanied the final collision and amalgamation of the Gondwana supercontinent during the latest Neoproterozoic.

Thrust or detachment? Exhumation processes in the Aegean: Insight from a field study on Ios (Cyclades, Greece)

Whether back‐arc extension in the Aegean is symmetric or asymmetric at the regional scale is a key question to the understanding of the dynamics of back‐arc extension. New field observations, structural analysis of finite and instantaneous strain, and their relationship with metamorphism on Ios island (Cyclades) lead us to investigate the significance of the South Cycladic Shear Zone (SCSZ). Two successive tectonometamorphic events have been documented in the hanging wall and the footwall of the SCSZ. First, penetrative top‐to‐the‐south shearing is observed in the upper Cycladic Blueschists and the lower Cycladic Basement close to their contact. It is expressed in the hanging wall by shear bands and pressure shadows associated to blueschist facies minerals, contemporaneous with overthrusting of the Cycladic Blueschists over the basement unit. Second, large extensional top‐to‐the‐north shear zones lately crosscut the whole stack. These constraints allow us to reinterpret the kinematics of the Aegean domain during synorogenic and postorogenic periods. When replaced in the reconstructed geometry of the Hellenic Eocene orogenic wedge, the observed top‐to‐the‐south thrusting is coeval with extensional shear zones observed in Syros that could represent the roof of the Cycladic Blueschists. An extrusion structure is then proposed for the exhumation of the Cycladic Blueschists in the subduction zone. Top‐to‐the‐north asymmetric extension together with the Andros‐Tinos‐Mykonos and Paros‐Naxos detachments can be correlated to Oligo‐Miocene extension in the back‐arc domain.

K–Ar and 40Ar/39Ar phengite ages of Sanbagawa schist clasts from the Kuma Group, central Shikoku, southwest Japan

AbstractConglomerates of the Kuma Group, central Shikoku, southwest Japan contain Sanbagawa schist clasts with a variety of metamorphic grades and lithologies. K–Ar and 40Ar/39Ar dating of phengite show all the pelitic schist clasts from low‐ to high‐grade zones have similar phengite ages (82–84 Ma) that are significantly older than those from the in situ Sanbagawa sequence of central Shikoku. This is because the Kuma–Sanbagawa sequence was exhumed earlier than the in situ Asemi sequence with an exhumation process intermediate between those for the Kanto Mountains and the in situ Asemi sequences. 40A/39Ar plateau ages (103 and 117 Ma) of phengite in amphibolites indicate the timing of the early stage of the exhumation of the metamorphic pile, probably close to the peak metamorphic age.

Mountain building and exhumation processes through time: inferences from nature and models

AbstractOrogenic belts involving initially hot lithospheres, as exemplified by accretionary‐type orogens, commonly show distributed deformation and retrograde PT paths with a concomitant decrease in pressure and temperature. Paths may track geotherms, indicating thermal equilibrium, consistent with slow strain and exhumation rates, limited strain localization, and consequently limited topographical gradients and distributed erosion. Such patterns are more common in Precambrian times than in younger periods of the Earth history. In contrast, orogens involving initially stiff lithospheres show exhumation PT paths that track isothermal decompression reflecting high strain rates along major shear zones, a feature typical of Phanerozoic collision belts. Field evidence, analogue and numerical models emphasize that strain localization has first‐order consequences for the tectonic evolution of orogenic zones including structure, metamorphism, exhumation processes, topography, erosion and sedimentation modes.

Igneous, Alteration and Exhumation Processes Recorded in Abyssal Peridotites and Related Fault Rocks from an Oceanic Core Complex along the Central Indian Ridge

