Exhumation Processes Research Articles

Evidence of two-stage extensional tectonics from the northern edge of the Edremit Graben, NW Turkey

Western Turkey is a place of active continental extension, characterized by the occurrence of several WNW-ESE-trending major grabens. The central part of the northern edge of the Edremit Graben is delineated by various geological units, namely the metamorphic Kazdağ Massif, the Mid-Cretaceous Çetmi mélange, the sedimentary Küçükkuyu formation, and loose Plio-Quaternary deposits. Detailed structural and sedimentological study suggests a two-stage extensional evolution of the area, separated by a short break in the tectonic regime. The first stage, possibly related to back-arc extension and/or orogenic collapse, is marked by the activity of a newly described low-angle detachment fault, the şelale detachment fault, from the latest Oligocene onward. The fault plane, separating the mylonitized rocks of the Kazdağ Massif in the footwall from the unmetamorphosed Çetmi mélange and Küçükkuyu formation in the hanging wall, must have played a significant role in the initial exhumation processes of the Kazdağ Massif at that time. The Lower Miocene syntectonic Küçükkuyu formation has recorded the initiation and filling up of a small basin, which has developed in a typical supra-detachment basin, above the detachment fault. After a short phase of possible compression and erosion, the second stage—which marks the onset of neotectonic activity—is marked by the development of Plio-Quaternary step-like normal faults, which cut through all the previous units. Coarse, loose sediments were deposited following the fault activity. These local results are extrapolated to apply to the entire Edremit Graben. In that case, its evolution is seen as the succession of two extensional stages, characterized by distinct structural and sedimentological patterns, and possibly separated by a short compressional phase.

Cooling history of the Upper Cretaceous Palgongsan Granite, Gyeongsang Basin, SE Korea and its tectonic implication for uplift on the active continental margin

Apatite and zircon fission track analyses were carried out to reconstruct the cooling and inferred uplift history of the Cretaceous Palgongsan Granite, Gyeongsang Basin, Korea. The Palgongsan Granite is one of the Bulguksa intrusive rocks that formed by arc-related plutonism during Late Cretaceous to Early Tertiary time. Fission track dating of the Palgongsan Granite yielded nearly concordant ages of 53 and 65 Ma for apatite and zircon, respectively. The Palgongsan Granite also shows a simple cooling pattern, which suggests that it has not been affected by any thermal event after emplacement. The cooling history derived from fission track data combined with other thermochronometric data indicates that the Palgongsan Granite experienced relatively rapid cooling in earlier stage (> 30 °C/Ma). The initial rapid cooling rate during the Late Cretaceous has been caused by the large thermal contrast between the granite body and the country rocks. After reaching thermal equilibrium with the surrounding country rocks, the cooling rate of the Palgongsan Granite was abruptly decreased in late stage. In this late stage, the decelerated cooling rate is interpreted to have been controlled by uplift and erosion processes, and the average exhumation rate is calculated to be ca. 50 m/my over the temperature range from 100 °C to the surface temperature. The cooling history of the Palgongsan Granite is in good agreement with that of the Ryoke Granitic Belt in Southwest Japan, as well as those of the Taebaeksan Range and other Bulguksa intrusive rocks in the Gyeongsang Basin. This suggests that such cooling was probably caused by regional uplift and exhumation processes on the East Asian active continental margin. Compared with the uplift rates of the Andes, the uplift rates on the eastern Pacific margin appear to be higher than those on the western Pacific margin.

Amphibole-bearing assemblages as indicators of microdomain-scale equilibrium conditions in metabasites: an example from Alpine ophiolites of the Meliata Unit, NE Hungary

The polymetamorphic evolution of metamorphosed Mesozoic ophiolitic rocks in NE Hungary was studied by microtextural and mineral chemical analyses of metagabbros and metabasalts. Both rock types preserved their original textures remarkably well. By contrast, magmatic minerals (especially clinopyroxene) were preserved only in the metagabbros. The original magmatic whole-rock and mineral chemical features had a strong influence on the formation and preservation of the various metamorphic assemblages as testified by the chemically different amphiboles (and other major minerals) formed in various microdomains. The calculated metamorphic P-T conditions are in part different for metabasalts and metagabbros. These results are in accordance with the observed petrographic features, among which the sequence of crystallisation of Na-amphibole and actinolite in the two rock types is most characteristic. Crystallisation of actinolite is followed by Na-amphibole in metabasalts, while Na-amphibole formed first in the metagabbros. Overall equilibrium conditions were not attained throughout the NE Hungarian Mesozoic ophiolite unit during Alpine metamorphic evolution. Additionally, various disequilibrium metamorphic assemblages with several generations of chemically complex amphiboles were formed even on the thin section scale. Various P-T paths during the tectonic evolution of the accretionary wedge were experienced by parts of the dismembered ophiolite sequence, which originally formed a coherent unit, and were brought back into close spatial relationship only during the post-metamorphic exhumation processes.

