Continental Collision Zone Research Articles

The evolving continents: understanding processes of continental growth – introduction

We have organized and edited this Special Publication of the Geological Society of London to honour the career of Brian F. Windley, who has been hugely influential in helping to achieve our current understanding of the evolution of the continental crust, and who has inspired many students and scientists to pursue studies on the evolution of the continents. Brian has studied processes of continental formation and evolution on most continents and of all ages, and has educated and inspired two generations of geologists to undertake careers in studies of continental evolution. The contributions in this volume represent only a small percentage of studies that Brian has influenced, yet the scope and significance of these papers are clear, and stand as a testimony to Brian’s contributions to understanding processes of continental evolution, growth, and stabilization. The volume is organized into six sections: oceanic and island arc systems and continental growth; tectonics of accretionary orogens and continental growth; growth and stabilization of continental crust: collisions and intraplate processes; Precambrian tectonics and the birth of continents; active tectonics and geomorphology of continental collision and growth zones. The first section, oceanic and island arc systems and continental growth, begins with a paper by Stern, who summarizes the current state of knowledge about intra-oceanic arc systems from petrological, geophysical, and tectonic viewpoints and emphasizes that these systems have been the most important sites of juvenile continental crust formation for as long as plate tectonics has operated (the time of the start of plate tectonics is a matter of debate between some geologists). Stern describes the main components and zonation of intra-oceanic arc systems, including the trench, forearc, volcanic–magmatic arc, and back-arc, typically forming a system about 200 km wide, and strongly influenced by hydrous melting process in the underlying mantle. He then describes differences between the various stages of intra-oceanic arc systems, including juvenile arc lithosphere preserved in many forearcs, to mature arc systems where magmatism is concentrated along the magmatic–volcanic front. Mature intra-oceanic arc systems are typically extensional with volcanism and sea-floor spreading developing in the back-arc, and also show a transition in mantle types from serpentinized harzburgite beneath forearc sections, pyroxene-rich low-Vp mantle beneath the magmatic front, and lherzolite– harzburgite mantle beneath back-arc basins. In the second paper in this section, Xiao et al. describe the major differences between the types of arc systems found in the circum-Pacific region, including Mariana-, Japan-, Cordillera-, and Alaska-type arcs, and compare these systems with accreted terranes in accretionary orogens. They show how arcs are complex systems that can be different along strike (such as in the Alaska-type systems), and change with time. They suggest that some unresolved issues in accretionary and collisional orogens may be related to geologists not appreciating some of the complexities in modern arc systems, and use examples from the Altaids and other systems to demonstrate their points. Part 2 of the book, tectonics of accretionary orogens and continental growth, highlights a common thread of Brian Windley’s multi-year efforts of trying to work out the framework and evolution of accretionary orogens. His studies followed through early studies on the Tien Shan, Altai, and Central Asian Orogenic Belt in Mongolia, the Solonker suture in China, the Qilian Shan, the western Kun Lun Mountains and the Bei Shan in China. His most recent work in Asia has all been done in collaboration with Xiao Wenjiao, and in the Mona Complex in Anglesey and Lleyn in Wales with Shigenori Maruyama. Appropriately, the first paper in this section is by Maruyama et al. They recognize three types of strongly imbricated oceanic plate stratigraphy in the Neoproterozoic accretionary orogen on the island of Anglesey and the Lleyn Peninsula.

About this title ‐ The Evolving Continents: Understanding Processes of Continental Growth

This volume honours the career of Brian F. Windley, who has been hugely influential in helping to achieve our current understanding of the evolution of the continental crust, and who has inspired many students and scientists to pursue studies on the evolution of the continents. Brian has studied processes of continental formation and evolution on most continents and of all ages, and has educated and inspired two generations of geologists to undertake careers in studies of continental evolution. The volume is organized into six sections, including: oceanic and island arc systems and continental growth; tectonics of accretionary orogens and continental growth; growth and stabilization of continental crust; collisions and intraplate processes; Precambrian tectonics and the birth of continents; and active tectonics and geomorphology of continental collision and growth zones.