This paper presents the first detailed studies on the petrology of abyssal peridotites and related fault rocks recovered from an oceanic core complex (OCC) in the southern part of the Central Indian Ridge using the submersible SHINKAI 6500 of theJapan Agency for Marine^Earth Science andTechnology. Less deformed, statically serpentinized peridotites were recovered from the ridge-facing slope, whereas highly deformed rocks were recovered from sheet-like structures on the top surface of the OCC.The top surface of the OCC is interpreted to be the main detachment fault.The serpentinized peridotites are consistent with an origin as residues after moderate degrees (13^15%) of partial melting; these were later chemically modified as a result of the infiltration of evolved melts of probable granitic composition resulting in the formation of leucocratic veins. The deformed rocks from the detachment fault are divided into talc-rich and chlorite-rich parts, probably formed as a result of interaction of hydrothermal fluids with peridotite and gabbro precursors along the detachment fault, respectively. Deformation and alteration were locally concentrated along the detachment fault, resulting in mechanical mixing of both altered gabbros and serpentinized peridotites in the deformed rocks during the exhumation of the OCC associated with long-lived fault activity. Our results reveal that gabbros and peridotites are tectonically exposed in oceanic core complexes on the seafloor along the intermediate-spreading CIR, as well as in slow-spreading regions. Fluid-mobile elements such as Li, Rb, Ba, Pb, Sr and U are higher in serpentines than their precursor mantle minerals.The uranium content in serpentine is variable but is abundant in the outermost margin of the precursor minerals.The trace element compositions of serpentine appear to have been continuously

Two-stage collision-related extrusion of the western Dabie HP–UHP metamorphic terranes, central China: Evidence from quartz c-axis fabrics and structures

The western Dabie orogen (also known as the Hong'an block) forms the western part of the Dabie–Sulu HP–UHP belt, central China. Rocks of this orogen have been subjected to pervasive ductile deformation, and include numerous quartz schists and felsic mylonites cropping out in ductile shear zones. Quartz textures in these mylonites contain important clues for understanding the movement sense of late-collisional extrusion and exhumation of high-pressure–ultrahigh-pressure (HP–UHP) rocks from the lower crustal level to the upper crustal level during Middle Triassic and Early Jurassic. The orientation and distribution of quartz crystallographic axes were used to confirm the regional shear sense across the orogen. The asymmetry of c-axis patterns consistently indicates top-to-the-southeast thrusting across the orogen in early structural stages. Later stages of deformation show different senses of movement in northern and southern parts of the orogen, with top-to-the-northwest sinistral shearing recorded in rocks north of the Xinxian HP–UHP eclogite-facies belt, and top-to-the-southeast dextral shearing south of the same unit. Based on our study on quartz c-axis fabrics and marco- to micro-scale structures, simultaneous southeastward shearing within a large part of the orogen and normal faulting north of the Xinxian HP–UHP unit is explained by upward extrusion in early stages of deformation. The extrusion process has been attributed to syn- and late-collisional processes, accounting for some earlier deformation in the western Dabie orogen such as metamorphic sequences around the core of the Xinxian HP–UHP eclogite-facies unit. Much higher pressure of deformation is also indicated in the aligned glaucophane and omphacite from blueschist and eclogite in the field. An orogen-parallel eastward extrusion of the Xinxian HP–UHP eclogite-facies unit, however, occurred diachronously in later stages of deformation. Therefore, a tectonic model combining an early upward extrusion with a later eastward extrusion is presented. Two different stages and types of extrusion for exhumation of HP–UHP rocks are suitable to all of east central China. Geochronological data shows that the first, upward extrusion occurred during Middle Triassic, the second, eastward extrusion occurred during Late Triassic to Early Jurassic. These two extrusions are correlative with two stages of rapid exhumation of the Dabie HP–UHP rocks, respectively. These two-stage late-collisional (Middle Triassic to Early Jurassic) extrusion events bridge the gap between syn-collisional (Early to Middle Triassic) vertical extrusion and post-collisional (Cretaceous) eastward-directed lateral escape and provide vital clues to understanding the more detailed processes of exhumation of HP–UHP rocks.