Metamorphic evolution and zircon geochronology of early Proterozoic granulites in the Aravalli Mountains of northwestern India

Granulites including a charnockite suite, mafic granulites, pelitic granulites, metanorite dykes and their retrograde varieties occur as discontinuous shear zone-bounded bodies within the Archaean basement comprising a granite gneiss–amphibolite–metasedimentary rock association in the central part of the Aravalli Mountains, northwest India. The entire suite, named the Sandmata Complex, preserves a complex history of tectonothermal evolution. Except for their strongly foliated margins, the granulite bodies are largely massive. Partial melting in the ‘country rocks’ led to the development of migmatite gneisses close to the contact of the granulite, a feature not as common in the rocks further away from the granulite contact. Geothermobarometry of massive granulites indicates Tmax>900°C and Pmax∼7.5 kbar. The retrograde granulites, which formed at lower amphibolite/upper greenschist-facies conditions, experienced channelized hydration reactions concomitant with shearing. These rocks locally appear as hornblende–biotite-bearing foliated granulite with or without Cpx or Opx. The rocks seem to have followed an inverse PTt path and have undergone an earlier phase of near-isobaric cooling. Our single zircon Pb–Pb ages indicate that the exhumation of granulites to the shallower amphibolite-facies levels with concomitant melting in the country rocks took place between 1690 Ma and 1621 Ma. Assuming that the granulite-facies metamorphism took place at around 1725 Ma, we relate the entire process of granulite metamorphism and exhumation covering an age range between 1725 and 1621 Ma to the rift basin opening stages of the Delhi Orogenic cycle that culminated at c. 1450 Ma.

Erosion and exhumation in accretionary orogens: Experimental and geological approaches

This study examines the role of erosion on exhumation and deformation history of accretionary orogens through experimental and geological methods. Fault propagation, kinematics, and exhumation rates in an eroded thrust wedge are analyzed through scaled sandbox simulations by testing different parameters (friction, rheologic layering, and sediment input/output ratio). A model Coulomb wedge is submitted to erosion under flux steady state conditions; that is, the volume of eroded material remains equal to the volume of newly accreted material, maintaining a constant surface slope during shortening. Results show that the way of exhumation depends on the internal dynamics of thrust wedges and, conversely, on how this dynamics is modified by erosion. In the eroded thrust wedges the diversity of exhumation patterns is controlled by the mode of fault propagation, depending on the basal friction (high or low). The vertical component of exhumation is generally higher for the wedges with high basal friction than for low‐friction wedges. The uplift of material occurs along a cluster of subvertical thrusts in the middle part of the eroded thrust wedge with low basal friction. The material is exhumed along a series of inclined (20°–50°) thrusts in the rear of the high‐friction wedge. The zone of maximum exhumation is generally localized in the middle part of the thrust wedge and migrates toward the backstop with continued shortening. The vertical exhumation rate increases with time, and the material accreted later is rapidly transferred to the main exhumation zone compared to the material accreted during the early stages. The exhumation occurs at rates 2 times faster in the wedges with half the thickness than in full thickness wedges. The total quantity of eroded material at the end of convergence constitutes 36–50% of initial model area for the thrust wedges with low basal friction and 20–40% for the high basal friction wedges. The extent of basal underplating increases with total shortening. The area of basal underplated material constitutes up to 30 and 40% of the eroded thrust wedge area for the models with low and high basal friction, respectively. Presence of décollements in the accreted series allows underplating of thrust units developing an anticlinal stack, whose growth and location is favored by erosion. Our results are compared to different present‐day active and ancient convergent orogens (Himalaya, Taiwan, Southern Alps of New Zealand, and Cascadia orogen), allowing the different specific exhumation processes to be characterized.

