Late Cretaceous ophiolite obduction and Paleocene India-Asia collision in the westernmost Himalaya

Abstract

Collision of the Kohistan island arc with Asia at ~100 Ma resulted in N-S compression within the Neo-Tethys at a spreading center north of the Indo-Pakistani craton. Subsequent India-Asia convergence converted the Neo-Tethyan spreading center into a short-lived subduction zone. The hanging wall of the subduction zone became the Waziristan, Khost and Jalalabad igneous complexes. During the Santonian- Campanian (late Cretaceous), thrusting of the NW IndoPakistani craton beneath Albian oceanic crust and a Cenomanian volcano-sedimentary complex, generated an ophiolite-radiolarite belt. Ophiolite obduction resulted in tectonic loading and flexural subsidence of the NW Indian margin and sub-CCD deposition of shelf-derived olistostromes and turbidites in the foredeep. Campanian-Maastriehtian calci- clastic and siliciclastic sediment gravity flows derived from both margins filled the foredeep as a huge allochthon of Triassic-Jurassic rise and slope strata was thrust ahead of the ophiolites onto the Indo-Pakistani craton. Shallow to intermediate marine strata covered the foredeep during the late Maastrichtian. As ophiolite obduction neared completion during the Maastrichtian, the majority of India-Asia convergence was accommodated along the southern margin of Asia. During the Paleocene, India was thrust beneath a second allochthon that included open marine middle Maastrichtian colored mélange which represents the Asian Makran-Indus-Tsangpo accretionary prism. Latérites that formed on the eroded ophiolites and structurally higher colored mélange during the Paleocene wei’e unconformably overlapped by upper Paleocene and Middle Eocene shallow marine limestone and shale that delineate distinct episodes of Paleocene collisional and Early Eocene post-collisional deformation.     

Fission track and fault kinematics analyses for new insight into the Late Cenozoic tectonic regime changes in West-Central Sulawesi (Indonesia)

A 3-D density model for the Cretan and Libyan Seas and Crete was developed by gravity modelling constrained by five 2-D seismic lines. Velocity values of these cross-sections were used to obtain the initial densities using the Nafe–Drake and Birch empirical functions for the sediments, the crust and the upper mantle. The crust outside the Cretan Arc is 18 to 24 km thick, including 10 to 14 km thick sediments. The crust below central Crete at its thickest section, has values between 32 and 34 km, consisting of continental crust of the Aegean microplate, which is thickened by the subducted oceanic plate below the Cretan Arc. The oceanic lithosphere is decoupled from the continental along a NW–SE striking front between eastern Crete and the Island of Kythera south of Peloponnese. It plunges steeply below the southern Aegean Sea and is probably associated with the present volcanic activity of the southern Aegean Sea in agreement with published seismological observations of intermediate seismicity. Low density and velocity upper mantle below the Cretan Sea with ρ  3.25 × 10³ kg/m³ and V_p velocity of compressional waves around 7.7 km/s, which are also in agreement with observed high heat flow density values, point out at the mobilization of the upper mantle material here. Outside the Hellenic Arc the upper mantle density and velocity are ρ ≥ 3.32 × 10³ kg/m³ and V_p = 8.0 km/s, respectively. The crust below the Cretan Sea is thin continental of 15 to 20 km thickness, including 3 to 4 km of sediments. Thick accumulations of sediments, located to the SSW and SSE of Crete, are separated by a block of continental crust extended for more than 100 km south of Central Crete. These deep sedimentary basins are located on the oceanic crust backstopped by the continental crust of the Aegean microplate. The stretched continental margin of Africa, north of Cyrenaica, and the abruptly terminated continental Aegean microplate south of Crete are separated by oceanic lithosphere of only 60 to 80 km width at their closest proximity. To the east and west, the areas are floored by oceanic lithosphere, which rapidly widens towards the Herodotus Abyssal plain and the deep Ionian Basin of the central Mediterranean Sea. Crustal shortening between the continental margins of the Aegean microplate and Cyrenaica of North Africa influence the deformation of the sediments of the Mediterranean Ridge that has been divided in an internal and external zone. The continental margin of Cyrenaica extends for more than 80 km to the north of the African coast in form of a huge ramp, while that of the Aegean microplate is abruptly truncated by very steep fractures towards the Mediterranean Ridge. Changes in the deformation style of the sediments express differences of the tectonic processes that control them. That is, subduction to the northeast and crustal subsidence to the south of Crete. Strike-slip movement between Crete and Libya is required by seismological observations.     