Exhumation processes during post-collisional stage in the Variscan belt revealed by detailed 40Ar/39Ar study (Tanneron Massif, SE France)

Detailed 40Ar/39Ar geochronology on single grains of muscovite was performed in the Variscan Tanneron Massif (SE France) to determine the precise timing of the post-collisional exhumation processes. Thirty-two plateau ages, obtained on metamorphic and magmatic rocks sampled along an east–west transect through the massif, vary from 302 ± 2 to 321 ± 2 Ma, and reveal a heterogeneous exhumation of the lower crust that lasted about 20 Ma during late Carboniferous. In the eastern part of the massif, the closure of the K–Ar isotopic system is at 311–315 Ma, whereas in the middle part of the massif it closes earlier at 317–321 Ma. These cooling paths are likely to be the result of differential exhumation processes of distinct crustal blocks controlled by a major ductile fault, the La Moure fault that separates both domains. In the western part of the massif, the ages decrease from 318 to 303 Ma approaching the Rouet granite, which provides the youngest age at 303.6 ± 1.2 Ma. This age distribution can be explained by the occurrence of a thermal structure spatially associated to the magmatic complex. These ages argue in favour of a cooling of the magmatic body at around 15 Ma after the country rocks in the western Tanneron. The emplacement of the Rouet granite in the core of an antiform is responsible for recrystallization and post-isotopic closure disturbances of the K–Ar chronometer in the muscovite from the host rocks. These new 40Ar/39Ar ages clearly outline that at least two different processes may contribute to the exhumation of the lower crust in the later stage of collision. During the first stage between 320 and 310 Ma, the differential motion of tectonic blocks limited by ductile shear zones controls the post-collisional exhumation. This event could be related to orogen parallel shearing associated with crustal-scale strike-slip faults and regional folding. The final exhumation stages at around 300 Ma take place within the tectonic doming associated to magmatic intrusions in the core of antiformal structures. Local ductile to brittle normal faulting is coeval to Upper Carboniferous intracontinental basins opening.

Vertical ductile thinning and its contribution to the exhumation of high-pressure rocks: the Cycladic blueschist unit in the Aegean

The contribution of vertical ductile thinning to the exhumation of high-pressure rocks is evaluated by estimating finite strain in 75 exhumed high-pressure rocks of the Cycladic blueschist unit in the Aegean Sea, Greece, and western Turkey. Strain data indicate heterogeneous deformation; principal stretches are 1.24–5.03 for S X , 0.63–2.53 for S Y and 0.10–0.81 for S Z , with a tensor average of S X : S Y : S Z =1.52:1.28:0.51. A 1D numerical model, which integrates velocity gradients along a vertical flow path with a steady-state orogen, is used to estimate the contribution of ductile thinning of the overburden of the high-pressure rocks to exhumation. Using a strain-rate law that is proportional to depth, averaged results show that ductile flow contributed c . 20% to exhumation. A major implication is that the vertical strain in the exhumed rocks is an overestimate of the contribution that ductile flow makes to the total exhumation. A proportional strain-rate law that scales linearly with depth implies that material points rising towards the surface move quickly out of the more rapidly deforming part of the orogen. Therefore, very large vertical strains >90% in deeply exhumed rocks are needed for vertical ductile thinning to be a major exhumation process.

Building Peace in Angola: The Role of the Exhumation Process in Kuito

Angola endured more than 40 years of nearly constant war and is now in the process of building peace. The city of Kuito paid an especially heavy price when 20,000 to 30,000 people lost their lives there during an 18-month siege (1993-1994) and were buried in hastily dug graves scattered around the city. The exhumation of the bodies and their reburial in a special cemetery have allowed for proper mourning of the dead, offering the living the opportunity to move forward with their lives. The power of the exhumations in Kuito differs from recent cases in which countries have dug up the dead in search of truth and/or justice. Instead, the reburial and mourning of those who died in Kuito's siege was central to a process of individual and collective reconciliation. At the same time, the deceased took their rightful place as national martyrs and, thus, transformed the image of Kuito as a city of the dead to one in which the living have put the past in its proper place. In this article, I describe Kuito's exhumat...