Thermal maturity of the axial zone of the southern Apennines fold‐and‐thrust belt (Italy) from multiple organic and inorganic indicators

AbstractThe reconstruction of the thermal history of folded and thrust units is crucial to define the pattern of tectonic loading and the time‐space evolution of an orogen where tectonic exhumation processes occurred at shallow crustal levels. In the present study, a well‐constrained reconstruction of the thermal maturity in the axial zone of the southern Apennines has been achieved by the combined use of different thermal indicators in diagenesis. The major results are: (i) documentation of a jump in thermal maturity from the Apenninic Platform derived tectonic unit (from immature to early mature stages of hydrocarbon maturation) to the Lagonegro Basin derived tectonic units (late diagenetic zone); (ii) documentation of along‐strike slighter variations in the Lagonegro units, concerning thermal maturity (thus maximum burial temperatures). This can be related to changes in amounts of tectonic burial and erosion/exhumation because of the lack of cylindricity of contractional structures; (iii) recognition of an independent thermal evolution of the allochthonous chain compared with the Apulian Platform tectonic unit with Mt Alpi area (in the late mature stage of hydrocarbon generation) interpreted as a sector of localized, intense exhumation within the External Zone of the orogen.

Hydroxyl induced eclogite fabric and deformation mechanism

Eclogites from orogens often show strong plastic deformation and high hydroxyl content. We have studied the correlation between crystallographic preferred orientations of garnet and omphacite from natural eclogites with their hydroxyl contents using the electron back-scattered diffraction technique. The results show: 1) Omphacite has typical L-type or SL-type crystrallographic preferred orientations, that is, [001] is distributed in a girdle in the foliation plane with a maximum parallel to lineation; (010) is distributed in a girdle normal to the lineation with a maximum parallel to the foliation plane, suggesting a shear dominant deformation regime. Omphacite fabrics do not vary significantly with hydroxyl content, although the hydrous component may cause lower flow strength. 2) Hydroxyl can influence significantly flow properties of garnet in eclogite. Garnets behave as rigid bodies under low temperature and dry conditions. Grain boundary processes will dominate the deformation and lower the flow strength of garnet under high water fugacity conditions. Garnets show no crystallographic preferred orientation in both cases. These results may have important implications for a better understanding of deformation mechanisms and associated fluid activities during deep subduction and exhumation processes.

Structural evolution of the Tuscan Nappe in the southeastern sector of the Apuan Alps metamorphic dome (Northern Apennines, Italy)

AbstractStructural analysis carried out in the Tuscan Nappe (TN) in the southeastern sector of the Apuan Alps highlights a structural evolution much more complex than that proposed so far. The TN has been deformed by structures developed during four deformation phases. The three early phases resulted from a compressive tectonic regime linked to the construction of the Apenninic fold‐and‐thrust‐belt. The fourth phase, instead, is connected with the extensional tectonics, probably related to the collapse of the belt and/or to the opening of the Tyrrhenian Sea.Our structural and field data suggest the following. (1) The first phase is linked to the main crustal shortening and deformation of the Tuscan Nappe in the internal sectors of the belt. (2) The second deformation phase is responsible for the prominent NW–SE‐trending folds recognized in the study area (Mt. Pescaglino and Pescaglia antiforms and Mt. Piglione and Mt. Prana synforms). (3) The direction of shortening related to the third phase is parallel to the main structural trend of the belt. (4) The interference between the third folding phase and the earlier two tectonic phases could be related to the development of the metamorphic domes. The two directions of horizontal shortening induced buckling and vertical growth of the metamorphic domes, enhancing the process of exhumation of the metamorphic rocks. (5) The exhumation of the Tuscan Nappe occurred mostly in a compressive tectonic setting.A new model for the exhumation of the metamorphic dome of the Apuan Alps is proposed.Its tectonic evolution does not fit with the previously suggested core complex model, but is due to compressive tectonics. Copyright © 2004 John Wiley & Sons, Ltd.