Seismic images of crustal variations beneath the East Anatolian Plateau (Turkey) from teleseismic receiver functions

Abstract We used teleseismic P-wave receiver functions recorded by the Eastern Turkey Seismic Experiment to determine the crustal structure across an active continent–continent collision zone. Moho depth and V p / V s variations in the region are mapped by incorporating crustal multiples and later two-dimsional (2-D) seismic profiles are produced using a common conversion point technique with our crustal V p / V s estimates. Moho depths do not correlate with surface topography and reveal a relatively thin crust consistent with the high plateau being supported by hot asthenosphere near the base of the crust. Under the Arabian plate, the crust is thinnest ( c . 35 km) and exhibits high V p / V s (≥1.8) associated with mafic compositions. In the east, the crust gradually becomes thicker towards the north and exceeds 45 km in the northeastern side whereas in the west, the crust thickens sharply near the Bitlis suture and displays pronounced Moho topography within the Anatolian plate that suggests the presence of multiple fragments. V p / V s variations show an anomalously high V p / V s corridor (≥1.85) along the North Anatolian Fault and near the youngest volcanic units ( c . 3 Ma) and support the presence of partial melt. This corridor is spatially limited from both north and south by low V p / V s regions implying a change in crustal composition. Near the Bitlis suture, a layered V p / V s model points to the source of low V p / V s in the lower crust that may be rich in quartz. Furthermore, the seismic profiles indicate a prominent low velocity zone in the lower crust across a large area beneath the plateau that may act as a decoupling zone between the crust and upper mantle.

Landscape development of the Himalayan–Tibetan orogen: a review

Abstract The Himalayan–Tibetan orogen provides one of the best natural laboratories in which to examine the nature and dynamics of landscape development within continent–continent collision zones. Many new tectonic–climatic–geomorphological theories and models have emerged and/or have been greatly influenced as a consequence of the study of the region and the quest to understand its geomorphological development. These include models of the interactions between tectonics, climate and surfice processes, notably, the influence of climate on surface uplift by denudational unloading; the limiting of topography by glaciation (the glacial buzz-saw model); localized uplift at syntaxes by enhanced fluvial and glacial erosion that, in turn, weaken the lithosphere, enhancing surface uplift and exhumation (the tectonic aneurysm model); climate-driven out-of-sequence thrusting and crustal channel flow; glacial damming leading to differential erosion and uplift; paraglaciation; and the influence of extreme events such as earthquakes, landslides, and floods as major formative processes. The development of new technologies, including satellite remote sensing and global positioning systems, and analytical methods such as numerical dating is now allowing these theories and models to be tested and will inevitably lead to new paradigms.

Carbonatites in collisional settings and pseudo-carbonatites of the Early Paleozoic Ol’khon collisional system

Carbonatites associated with syenites and subalkalic mafic rocks (lamprophyres) occur in the Himalayan continental collision zone, and this suggests their possible existence in other Phanerozoic collisional settings. The Early Paleozoic Ol’khon collisional system in Western Cisbaikalia is considered one of the possible occurrences. Subalkalic gabbroids as well as peculiar carbonate (brucite marbles) and calc-silicate rocks were found here, within the Tazheran massif of alkalic and nepheline syenites. Alkalic syenites, nepheline syenites, and calciphyres were dated at 471 Ma, 451–464 Ma, and 466 Ma, respectively, and their ages correspond to the main collisional events in the system. A geochemical description of igneous and carbonate rocks in the massif is provided. Close mapping showed unusual syenite and brucite marble combinations and the frequent vein or pipe-like form of carbonates and calciphyres corresponding to their magmatic intrusion. But carbonatite nature of the marble mentioned above does not fit their typical crustal geochemical features. Not ruling out the possibility of total change of geochemical signatures of mantle carbonatite in the collisional medium, we offer two other possible explanations for these facts: (1) melting of carbonate masses by syenite and mafic magmas, followed by carbonate melt intrusion into the upper crust, and (2) protrusion of carbonates into syenites and gabbroids at the late stages of the contact action of a silicate magma. In this case the above-mentioned carbonate structural features result from late recrystallization, whose mechanism is yet to be explained.