Omphacite textures in eclogites of the Tauern Window: Implications for the exhumation of the Eclogite Zone, Eastern Alps

We discuss the relationship of omphacite lattice preferred orientation (LPO) patterns in eclogite and kinematic indicators in the matrix surrounding the eclogite during the very rapid exhumation of the deeply buried Eclogite Zone in the Tauern Window of the Eastern Alps. LPO patterns are presented from profiles parallel to the eclogite stretching lineation across the Eclogite Zone. The omphacite textures show symmetric patterns across the entire Eclogite Zone; only in few cases slight asymmetries are observed. The patterns are characterized by an alignment of {110} with the foliation and a maximum of 〈001〉 close to the lineation and are indicative of deformation by intracrystalline glide on the (010) [001] and (110) [001] slip systems. The patterns have close resemblance to S-type LPO patterns and probably developed during the burial and accretion of the eclogites. We argue that the LPO patterns are not related to deformation associated with the very rapid exhumation of the Eclogite Zone. We conclude that exhumation-related deformation was concentrated in the metasedimentary rocks that surround the eclogites, allowing the eclogites to preserve their burial LPO signature during the exhumation process. Kinematic indicators show top-N thrusting at the base of the Eclogite Zone and sinistral strike-slip faulting at the top. Most of the kinematic indicators record blueschist- and greenschist-facies metamorphic conditions. However, because of the very rapid exhumation of the Eclogite Zone with rates exceeding 36 km Myr −1, the kinematic indicators are interpreted to reflect the entire exhumation process from eclogite-facies conditions. We discuss a model according to which the Eclogite Zone was exhumed in an oblique extrusion wedge.

Dynamics and structural development of metamorphic core complexes

The development of metamorphic core complexes (MCCs) in a thickened continental lithosphere is studied using fully coupled thermomechanical numerical code, accounting for elastic‐brittle‐ductile properties of constituent rocks. For MCCs to develop, the middle lower crust and the sub‐Moho mantle are required to be weak enough to flow laterally, so as to simultaneously feed the exhuming dome and enable the Moho to keep a flat geometry. The conditions are satisfied with initial Moho temperatures of 800°C or higher, with crustal thicknesses of 45 km or greater, and with initial effective viscosities lower than 1020 Pa s and 1022 Pa s in the lower crust and the underlying mantle, respectively. A compositional (mainly density) anomaly with the properties of granite is placed centrally in the crust to localize strain at the onset of deformation. During a first stage of “upper crust necking,” the deformation pattern is relatively symmetrical and dominated by graben formation in the upper crust. When the first ductile layers reach the surface, a second stage of “dome amplification and widening” occurs. Dome amplification is accommodated by horizontal flow in the ductile crust, giving an early symmetrical pattern of conjugate shear zones with no obvious detachment zone. The system then rapidly becomes asymmetric, with the localization of a detachment zone along one dome limb, further accommodating dome widening. Thus the exhumation process of a metamorphic dome results in the progressive development of a detachment zone. Depending on initial Moho temperature, the detachment zone can migrate in space or die out and be replaced by a new one with an opposite dip.

Introduction to recent advances in regional geological mapping (1:250, 000) and new results from southern Qinghai-Tibet Plateau

A large volume of new geological data has been obtained in the process of new round of geological surveys (at a scale of 1:250, 000) and other geoscientific studies in southern QinghaiTibet Plateau ( Wang et al. 2004). The new results and achievements provide stable bases that are beneficial for understanding the principles governing the geological processes and verifying them in the Qinghai-Tibet Plateau. Significant data on the spatio-temporal distribution of the regional primary junction belts (suture zones) and faults also serve as the base for basic geological information required to establish a workable geotectonic framework for Qinghai-Tibet Plateau. The discovery of high-pressure granulites and lots of data on isotope chronology of Precambrian metamorphic rocks offer new insights to discuss the formation mechanism and exhumation process of the metamorphic basement in the southern Tibet. Findings of new strata and confirmation of already known strata containing abundant fossils provide new evidences to establish a regional stratigraphic system that will be used for geological classification and comparison. Likewise, discovery of some magmatic rock types and isotope-based age data on them enable to reconstruct the process of evolution and also propose a spatiotemporal framework for the magmatic rocks. Newly discovered unconformities and sedimentary facies or the verification of existing ones together with the associated geological information are fundamental for the analysis of sedimentary basin and for the paleogeographic reconstruction of tectonic lithofacies. The results and progresses in Quaternary geological mapping at regional scale and also in studying the neotectonic movements will be useful for deciphering the history of uplift of Plateau, the changes in the paleoenvironments and also the development of lakes in the geological past. Additional progress in finding the relics of ancient human activity, deciphering zoological environments and also studying the geology of regions of touristic importance has been made. These will be definitely valuable for research on zoological environment and ancient human civilization of Qinghai-Tibet Plateau and also for regional economic development. Besides these discoveries, new information on mineralization gained through the regional geological surveys will serve as guidelines for undertaking necessary action to explore and develop the mineral resources and also for appraisal of important metallogenic regions and belts in Qinghai-Tibet Plateau.