Complete Tertiary exhumation history of the Menderes massif, western Turkey: an alternative working hypothesis

AbstractThe main exhumation of the Menderes massif, western Turkey, occurred along an originally N‐dipping Datça–Kale main breakaway fault that controlled depositions in the Kale and the Gökova basins during the Oligocene – Early Miocene interval. The isostatically controlled upward bending of the main breakaway fault brings the lower plate rocks to the surface. In the Early Miocene, E–W‐trending N‐ and S‐dipping graben‐bounding faults fragmented the exhumed, dome‐shaped massif. The development of half grabens by rolling master fault hinges has allowed further exhumation of the central Menderes massif. After the Pliocene, high‐angle normal faults cut all of the previous structures. This model suggests that the Menderes massif is a single large metamorphic core complex that has experienced a two‐stage exhumation process.

Exhumation process of the Nago subduction-related metamorphic rocks, Okinawa, Ryukyu island arc

The Kunigami zone in Okinawa is an extension of the Shimanto zone, Japan. The rocks make up the main part of the Nago metamorphic rocks, and such metamorphic rocks are exceptional in the Shimanto zone. The Anne complex, in the older Motobu zone, is also metamorphosed. The reason for why and how this kind of the metamorphism occurred, and especially why and how the metamorphic rocks were exhumed, is yet uncertain and unresolved. To understand the metamorphic and exhumation process in Okinawa, a structural study is undertaken, and its relation to the Eocene ridge subduction is discussed. We believe exhumation was performed by formation of a D2 extrusion wedge, made up of the Nago metamorphic rocks. The base for this wedge is a subduction thrust, and the roof is a detachment fault. Internally, there exists another Kijoka detachment fault, which is a brittle low-angle fault with top to the northwest shear sense, and the D2 major recumbent folds and thrusts show top to the southeast opposite shear sense in the Kunigami zone. This is the first report that finds detachment faults from the typical and ancient accretionary complex. M2 is mostly retrograde related to exhumation, but its medium P/T-type prograde metamorphism, abnormal at subduction zones, represents a high thermal gradient during ridge subduction. As a result, this ridge subduction is responsible for exhumation. At the time of accretion of the Kunigami zone, D1 ductile contraction and constriction exhibited top to the southeast shear sense, but an opposite and extensional shear sense is recognized in the proto-wedge. During D1, the wedge had already been active and begun to exhume. M1 of the Miyagi complex is accretion related and also of medium P/T-type metamorphism, and is a consequence of Cretaceous ridge subduction without any ability to cause much exhumation.

Late tectonic evolution of the Northern Apennines: the role of contractional tectonics in the exhumation of the tuscan units

Structural analysis performed in the continental tuscan units of the Northern Apennines (Italy) highlight the presence of transversal and parallel structures, with respect to the main trend of the belt (NW-SE), affecting the tectonic nappe pile. These structures, following the main synmetamorphic phases associated to isoclinal folds with axial plane foliations, are upright folds, brittle and brittle-ductile shear zones. Nearly upright folds are related to NW-SE and NE-SW directions of shortening. The two perpendicular directions of shortening, associated with a contraction setting, induced the vertical growth of the metamorphic domes enhancing the process of exhumation of the metamorphites. The presence of later NE-verging brittle-ductile shear zones crosscutting both systems of folds testifies that contraction long lasted during the orogenesis. Fluid inclusion analyses, performed in syntectonic veins coeval with NW-SE trending upright folds, point out that, after the first tectonic phase, pressure values decrease supporting that this tectonic unit was exhuming during the development of upright folds. Post-collisional extensional tectonics gave rise to collapse folds and low- and high-angle normal faults in the latest stages of deformation.

Southward extrusion of eclogite-bearing mafic–ultramafic bodies in the Sanbagawa belt, central Shikoku, Japan

