Georges Mascle

Dating the Indian continental subduction and collisional thickening in the northwest Himalaya: Multichronology of the Tso Morari eclogites

Multichronometric studies of the low-temperature eclogitic Tso Morari unit (Ladakh, India) place timing constraints on the early evolution of the northwest Himalayan belt. Several isotopic systems have been used to date the eclogitization and the exhumation of the Tso Morari unit: Lu-Hf, Sm-Nd, Rb-Sr, and Ar-Ar. A ca. 55 Ma age for the eclogitization has been obtained by Lu-Hf on garnet, omphacite, and whole rock from mafic eclogite and by Sm-Nd on garnet, glaucophane, and whole rock from high-pressure metapelites. These results agree with a previously reported U-Pb age on allanite, and together these ages constrain the subduction of the Indian continental margin at the Paleocene-Eocene boundary. During exhumation, the Tso Morari rocks underwent thermal relaxation at about 9 ± 3 kbar, characterized by partial recrystallization under amphibolite facies conditions ca. 47 Ma, as dated by Sm-Nd on garnet, calcic amphibole, and whole rock from metabasalt, Rb-Sr on phengite, apatite, and whole rock, and Ar-Ar on medium-Si phengite from metapelites. Ar-Ar analyses of biotite and low-Si muscovite from metapelites, which recrystallized at <5 kbar toward the end of the exhumation, show that the Tso Morari unit was at upper crustal levels ca. 30 Ma. These results indicate variable exhumation rates for the Tso Morari unit, beginning with rapid exhumation while the Indian margin subduction was still active, and later proceeding at a slower pace during the crustal thickening associated with the Himalayan collision.

Sedimentary record of the northward flight of India and its collision with Eurasia (Ladakh Himalaya, India)

Abstract— Stratigraphic and petrographic analysis of the Cretaceous to Eocene Tibetan sedimentary succession has allowed us to reinterpret in detail the sequence of events which led to closure of Neotethys and continental collision in the NW Himalaya.During the Early Cretaceous, the Indian passive margin recorded basaltic magmauc activity. Albian volcanic arenites, probably related to a major extensional tectonic event, are unconformably overlain by an Upper Cretaceous to Paleocene carbonate sequence, with a major quartzarenite episode triggered by the global eustatic sea-level fall at the Cretaceous/Tertiary boundary. At the same time, Neotethyan oceanic crust was being subducted beneath Asia, as testified by calc-alkalic volcanism and forearc basin sedimentation in the Transhimalayan belt.Onset of collision and obduction of the Asian accretionary wedge onto the Indian continental rise was recorded by shoaling of the outer shelf at the Paleocene/Eocene boundary, related to flexural uplift of the passive ...

Neogene shortening contribution to crustal thickening in the back arc of the Central Andes

To illustrate the Neogene shortening distribution in the back-arc units of the Central Andes and to estimate the contribution of the shortening to crustal thickening, two balanced crustal cross sections have been constructed across the northern and southern branches of the Bolivian orocline. Total Neogene shortening, which varies from 191 to 231 km, is accommodated by a crustal duplex below the Cordillera Oriental, but is insufficient to produce the 70 km of crustal thickness evidenced by geophysical data below the Altiplano. The best explanation for this anomalous thickening seems to be crustal underplating by material tectonically eroded from the continental margin; this process probably caused the Altiplano uplift. The subduction of oceanic lithosphere coupled with this underplating and a brief episode of gravity spreading of the Altiplano constituted the driving forces that produced Neogene shortening and development of the Central Andes.

Oceanic plateau and island arcs of southwestern Ecuador: their place in the geodynamic evolution of northwestern South America

Abstract Coastal Ecuador is made up of an oceanic igneous basement overlain by Upper Cretaceous to Lower Paleocene (≈98–60 Ma) volcaniclastic and volcanic rocks of island-arc affinities. The igneous basement, known as the Pinon Formation, locally dated at 123 Ma, consists of olivine-free basalts and dolerites. Relative to N-MORB, both types of rocks exhibit high concentrations in Nb (0.3–10.75 ppm), Ta (0.03–0.67 ppm), Th (0.11–1.44 ppm), light and medium rare earth elements, and low Zr (22–105 ppm) and Hf (0.59–2.8 ppm) contents, thus showing oceanic plateau basalts affinities. Most of these oceanic plateau basalts tholeiites display rather homogeneous eNd (T = 123 Ma) ratios (∼+7), with the exception of two rocks with higher (+10) and lower (+4.5) eNd (T = 123 Ma), respectively. All these basalts plot, with one exception, within the ocean island basalts field. Their (87Sr/86Sr)i ratios are highly variable (0.7032–0.7048), probably due to hydrothermal oceanic alteration or assimilation of altered oceanic crust. The rocks of the Pinon Formation are geochemically similar to the oceanic plateau tholeiites from Nauru and Ontong Java Plateaus and to the Upper Cretaceous (92–88 Ma) Caribbean Oceanic Plateau lavas. The basalts and dolerites of the Upper Cretaceous–Lower Paleocene island arcs show calc-alkaline affinities. The eNd ratios (+6.1 to +7.1) of these arc-rocks are very homogenous and fall within the range of intra-oceanic island-arc lavas. The Upper Cretaceous–Lower Paleocene calc-alkaline and tholeiitic rocks from coastal Ecuador share similar high eNd ratios to Cretaceous intra-oceanic arc rocks from north, central and South America and from the Greater Antilles. Since the Pinon oceanic plateau tholeiites are locally overlain by early-Late Cretaceous sediments (∼98–83 Ma) and yielded locally an Early Cretaceous age, they do not belong to the Late Cretaceous Caribbean Oceanic Plateau. The basement of coastal Ecuador is interpreted as an accreted fragment of an overthickened and buoyant oceanic plateau. The different tectonic units of coastal Ecuador cannot be easily correlated with those of western Colombia, excepted the Late Cretaceous San Lorenzo and Ricaurte island arcs. It is suggested that northwestern South America consists of longitudinally discontinuous terranes, built by repeated accretionary events and significant longitudinal displacement of these terranes.