Exhumation of the Dabie UHP Terrane, China

Exhumation of deeply buried ultrahigh-pressure (UHP) terranes remains a puzzle in understanding lithospheric dynamic process. New evidence that constrains the exhumation process of the Dabie UHP terrane includes the following. (1) Structural data record the architecture of the Dabie UHP terrane. Three slices are recognized—i.e., from top to bottom, the Susong blueschist slice, the Huangzhen-Huangweihe (HH) HP slice, and the Jinheqiao-Shuanghe-Bixiling (JSB) UHP slice. These slices have been stacked and domed. (2) Geophysical data depict two offsets in the Moho. Offset-I separates the Dabie UHP terrane from the North China craton, and has served as a subduction channel, as well as an exhumation channel. Offset-II, separating the Dabie UHP terrane from the foreland belt, developed due to geodynamic regime reorganization. (3) Geochronological data reveal that the HP slice was exhumed earlier that the UHP slice. A northward-younging exhumation polarity is implied, with younger exhumation age closer to the exhumation channel. These pieces of evidence point to a multistage exhumation process. The Yangtze craton collided with and subducted beneath the North China craton at the very beginning of Triassic time. By 230 Ma, the Susong blueschist slice and the southern end of the HH HP slice had arrived at a middle-crustal level, while its northern part was still at mantle depth. Such processes continued until the JSB UHP slice was exhumed to mid-crustal levels at 210 Ma. After geodynamic regime reorganization during 210—180 Ma, continuing compression of the Yangtze craton and underthrusting along offset-II in the Moho caused the dome structure, and eventually brought the UHP rocks to the Earth's surface.

Structural development of the Tso Morari ultra-high pressure nappe of the Ladakh Himalaya

A continental subduction-related and multistage exhumation process for the Tso Morari ultra-high pressure nappe is proposed. The model is constrained by published thermo-barometry and age data, combined with new geological and tectonic maps. Additionally, observations on the structural and metamorphic evolution of the Tso Morari area and the North Himalayan nappes are presented. The northern margin of the Indian continental crust was subducted to a depth of > 90 km below Asia after continental collision some 55 Ma ago. The underthrusting was accompanied by the detachment and accretion of Late Proterozoic to Early Eocene sediments, creating the North Himalayan accretionary wedge, in front of the active Asian margin and the 103–50 Ma Ladakh arc batholith. The basic dikes in the Ordovician Tso Morari granite were transformed to eclogites with crystallization of coesite, some 53 Ma ago at a depth of > 90 km (> 27 kbar) and temperatures of 500 to 600 °C. The detachment and extrusion of the low density Tso Morari nappe, composed of 70% of the Tso Morari granite and 30% of graywackes with some eclogitic dikes, occurred by ductile pure and simple shear deformation. It was pushed by buoyancy forces and by squeezing between the underthrusted Indian lithosphere and the Asian mantle wedge. The extruding Tso Morari nappe reached a depth of 35 km at the base of the North Himalayan accretionary wedge some 48 Ma ago. There the whole nappe stack recrystallized under amphibolite facies conditions of a Barrovian regional metamorphism with a metamorphic field gradient of 20 °C/km. An intense schistosity with a W–E oriented stretching lineation L 1 and top-to-the E shear criteria and crystallization of oriented sillimanite needles after kyanite, testify to the Tso Morari nappe extrusion and pressure drop. The whole nappe stack, comprising from the base to top the Tso Morari, Tetraogal, Karzok and Mata–Nyimaling-Tsarap nappes, was overprinted by new schistosities with a first N-directed and a second NE-directed stretching lineation L 2 and L 3 reaching the base of the North Himalayan accretionary wedge. They are characterized by top-to-the S and SW shear criteria. This structural overprint was related to an early N- and a younger NE-directed underthrusting of the Indian plate below Asia that was accompanied by anticlockwise rotation of India. The warping of the Tso Morari dome started already some 48 Ma ago with the formation of an extruding nappe at depth. The Tso Morari dome reached a depth of 15 km about 40 Ma ago in the eastern Kiagar La region and 30 Ma ago in the western Nuruchan region. The extrusion rate was of about 3 cm/yr between 53 and 48 Ma, followed by an uplift rate of 1.2 mm/yr between 48 and 30 Ma and of only 0.5 mm/yr after 30 Ma. Geomorphology observations show that the Tso Morari dome is still affected by faults, open regional dome, and basin and pull-apart structures, in a zone of active dextral transpression parallel to the Indus Suture zone.