Several mafic rock masses, which have experienced eclogite facies metamorphism, are distributed in flat-lying non-eclogitic schists in an intermediate structural level (thermal core) of the Sanbagawa belt. The largest, Iratsu mass, and an associated peridotite, the Higashi-Akaishi mass, extend E–W for about 8 km, and N–S for about 3 km, and are surrounded by pelitic, basic and quartz schists. The Iratsu mass consists of metabasites of gabbroic and basaltic origin, with intercalations of ultramafic rocks, felsic gneiss, quartz schist and metacarbonate. The Iratsu mass can be divided into two layers along a WNW-trending metacarbonate layer. The Higashi-Akaishi mass consists of peridotite with intercalations of garnet clinopyroxenite. It is situated beneath the western half of the Iratsu mass, and their mutual boundary dips gently or steeply to the N or NE. These masses underwent eclogite, and subsequent epidote-amphibolite facies metamorphism as has been reported elsewhere. The Iratsu–Higashi-Akaishi masses and the surrounding rocks underwent ductile deformation under epidote-amphibolite facies (or lower P– T) metamorphic conditions. Their foliation generally trends WNW and dips moderately to the NNE, and the mineral lineation mostly plunges to the N and NE. In non-eclogitic schists surrounding the Iratsu–Higashi-Akaishi masses, the foliation generally trends WNW and dips gently or steeply to the N or S and the mineral lineation mostly plunges to the NW, N and NE. Kinematic analysis of deformation structures in outcrops and oriented samples has been performed to determine shear senses. Consistent top-to-the-north, normal fault displacements are observed in peridotite layers of the Higashi-Akaishi mass and eclogite-bearing epidote amphibolite layers of the Iratsu mass. Top-to-the-northeast or top-to-the-northwest displacements also occur in non-eclogitic pelitic–quartz schists on the northern side of the Iratsu mass. In the structural bottom of the Iratsu–Higashi-Akaishi masses and to the south, reverse fault (top-to-the-south) movements are recognized in serpentinized peridotite and non-eclogitic schists. These observations provide the following constraints on the kinematics of the rock masses: (1) northward normal displacement of Iratsu relative to Higashi-Akaishi, (2) northward normal displacement of non-eclogitic schists on the north of the Iratsu mass and (3) southward thrusting of the Iratsu–Higashi-Akaishi masses upon non-eclogitic schists in the south. The exhumation process of the Iratsu–Higashi-Akaishi masses can be explained by their southward extrusion.

The high-resolution sequence stratigraphy and the mammal fossil record: a test in the Middle–Upper Pleistocene deposits of the Roman Basin (Latium, Italy)

In the Campagna Romana area Pleistocene vertebrate fossil remains are preserved both in continental and marine sedimentary successions. This fact favoured the identification of faunal complexes in agreement with the biochronologic setting proposed for the Plio–Pleistocene large mammal faunas of the Italian peninsula. At the same time, the detailed facies and sequence stratigraphic analyses carried out in the last years on such deposits have allowed establishment of a regional and local chronostratigraphic framework which sets some physical and temporal limits to the occurrence of faunal complexes. In this light, the study of the Pleistocene deposits of the Roman Basin, based on sequence stratigraphy and mammal biochronology approach, can constitute a significant tool to test the relationships among physical stratigraphic units, faunal complexes and faunal units. In the examined area, the fossiliferous levels are essentially found in transgressive fluvial and fluvio-palustrine deposits filling incised valleys (TST) and in highstand barrier island–lagoon deposits (HST). Excluding some cases where bones show a probable exhumation process, whole discovered mammal remains are mostly coeval with deposits containing them. Based on taphonomic considerations and on depositional and physical stratigraphic settings, a correlation scheme among Roman Pleistocene sequence-stratigraphic units, local faunal complexes and Italian mammal biochrons is proposed. Nevertheless, many problems remain to be clarified before either the correlation between events and the geochronological framework, or the significance and the biochronological value of each faunal assemblage can be considered as definitive.

Exhumation of the ultrahigh‐pressure Tso Morari unit in eastern Ladakh (NW Himalaya): A case study

Exhumation processes of the ultrahigh‐pressure (UHP) Tso Morari dome (NW Himalaya) are investigated using structural, petrological, and geochronological data. The UHP Tso Morari unit is bounded by the low‐grade metamorphic Indus Suture Zone to the NE and Mata unit to the SW. Three deformation phases (D1, D2, and D3) are observed. Only D3 is common to the UHP unit and surrounding units. In the UHP unit, the first deformation phase (D1) produced upright folds, under eclogitic conditions (>20 kbar; 580 ± 60°C). D1 is overprinted by D2 structures related to a NW‐SE trending open anticline. D2 is characterized by blueschist mineral associations, and corresponds to the quasi‐isothermal decompression from a depth of 90 km up to 30–40 km. The final exhumation phase of the Tso Morari unit is dominated by tectonic denudation and erosion (D3), associated with a slight temperature increase. Radiochronological analyses indicate that the UHP exhumation process began during the Eocene. Exhumation was fast during D1‐D2 and slowed down through D3 during the Oligocene. The change in the deformation style from D1‐D2 to D3 in the Tso Morari unit coincides with changes in the exhumation rates and in the metamorphic conditions. These changes may reflect the transition from an exhumation along the subduction plane in a serpentinized wedge, to the vertical uplift of the Tso Morari unit across the upper crust.