Enhancement of subduction/obduction due to hurricane-induced mixed layer deepening

Ventilation, including subduction and obduction, in the North Pacific is re-examined, based on SODA outputs and the Eulerian definition. The annual subduction rate averaged from 2001 to 2004 is estimated at 49.8 Sv, whereas the annual obduction rate is 26.7 Sv.Furthermore, the annual subduction/obduction rate enhancement induced by tropical cyclones in the North Pacific, defined as the difference between the annual subduction/obduction rate for the cases including the mixed layer depth perturbations induced by tropical cyclones and that for the cases without the perturbations, is estimated. Based on SODA outputs and the mixed layer deepening obtained from a hurricane-ocean coupled model, the annual tropical cyclone-induced subduction rate enhancement averaged from 2001 to 2004 is estimated at 4.4 Sv and the obduction rate enhancement 5.2 Sv, and such enhancement is mainly concentrated in the latitudinal band from 10°N to 30°N.     

The geodynamic evolution of the South-Tethyan,margin in Zanskar,NW-Himalaya,as revealed by the Spongtang ophiolitic melanges

Abstract

— The composite nappe of the Spongtang ophiolite is thrust over Mesozoic and Cenozoic sediments of the North-Indian margin and shelf. The ophiolitic peridotiles tectonic-ally overlie a sedimentary melange, the detailed stratigraphy of which reveals the evolution of the South-Tethyan margin from its opening to its closure. The matrix of the melange is dated as Upper Campanian to Lower Eocene. Volcano-sedimentary olistoliths indicate carbonate platform sedimentation associated with alkaline lavas in the Permian, followed by more pelagic sedimentation of Upper Triassic and mid- Cretaceous age. They derived from external platforms near to the Indian shelf, but separated from it. Composite olistoliths of mid to late Cretaceous age, containing Permian elements, are found in the upper Cretaceous to lower Eocene wildflysch. Primary contacts of melange upon serpentinites indicates that the deeply eroded oceanic crust served partly as a substratum for the melange.     

Genesis of the Ibero-Armorican arc

Abstract

The Ibero-Armorican arc is continuous between Iberia and Armorica; its curvature increased with time due to subduction followed by continental collision; indentation produced left lateral transpression in Iberia and right-lateral transpression in Armorica. It is argued that whereas the antithetic shear is predominant in Iberia, in Armorica a synthetic shear prevailed because the identer rotated anticlockwise between the opposed forelands of the Variscan Fold Belt. It is proposed that the major Rheic ocean, closed by subduction towards the inner part of the arc, solving the space problem of centripetal vergences.     

白龙江地区的逆冲推覆构造

白龙江地区是由亚尔玛－舟曲后缘逆冲带、西倾山－武坪南逆冲岩席－飞来峰构造带、波海－白依－迭部－武都、略阳逆冲带、玛曲－南坪前缘挤压逆冲带及尕海－郎木寺沉降盆地等基本构造单元组合成的一个复杂的逆冲推覆构造体系，逆冲构造由北向南前展式扩展，逆冲作用主要导源于海西－印支期以来特提斯洋壳及若尔盖地块向中朝地块的后退式俯冲机制以及喜山期以来青藏高原被印度板块向北东方向的推挤作用，各逆冲构造单元对区域不同成因的金矿床（点）、带的控制作用，是研究区域矿产及成矿预测的重要地质依据。     