40Ar/39Ar ages (600-570 Ma) of the Serra do Azeite transtensional shear zone: evidence for syncontractional extension in the Cajati area, southern Ribeira belt

This paper presents 40Ar/39Ar ages of the rocks from the Serra do Azeite transtensional shear zone in the southern part of the Ribeira belt, between the States of São Paulo and Paraná, and also discusses the regional correlations and the tectonic implications for other parts of the belt. The geochronological data suggest that transtensional deformation was active between 600 and 580 Ma (hornblende and muscovite apparent ages, respectively). This time span is considerably older than previous proposals for the period of activity of these structures (520-480 Ma) in the northern segment of this belt and in the Araçuaí belt. Kinematic analysis of the dated mylonites shows extensional structures with top-down movement to ESE compatible with structures found in other tectonic segments in the eastern portion of the Quadrilátero Ferrífero and in the Rio Doce Valley region. Our ages are situated in the same time interval defined for the alkaline magmatism of the Serra do Mar suite. We suggest that the regional tectonic framework was developed during continental-scale extension. This process has been coeval with convergent strain in the adjacent Neoproterozoic shear zones of the Apiaí/Ribeira and Araçuaí belts, which make up significant segments of these belts. The available data show that these structures may not be simply related to post-orogenic gravitational collapse, but must involve a more complex process probably related to dynamic balance between crustal thickening and thinning during tectonic convergence, basin formation and exhumation processes.

Thermochronologic evidence for the exhumational history of the Alpi Apuane metamorphic core complex, northern Apennines, Italy

The Apennine Range is a young convergent orogen that formed over a retreating subduction zone. The Alpi Apuane massif in the northern Apennines exposes synorogenic metamorphic rocks, and provides information about exhumation processes associated with accretion and retreat. (U‐Th)/He and fission‐track ages on zircon and apatite are used to resolve exhumational histories for the Apuane metamorphic rocks and the structurally overlying, very low grade Macigno Formation. Stratigraphic, metamorphic, and thermochronologic data indicate that the Apuane rocks were structurally buried to 15–30 km and ∼400°C at about 20 Ma. Exhumation to 240°C and 9 km depth (below sea level) occurred at 10–13 Ma. By 5 Ma the Apuane rocks were exhumed to 70°C and ∼2 km. The Macigno and associated Tuscan nappe were also structurally buried and the Macigno reached its maximum depth of 7 km at ∼15 to 20 Ma. Stratigraphic evidence indicates that the Apennine wedge was submarine at this time. Thus we infer that initial exhumation of the Apuane was coeval with tectonic thickening higher in the wedge, as indicated by synchronous structural burial of the Tuscan nappe. From 6 to 4 Ma, thinning at shallow depth is indicated by continued differential exhumation between the Apuane and the Tuscan nappe at high rates. After 4 Ma, differential exhumation ceased and the Apuane and the Tuscan nappe were exhumed at similar rates (∼0.8 km/Ma), which we attribute to erosion of the Apennines, following their emergence above sea level.