Heterogeneity of water in UHP eclogites from Bixiling in Dabieshan: Evidence from garnet

Garnets in ultrahigh pressure (UHP) eclogites from Bixiling in Dabieshan were investigated by Fourier transform infrared spectrometer (FTIR). The results demonstrate that all garnets contain structural water which occurs as hydroxyl (OH), with contents ranging from 164 to 2034 ppm (H2O wt.) and mostly higher than 500 ppm. Like omphacite which is another major OH-rich mineral in eclogites, garnet is an important carrier that can recycle the surface water into deep mantles. Heterogeneity of water in garnets exists not only among different samples of the same outcrop (∼150 m), but also among different crystals of the same sample (∼1 cm). This indicates that the mobility of fluids during UHP metamorphism is very limited (possibly on centimeter scales), and that both subduction and exhumation processes of UHP rocks are very fast.

Deformation history and exhumation process of the Horoman Peridotite Complex, Hokkaido, Japan

The Horoman Peridotite Complex is an Alpine-type orogenic peridotite massif in the Hidaka metamorphic belt, Hokkaido, Northern Japan. Because of wide exposure and extremely limited serpentinization, the complex provides important information on uplift and emplacement processes of an Alpine-type peridotite massif into the crust. Based on microstructures, the massif can be divided into five structural units parallel to the lithological layering as follows; (1) Equigranular Zone, (2) Internal Shear Zone (ISZ), (3) Transition Zone, (4) Porphyroclastic Zone and (5) Basal Shear Zone (BSZ). A top-to-the-north sense of shear deformation in the Porphyroclastic Zone and the Basal Shear Zone implies that the Horoman Peridotite Complex had uplifted from the upper mantle to the lower crust along a northward dipping extensional shear-zone systems. After incorporation of the mantle peridotite with lower crustal rocks, the upper part of the massif (i.e. the Equigranular Zone and the Internal Shear Zone) was overprinted by a top-to-the-south sense of shear deformation that was comparable with the sub-horizontal displacement of the crustal granulite sequences in the Hidaka metamorphic belt under transpressive tectonic environment.

Quantitative resolution of the debate over antiquity of the central Australian landscape: implications for the tectonic and geomorphic stability of cratonic interiors

We report the first measure of long- (∼100 Myr) and short- (∼1 Myr) term denudation rates from key geologically stable landforms in the Davenport Range, central Australia. These landforms have previously been assigned a Cambrian age, which arguably places them amongst the oldest persistent landforms on the continent, if not on Earth. Our results from combined apatite fission track thermochronology and in situ cosmogenic radionuclide analyses using 10Be and 26Al show that while average exhumation rates are low, the denudation history for this cratonic region is incompatible with extreme, sub-aerial longevity and long-term tectonic and geomorphic stability. Our revised model for the landscape evolution of this region is consistent with one of maximum burial prior to and during the Mesozoic, followed by a phase of kilometre-scale exhumation that was largely complete by the beginning of the Cainozoic. We suggest that a similar process of burial and exhumation has probably been responsible for the sub-aerial preservation of seemingly ancient landforms elsewhere in Australia.

THE SUBDUCTION-EXHUMATION CYCLE IN THE LIGURIAN ALPS: NEW RADIOMETRIC CONSTRAINTS FROM THE VOLTRI MASSIF AND THE TERTIARY PIEMONTESE BASIN