Morphotectonics and strain partitioning at the Iberia–Africa plate boundary from multibeam and seismic reflection data

The Gulf of Cadiz, off SW Iberia and the NW Moroccan margin, straddles the cryptic plate boundary between Africa and Eurasia, a region where the orogenic Alpine compressive deformation in the continental collision zone passes laterally to the west to strike-slip deformation. A set of new multibeam bathymetry, multi-channel and single-channel seismic data presented here image the main morphological features of tectonic origin of a significant part of the Gulf of Cadiz from the continental shelf to the abyssal plain. These morphotectonic features are shown to result from the reactivation of deeply rooted faults that changed their kinematics from the early Mesozoic rifting, through the Late Cretaceous–Paleogene collision, to the Pliocene–Quaternary thrusting and wrenching. The old faults control deep incised, more than 100 km long canyons and valleys. Several effects of neotectonics on deep water seabed are shown. These include: i) the complex morphology caused by wrenching on the 230 km long WNW–ESE faults that produced en echelon folds on the sediments; ii) the formation of up to 5 km wide crescent shaped scours at roughly 4 km water depth by reactivation of thrusts; iii) 10 km long creep folds on the continental slope; and iv) the formation of landslides on active fault escarpments. The present day deformation is partitioned on NE–SW thrusts and WNW–ESE to W–E strike-slip faults and is propagating northwards on N–S trending thrusts along the West Iberia Margin from 35.5°N to 38°N, which should be considered for seismic hazard.

Fluid regime in continental subduction zones: petrological insights from ultrahigh-pressure metamorphic rocks

Abstract: Intensive studies of ultrahigh-pressure (UHP) metamorphic rocks in continental collision zones have provided petrological insights into the fluid regime during subduction and exhumation. Recent studies in the Dabie–Sulu orogen of China have made important contributions to understanding fluid action in such systems, hence these rocks are the main focus of this review. Sources of metamorphic fluid can be deduced from study of hydrous minerals, structural hydroxyl and molecular water in nominally anhydrous minerals. Fluid and solid inclusions of UHP metamorphic minerals are also a record of fluid action in deep subduction zones. The presence or absence of metamorphic fluid during subduction and exhumation dictates zircon growth and recrystallization, making available materials for U–Pb dating. Isotopic dating demonstrates that after peak UHP metamorphism, retrograde fluid with deuteric origin became active. The decomposition of hydrous minerals and the exsolution of molecular water and structural hydroxyl during exhumation can provide sufficient amounts of aqueous fluid for quartz veining and amphibolite-facies retrogression within UHP slabs. Aqueous fluid released by the exsolution of molecular water and structural hydroxyl from UHP nominally anhydrous minerals is characterized by low salinity and light hydrogen and oxygen isotope compositions. Therefore, one part of the UHP slab is a source of aqueous fluid, whereas another part is a sink. In both cases, the fluid action during subduction and exhumation is episodic rather than continuous. Supplementary material: A description of the Dabie–Sulu UHP metamorphic rocks, analytical methods, LA-ICPMS zircon U–Pb isotope data and rare earth element compositions are available at http://www.geolsoc.org.uk/SUP18355 .

Caracterização geoquímica de rochas metamáficas e metaultramáficas da Faixa Pajeú-Paraíba, Província Borborema - NE Brasil

Concordant lenses of high pressure metaultramafic and metamafic rocks are positioned in Paleoproterozoic migmatites and orthogneisses of the Pajeú-Paraíba belt, Borborema Province, northeast Brazil. The geochemical signature of theses eclogites facies rocks points to a tholeiitic basaltic composition related to an island arc evolution. They are probably associated with a enriched mantle source underlying a subduction zone. This mantelic source was melted and further underwent in-situ differentiation, producing as high-MgO as high-FeO-TiO2 rocks. Cumulatic pyroxenites could be residual product of this process. The high-FeO-TiO2 group seems to represent more differentiated member submits to crustal contamination event. Reported positive and negative gravimetric anomalies in these metaultramafic and metamafic rocks occurrence area suggest the presence of possible NE-SW to E-W continental collision zone in the Borborema Province.