Age of the metamorphic sole of the Papuan Ultramafic Belt ophiolite, Papua New Guinea

The Papuan Ultramafic Belt (PUB) ophiolite is former oceanic crust and upper mantle emplaced onto continental crust in Papua New Guinea (PNG) in a zone of general convergence between the Pacific and Australian plates. The metamorphic sole beneath the ophiolite is best exposed in the Musa–Kumusi divide and comprises a 40- to 300-m-thick body of granulite and amphibolite facies rocks. Geochronological studies on the metamorphic sole, using amphiboles from the granulites and amphibolites, yield measured K–Ar ages ranging from 65.0±0.7 to 57.2±0.6 Ma and average ⁴⁰Ar–³⁹Ar direct total fusion ages ranging from 67.0±0.7 to 59.5±0.2 Ma. Five of the six ⁴⁰Ar–³⁹Ar plateau ages, derived from age spectra, lie between 58.6±0.2 and 57.8±0.2 Ma, with an overall mean age of 58.3±0.4 Ma. The large spread in measured K–Ar and ⁴⁰Ar–³⁹Ar total fusion ages is thought to be caused by the presence of variable amounts of excess argon. The mean plateau age for five samples of 58.3±0.4 Ma is interpreted to mark the time of cooling of the metamorphic sole following peak metamorphism. We suggest that the development of the metamorphic sole and emplacement of the PUB ophiolite onto the PNG crust occurred in a relatively short time interval in the Paleocene.     

The East Anatolia-Lesser Caucasus ophiolite:An exceptional case of large-scale obduction,synthesis of data and numerical modelling

We present new,geological,metamorphic,geochemical and geochronological data on the East Anatolian-Lesser Caucasus ophiolites.These data are used in combination with a synthesis of previous data and numerical modelling to unravel the tectonic emplacement of ophiolites in this region.All these data allow the reconstruction of a large obducted ophiolite nappe,thrusted for>100 km and up to 250 km on the Anatolian-Armenian block.The ophiolite petrology shows three distinct magmatic series,highlighted by new isotopic and trace element data:(1)The main Early Jurassic Tholeiites(ophiolite s.s.)bear LILEenriched,subduction-modified,MORB chemical composition.Geology and petrology of the Tholeiite series substantiates a slow-spreading oceanic environment in a time spanning from the Late Triassic to the Middle-Late Jurassic.Serpentinites,gabbros and plagiogranites were exhumed by normal faults,and covered by radiolarites,while minor volumes of pillow-lava flows infilled the rift grabens.Tendency towards a subduction-modified geochemical signature suggests emplacement in a marginal basin above a subduction zone.(2)Late Early Cretaceous alkaline lavas conformably emplaced on top of the ophiolite.They have an OIB affinity.These lavas are featured by large pillow lavas interbedded a carbonate matrix.They show evidence for a large-scale OIB plume activity,which occurred prior to ophiolite obduction.(3)Early-Late Cretaceous calc-alkaline lavas and dykes.These magmatic rocks are found on top of the obducted nappe,above the post-obduction erosion level.This series shows similar Sr-Nd isotopic features as the Alkaline series,though having a clear supra-subduction affinity.They are thus interpreted to be the remelting product of a mantle previously contaminated by the OIB plume.Correlation of data from the Lesser Caucasus to western Anatolia shows a progression from back-arc to arc and fore-arc,which highlight a dissymmetry in the obducted oceanic lithosphere from East to West.The metamorphic P-T-t paths of the obduction sole lithologies define a southward propagation of the ophiolite:(1)P-T-t data from the northern Sevan-Akera suture zone(Armenia)highlight the presence and exhumation of eclogites(1.85±0.02 GPa and 590±5℃)and blueschists below the ophiolite,which are dated at ca.94 Ma by Ar-Ar on phengite.(2)Neighbouring Amasia(Armenia)garnet amphibolites indicate metamorphic peak conditions of 0.65±0.05 GPa and 600±20 C with a U-Pb on rutile age of 90.2±5.2 Ma and Ar-Ar on amphibole and phengite ages of 90.8±3.0 Ma and 90.8±1.2 Ma,respectively.These data are consistent with palaeontological dating of sediment deposits directly under(Cenomanian,i.e.>93.9 Ma)or sealing(Coniacian-Santonian,i.e.,≤89.8 Ma),the obduction.(3)At Hinis(NE Turkey)PT-t conditions on amphibolites(0.66±0.06 GPa and 660±20℃,with a U-Pb titanite age of80.0±3.2 Ma)agree with previous P-T-t data on granulites,and highlight a rapid exhumation below a top-to-the-North detachment sealed by the Early Maastrichtian unconformity(ca.70.6 Ma).Amphibolites are cross-cut by monzonites dated by U-Pb on titanite at 78.3±3.7 Ma.We propose that the HT-MP metamorphism was coeval with the monzonites,about 10 Ma after the obduction,and was triggered by the onset of subduction South of the Anatolides and by reactivation or acceleration of the subduction below the Pontides-Eurasian margin.Numerical modelling accounts for the obduction of an"old"~80 Myr oceanic lithosphere due to a significant heating of oceanic lithosphere through mantle upwelling,which increased the oceanic lithosphere buoyancy.The long-distance transport of a currently thin section of ophiolites(<1 km)onto the Anatolian continental margin is ascribed to a combination of northward mantle extensional thinning of the obducted oceanic lithosphere by the Hinis detachment at ca.80 Ma,and southward gravitational propagation of the ophiolite nappe onto its foreland basin.     