We present 39Ar-40Ar dating of phengite, muscovite and paragonite of samples from the HP Voltri massif (Western Alps) and from the conglomerates of the Tertiary molasses which overlie the metamorphic basement. The phengite bearing rocks analysed display peak eclogitic and blueschist facies parageneses, locally showing complex greenschist - facies replacement textures. 39Ar-40Ar dating of white micas yielded Eocene ages interpreted as the time of different metamorphic equilibrations. In particular, high-Si phengites from eclogite clasts record a ca. 49 Ma age for the eclogite peak and ca. 43 Ma for the blueschist retrogression; phengites from a blueschist basement sample record a ca. 40 Ma for the metamorphic peak; low-Si muscovite from a metasediment dates the formation of the greenschist paragenesis at ca. 33 Ma. The internal discordance of age spectra is proportional to the chemical complexity of the micas. Our data document that the rock samples analyzed reached peak HP conditions at different times over a time - span of ca. 10 Ma. The subduction to peak blueschist conditions is in fact still going on during the exhumation of higher-pressure, eclogite-facies rocks. Our results therefore fit a tectonic model with different ophiolitic slices subducted at different moments, over a time span ranging from Lower Eocene until the Eocene-Oligocene boundary. This implies that the subduction and exhumation processes occurred simultaneously, allowing the uprising HP-rocks to escape termal reequilibration. Our data require a decoupling of exhumation from erosion: exhumation was largely accomplished before significant erosion of the wedge. Initially, it was likely driven by the buoyancy effect of antigorite serpentinites; subsequently it may have been associated with the development of blueschist shear zones. Fast exhumation was not accompanied by a high uplift of the mountain chain, whose erosion and input into the sedimentary basin occurred more than 10 Ma later.

Consolidation and Break-up of the South American Platform in Southeastern Brazil: Tectonothermal and Denudation Histories

The different tectonic stages that occurred at the end of the Proterozoic and during the Phanerozoic have an important bearing on the tectonothermal history of the South American Platform and its consolidation. Geochronological data (U/Pb monazite, 40Ar/ 39Ar whole rock) and apatite fission-track analysis, from Precambrian rocks of the southeastern Brazilian coastline, permit the modeling of a long-term thermal history of the crust and constrain variable denudation rates. Using these data, a temperature-time diagram reflects a period of accelerated exhumation during the end of the Brasiliano Orogeny, followed by long stability and reactivation of the platform during the Rifting Phase of the South Atlantic Ocean. U/Pb zircon and monazite (blocking temperature of ca. 650° C) data from a series of igneous bodies suggest that a tangential and transpressional tectonic regime occurred between 625 and 610 Ma. During the following escape tectonics, between 610 and 590 Ma the exhumation process indicates cooling rates of ca. 12°C/Ma. 40Ar/ 39Ar biotite ages between 540 and 510 Ma (ca. 300°C) and a corrected fission-track age on apatites (100°C) of 480 Ma indicate an exhumation event related to block tectonics with huge vertical displacement along shear zones. A long stabilization phase, with low exhumation, and cooling rate around 0.25°C/Ma was recorded from the Cambro/Ordovician to the Mesozoic. At 65 Ma an acceleration of the exhumation through denudation and reworking of the South American surface with cooling rate of 1.5°C/Ma is observed. The uplift of the Mantiqueira and Serra do Mar mountain ranges along the southeast Brazilian coastline works as a climatic barrier provoking lateral erosional processes causing long-term scarp retreat, combined with intense, but progressive denudation towards the continent. A denudation of 2.5 to 4 km was calculated for such processes. This lateral retreat of escarpments and flexural response can provide important insights regarding marginal isostatic uplift and the evolution of offshore sedimentary basins of southeast Brazil.

Study of Aeromagnetic Data Over Part of Eastern Ghat Mobile Belt and Bastar Craton

A portion of the aeromagnetic anomaly map of India, from 17 0 to 20 0 N and 78 o to 84 o E has been analysed to understand the tectonics of the region. The distribution of magnetic sources in the study region are clearly brought out in the analytic signal map and found to be associated with charnockitic rocks, iron formation and trap flows. The Godavari Graben is devoid of any magnetic sources. High-grade charnockitic rocks on surface and sub-surface, flank the shoulders of the Godavari Graben on either side. From the analysis of magnetic data, Sileru Shear Zone (SSZ) is identified as the contact of the Bastar craton and the Eastern Ghat Mobile Belt (EGMB). The Eastern Ghat is divided into two blocks: Block-N north of Srikakulam is devoid of magnetic sources while the charnockitic rocks are the main magnetic carriers in Block-S. The difference in magnetic characteristics of the two blocks has been attributed to the difference in metamorphic history. Block-N has an over print of amphibolite facies metamorphism while Block-S to the south depicts granulite facies metamorphism. The Euler solutions within the EGMB shows that the magnetic sources along SSZ is shallower than the south east implying that the exhumation process in the EGMB has a differential rate.