Xiongcun, Tibet: A telescoped system of veinlet-disseminated Cu (Au) mineralization and late vein-style Au (Ag)-polymetallic mineralization in a continental collision zone

The Xiongcun Cu–Au deposit occurs in the western part of the Gangdese porphyry and skarn Cu–Au mineralization belt in the Himalayan–Tibetan Orogen and was formed in an intracontinental orogenic environment related to the India–Asia collision. Sericite 40Ar/ 39Ar dating of the Xiongcun deposit indicates that ore formation occurred around 38.11 ± 0.90 Ma, much earlier than the main ore-forming ages (20 to 14 Ma) of the Gangdese Cu–Au metallogenic belt. The potassium feldspar K–Ar age of the sulfide-bearing pegmatoid dyke in the Xiongcun district is 47.62 ± 0.70 Ma. These ages indicate that the mineralization in Xiongcun is related to relaxation (ca. 52 to 40 Ma) and subsequent uplift (40 to 38 Ma) of the orogen. In the Xiongcun deposit, mineralization, hosted by Jurassic pyroclastic rocks and controlled by NW-striking faults, can be divided into two styles, i.e. veinlet-disseminated Cu ± Au mineralization and vein-style Au ± Ag-polymetallic mineralization. The former is associated with local remnant K-silicate alteration, extensive phyllic alteration and peripheral widespread propylitization. The latter, overprinting the former, is associated with strong silicification, chloritization and kaolinization. The early veinlet-disseminated Cu ± Au mineralization is similar to porphyry-type mineralization and has a mineral assemblage of quartz–sericite–pyrite–chalcopyrite–bornite–magnetite–pyrrhotite–sphalerite ± rutile ± anhydrite. The late vein-style Au ± Ag-polymetallic mineralization, characterized by a mineral assemblage of quartz–pyrite–pyrrhotite–sphalerite (rich in Fe)–chalcopyrite–galena–native gold–electrum–chlorite–kaolinite ± barite ± calcite ± siderite ± hematite, belongs to a sulfide-rich low-sulfidation-type epithermal system. The H, O and S isotopic compositions and high salinities of fluid inclusions indicate a magmatic contribution and increasing contents of meteoric water in the mineralized system from early to late stage. The wide range of homogenization temperatures (380 to 120 °C), homogenization pressures (45.92 to 1.94 bar) and salinities (36.61 to 1.23 wt.% NaCl equiv.) suggest rapid uplift and are consistent with the geological setting. Because of uplift, the hydrothermal system of Xiongcun evolved from a porphyry system to an epithermal system, resulting in an immature porphyry system and the telescoping of late vein-style Au ± Ag-polymetallic mineralization onto the early veinlet-disseminated Cu ± Au mineralization.

An Experimental Determination of the Effect of Bulk Composition on Phase Relationships in Metasediments at Near-solidus Conditions

Sediments of terrigenous origin represent a conspicuouspart of the cover of the oceanic and continental crust(Taylor & McLennan, 1985; Wedepohl, 1991). Pelitesand greywackes are involved in a variety of processes,from deep mass-transfer in subduction environments(e.g. Chopin, 1984; Smith, 1984; Sobolev & Shatsky, 1990;Reinecke, 1991) responsible for dehydration and fluidrelease during dehydration (Schmidt etal.,2004), to partialmelting in continental collision zones, which is at theorigin of intracrustal differentiation (Clemens & Vielzeuf,1987). The evolution of metapelites and metagreywackescan be reconstructed if the dependence of phase relation-ships on pressure, temperature and bulk composition iswell known and, under H