Obduction-type granites within the NE Jiangxi Ophiolite: Implications for the final amalgamation between the Yangtze and Cathaysia Blocks

Leucogranitic lenses are found within the Xiwan ophiolitic mélange in northeastern Jiangxi Province, South China. The leucogranites occur exclusively within the serpentinized peridotite unit of the ophiolite suite. SHRIMP U–Pb zircon dating results indicate that these granites were formed at 880 ± 19 Ma, and were overprinted by an Indosinian tectono-thermal event at ~ 230 Ma. The leucogranites are peraluminous (A/CNK = 1.0–1.24), characterized by high Al₂O₃ (14–18.33%) and Na₂O (6.5–10%) and clearly low εNd(T) values of 0.8 to − 3.9 compared with the other rock units of the ophiolite suite. On the basis of their REE characters, the leucogranites can be divided into three groups. Group I leucogranites show the most fractionated LREE-enrichment patterns (with La_N/Yb_N and La_N/Sm_N ratios of 30.1–75.0 and 2.3–3.9, respectively). Group II leucogranites have moderately fractionated LREE-enrichment patterns (with La_N/Yb_N and La_N/Sm_N ratios of 13.1–26.5 and 0.8–1.9, respectively). Group III leucogranites are characterized by obviously low total REE contents and flat REE patterns with significant positive Eu anomalies, probably due to small degrees of partial melting. All these leucogranites were likely formed by partial melting of sedimentary rocks from a marginal basin at the Yangtze side of the orogen, beneath a major thrust fault during the obduction of the ophiolite onto the continental crust. They are broadly similar to obduction-related granites within ophiolites identified in many places worldwide. Identification of the ca. 880 Ma obduction-type granites in the NE Jiangxi ophiolite provides a petrological constraint on the timing of the ophiolite obduction onto the continental crust. In combination with the termination of the Shuangxiwu arc magmatism at ca. 890 Ma, we interpret that the close of the Neoproterozoic back-arc basin and the termination of the continental amalgamation between the Yangtze and Cathaysia Blocks occurred at ca. 880 Ma.