Structural and metamorphic evolution of the Ambin massif (western Alps): toward a new alternative exhumation model for the Briançonnais domain

Abstract The basement domes of the central part of western Alps may result either from a multistage tectonic evolution with a dominant horizontal shortening component, an extensional behaviour, or both. The Ambin massif belongs to the “Briançonnais” domain and is located within the HP metamorphic zone. It was chosen for a reappraisal of the tectonic evolution of the Internal Alps in its western segment. Structural investigations have shown that Alpine HP rocks were exhumed in three successive stages. The D1 stage was roughly coeval with the observed peak metamorphic conditions and corresponds to a non-coaxial regime with dominant horizontal shortening and north movement direction. Petrological observations and P-T estimates show that the exhumation process was initiated during D1, the corresponding mechanism being still poorly understood. The D2 stage took place under low-blueschist facies conditions and culminated under greenschist facies conditions. It developed a retrogressive foliation and pervasive shear-zones at all scales that locally define major tectonic contacts. D2 shear zones show a top-to-east movement direction and correspond actually to large-scale detachment faults responsible for the juxtaposition of less metamorphic units above the Ambin basement and thus to a large part of the exhumation of HP rocks toward the surface. D2 shear zones were subsequently deformed by D3 open folds, large antiforms (e.g. the Ambin dome) and associated brittle-ductile D3 shear-bands. The D1 to D3 P-T conditions and P-T path of the blueschists occurring in the deepest part of the Ambin dome, was estimated by using the multi-equilibrium thermobarometric method of the Tweeq and Thermocalc softwares. Peak pressure conditions, estimated at about 14–16 Kb, 500oC, are followed by a nearly-isothermal decompression that occurred concurrently with the major D1–D2 change in the ductile deformation regime. Eastwards, the Schistes Lustrés units exhibit a similar geometry on top of the Gran Paradiso dome but exhibit opposite D2 movement direction. Lower-grade units are lying above higher-grade units, the shear zones occurring in between being similar to Ambin’s D2 detachments. Thus at regional scale, the D2 detachments seem to form together with the Ambin shear-zones, a network of conjugate detachments. Such a pattern suggests that the exhumation history is mostly controlled by a D2+D3 crustal-scale vertical shortening resulting in the thinning of the previous tectonic pile formed during D1. The slab-break off hypothesis may explain such an extensional behaviour within the western Pennine domain. It is suggested that the thermo-mechanical rebound of the residual European slab initiated between 35 and 32 Ma the fast exhumation of the previously thickened orogenic wedge (stack of D1 HP slices). It was immediately followed by a collapse of the wedge that may correspond to the E-W Oligocene extensional event responsible for the opening of rifts in the West European platform.

The carbonate tectonic units of northern Calabria (Italy): a record of Apulian palaeomargin evolution and Miocene convergence, continental crust subduction, and exhumation of HP–LT rocks

In northern Calabria (Italy), the metasedimentary succession of the Lungro–Verbicaro tectonic unit preserves mineral assemblages suggesting underthrusting to depths in excess of 40 km. Internal deformation of these rocks occurred continuously during the following decompression. Index mineral composition associated with progressively younger tectonic fabrics indicates that a substantial part of the structural evolution took place within the blueschist-facies P – T field. Despite their tectonic and metamorphic history, the rocks of the Lungro–Verbicaro Unit preserve significant sedimentary and palaeontological features allowing correlations with successions included in adjacent thrust sheets and the reconstruction of the Mesozoic continental margin architecture. The subduction–exhumation cycle recorded by the Lungro–Verbicaro Unit is entirely of Miocene age. This portion of the Apulia continental palaeomargin was involved in convergence-related deformation not earlier than the Aquitanian. The integration of our results with available constraints on the tectonic evolution of the Apennine–Calabrian Arc system suggests that subduction and most of the subsequent exhumation of the Lungro–Verbicaro Unit occurred, up to Langhian time, at maximum vertical rates in excess of 15 mm a −1 . The exhumation process was then completed, at much slower rates (<2 mm a −1 ) in Late Miocene time, as indicated by both apatite fission-track data and stratigraphic information.