The Mamonia Complex (SW Cyprus) revisited: remnant of Late Triassic intra-oceanic volcanism along the Tethyan southwestern passive margin

Upper Triassic volcanic and sedimentary rocks of the Mamonia Complex in southwestern Cyprus are exposed in erosional windows through the post-Cretaceous cover, where the Mamonia Complex is tectonically imbricated with the Troodos and Akamas ophiolitic suites. Most of these Upper Triassic volcanic rocks have been considered to represent remnants of Triassic oceanic crust and its associated seamounts. New Nd and Pb isotopic data show that the whole Mamonia volcanic suite exhibits features of oceanic island basalts (OIB). Four rock types have been distinguished on the basis of the petrology and chemistry of the rocks. Volcanism began with the eruption of depleted olivine tholeiites (Type 1) and oceanic island tholeiites (Type 2) associated with deep basin siliceous and/or calcareous sediments. The tholeiites were followed by highly phyric alkali basalts (Type 3) interbedded with pelagic Halobia-bearing limestones or white reefal limestones. Strongly LREE-enriched trachytes (Type 4) were emplaced during the final stage of volcanic activity. Nd and Pb isotopic ratios suggest that tholeiites and mildly alkali basalts derived from partial melting of heterogeneous enriched mantle sources. Fractional crystallization alone cannot account for the derivation of trachytes from alkaline basalts. The trachytes could have been derived from the partial melting at depth of mafic material which shares with the alkali basalts similar trace element and isotopic compositions. This is corroborated by the rather similar isotopic compositions of the alkali basalts and trachytes. The correlations observed between incompatible elements (Nb, Th) and {varepsilon}Nd and Pb isotopic initial ratios suggest that the Mamonia suite was derived from the mixing of a depleted mantle (DMM) and an enriched component of High µ (µ = 238U/204Pb, HIMU) type. Models using both Nd and Pb isotopic initial ratios suggest that the depleted tholeiites (Type 1) derived from a DMM source contaminated by an Enriched Mantle Type 2 component (EM2), and that the oceanic tholeiites (Type 2), alkali basalts (Type 3) and trachytes (Type 4) were derived from the mixing of the enriched mantle source of the depleted tholeiites with a HIMU component. None of the Mamonia volcanic rocks show evidence of crustal contamination. The Upper Triassic within-plate volcanism likely erupted in a small southerly Neotethyan basin, located north of the Eratosthenes seamount and likely floored by oceanic crust.

The Ivory Coast-Ghana transform margin: A marginal ridge structure deduced from seismic data

Abstract The Ivory Coast-Ghana (ICG) marginal ridge is a prominent feature of the ICG transform margin and includes a fossil ridge partially buried by a thick, undeformed sedimentary cover. The fossil ICG ridge is 130 km long and 25 km wide, and towers over the adjacent rifted basin (deep Ivorian basin, DIB) and the oceanic crust by 1.3 km and more than 4 km, respectively. It formed in three successive stages. 1. (1) During the rifting of the DIB, both vertical and horizontal motions between the DIB and the South American plate varied along the plate boundary. This relative motion occurred in an accommodation zone that tilted the northern slope of the ICG ridge along en-echelon, mainly strike-slip, faults. 2. (2) After the rifting of the DIB, the relative motion remained constant along the transform plate boundary. At this time strike-slip deformation was localized into a narrow and highly deformed belt that truncated the accommodation zone. 3. (3) Finally, the transform motion occurred between the DIB and an occanic plate. Thermal exchanges between the two adjacent plates induced thermal uplift of the ICG ridge that amplified previous tectonic tilting.

Channel profiles through the active thrust front of the southern Barbados prism

Submarine channels of the Orinoco River were analyzed using high-quality, dense- coverage bathymetric and seismic data provided by a recent marine survey on the southern Barbados prism. Analysis of the syntectonic sediments on seismic profiles shows that the four to five frontmost structures show evidence of recent tectonic movement. Slope analysis of the major channels was performed. From their headwaters to domains of little or no active tectonics, the channels display <0.2% slope and form levees. Slope and incision increase gradually in domains of moderate tectonics, but deep canyons with ∼2% mean slope form where the channels cross the active frontal folds of the prism. Detailed correlation between the active structures, their geometry, and canyon slope suggest that systematic variations in channel gradient highlight variations in substrate uplift rate. Steep slopes induced by uplift accelerate sediment flow and enhance incision. Nonetheless, such slope analysis is subject to complications introduced by variations in sediment flux and transient erosional conditions.

Recent movements along the Main Boundary Thrust of the Himalayas: Normal faulting in an over-critical thrust wedge?

Abstract The Main Boundary Thrust (MBT) is one of the major Himalayan thrusts occurring during the Cainozoic, and it is presently incorporated within the Himalayan thrust wedge (Lesser and Outer Himalayas) displaced above the Indian lithosphere. Nonetheless the MBT shows recent normal displacement along most of its length. We suggest that the orientation of the major principal stress within the Himalayan thrust wedge deviates significantly from the horizontal and when this deviation exceeds the dip of the vectors normal to back-tilted thrusts, the normal component of displacement may act along these faults. Steep north-dipping segments of the MBT therefore show a normal component of displacement if a geometrical definition is used, but they are faults in a compressional regime where the major principal stress axis has deviated from the horizontal. Micro-structural data recorded along the Surkhet-Ghorahi segment of the MBT are consistent with a strong deviation of the state of stress. The presence of such peculiar normal faulting along the MBT is used to calibrate the mechanical characteristics of the belt considered as a Coulomb wedge. The following characteristics are suggested: (a) very poor strength contrast between basal decollement and rocks in the wedge body, (b) a high pore fluid pressure ratio (probably close to 0.8–0.9) and a higher fluid pressure ratio (close to 1.0) along the active normal faults if a high internal friction angle (close to the Byerlee value) is considered. The strong deviation in principal stress direction may have recently increased, due to a taper of the Himalayan wedge exceeding the stability boundary and may be controlled by erosion and isostatic uplift rebound of the Himalayan range.

Offshore Frontal Part of the Makran Accretionary Prism: The Chamak Survey (Pakistan)

The Makran accretionary prism developed in the north-western part of the Indian Ocean as a consequence of the subduction of the Arabian Sea since Late Cretaceous times. It extends from southern Iran to the Baluchistan region of Pakistan where it joins the Chaman-Ornach-Nal left-lateral strike-slip fault systems to the north and the Owen Fracture Zone-Murray Ridge transtensional (right-lateral) system to the south in a complex triple junction near the city of Karachi. In September to October of 2004, we surveyed most of the accretionary complex off Pakistan with R/V Marion Dufresne. We achieved a nearly continuous bathymetric mapping of the prism and the subduction trench from 62°30′E to the triple junction near 62°30′E together with nearly 1000 km of seismic reflection (13 lines) and we took 18 piston cores in different geological settings. One of the main results is that the frontal part of the Makran accretionary prism is less two-dimensional than previously expected. We interpret the along-strike tectonic variation as a consequence of lateral variations in sediment deposition as well as a consequence of the under-thrusting of a series of basement highs and finally of the vicinity to the triple junction.

Control on thrust tectonics in the Himalayan foothills: a view from a numerical model

Abstract The evolution of an intracontinental accretionary wedge is controlled by: (a) the displacement of several thrust sheets along faults and an underlying decollement; (b) the vertical motion of the rigid basement beneath the accretionary wedge; and (c) superficial processes (erosion/sedimentation). A numerical model is proposed based on the Coulomb wedge theory, a 2.5-D forward kinematic system, progressive tilting of the underthrust basement, and an erosion/sedimentation model in order to characterise the sequence of fault activation in a thrust system and the syn-orogenic sedimentation in such a system. The Siwalik belt, the frontal thrust system of the Himalayas, represents a structure on which the model can be applied. It presents mainly simple structures made of one fault system branched on an underlying decollement. Conglomerate deposits of the Plio-Pleistocene (Upper Siwalik) are overthrust by Miocene formations (Lower Siwalik) at the front of the wedge and along inner faults. Structural maps show that thrust sheets are laterally relayed and simultaneously emplaced. The comparison between the deformation pattern in the Himalayan foothills and the modelling results supports the concept that the Himalayan thrust wedge has a steady-state-type evolution controlled by horizontal convergence and superficial mass transport, and characterised by an irregular spatial and temporal distribution of shortening within the whole wedge. During this evolution, piggy-back basins or strongly displaced internal sheets hide some thrust sheets. The exhumation path of these sheets is then discontinuous, though the shortening of the wedge is regular.