The fate of subducted Upper Continental Crust: An experimental study

The occurrence of ultra-high pressure metamorphic (UHPM) index minerals within rocks from continental collision zones indicates that continental rocks may descend into the Earth's upper mantle and leave specific geochemical signatures during the continental collision. It has long been suggested that continental crust is buoyant than that of the mantle rocks, and therefore might be gravitationally trapped during subduction. In order to understand the fate and exhumation of subducted continental crust, we conducted experimental studies up to 24 GPa and 1800 °C on natural UHPM gneisses in bulk compositions corresponding to average upper continental crust and terrigenous rocks, using piston-cylinder and multi-anvil apparatus. We found that the upper continental crust is no longer buoyant with respect to the surrounding mantle when it has been transported to a depth of 250 km (~ 8–9 GPa), which defines the “depth of no return” for the subducted upper continental crust. This in turn is consistent with the depth limit for most exposed UHPM rocks. Our experimental results suggest that the transformed subducted upper continental crust with a jadeite stishovitite lithology has the potential to sink into the transition zone. However, this density relationship is reversed at the base of the transition zone, leaving subducted upper continental crust being trapped at the bottom of the upper mantle, due to the presences of perovskite and magnesiowustite in the lower mantle.

The Himalayan Mianning–Dechang REE belt associated with carbonatite–alkaline complexes, eastern Indo-Asian collision zone, SW China

The Himalayan Mianning–Dechang REE belt, western Sichuan, SW China, is approximately 270 km long and 15 km wide, and contains total reserves of > 3 Mt of LREE, including one giant (Maoniuping), one large (Dalucao) and a number of small-medium REE deposits (Muluozhai, Lizhuang) and occurrences. The belt is tectonically located within a continental collision zone, i.e., the eastern Indo-Asian collision zone. REE mineralization is associated with Himalayan carbonatite–alkaline complexes, which consist of carbonatitic sills or dykes and associated alkaline syenite stocks. Available dating data define a Himalayan metallogenic epoch (40–10 Ma). The magmatic–hydrothermal REE systems were controlled by Cenozoic large-scale strike-slip faulting and the resultant tensional fissure zones, developed in a transform phase from a transpressional to a transtensional regime. Alteration is characterized by fenitization, which formed fenite halos enveloping the REE orebodies. Associated REE mineralization occurs as complex vein systems consisting of various veinlets, stringer and stockwork zones, and breccia-pipe systems. The REE orebodies show various shapes from plate-like, lenticular to pipe-like in different districts. Ore types are dominated by pegmatitic, carbonatitic, breccia, and stringer (stockwork), and disseminated ores, mainly composed of barite + fluorite + aegirine-augite + calcite + bastnaesite assemblages. Bastnaesite precipitation (275–325 °C) appears in three assemblages: early clastics-bearing barite + arfvedsonite + calcite + bastnaesite; intermediate quartz + fluorite + barite + bastnaesite; and late fluorite + barite + bastnaesite assemblages. These were followed by low-temperature pyrite + galena + sphalerite assemblages. Fluid inclusion studies indicate that gangue quartz and fluorite hosts numerous melt/fluid inclusions with a large proportion of sulfate (i.e., BaSO 4, K 2SO 4, and CaSO 4), in addition to CaCO 3 and CaF 2, yielding abnormally high (up to 750 °C) but wide-ranging homogenization temperatures (750–125 °C). The similarity of Sr–Nd isotopic compositions for gangue minerals with those of the host carbonatite–syenite suggests that ore-forming fluids, especially the F −, HCO 3 2−, SO 4 2− components, were derived from carbonatite–syenite magmas. The O–D–C isotopic data from gangue calcite, fluorite and quartz indicate that the fluids were of orthomagmatic origin, but also involved mixing with an external fluid. On the basis of our synthesis, we propose a possible genetic model for REE mineralization, in which the hydrothermal system temporally underwent a complicated evolution from separation of high-T sulfate-bearing NaCl–KCl brine resulting in effective precipitation of REE-fluorocarbonate and sulfate, to subsequent mixing with low-temperature meteoric water precipitating minor sulfide assemblages. Spatially, this sequence of events generated a three-tier REE mineralized architecture, in which pipe-like breccia orebodies grade downwards from shallow to deep structural levels into large veinlet ore systems, and veinlet-disseminated orebodies.

Hot granulite nappes — Tectonic styles and thermal evolution of the Proterozoic granulite belts in East Africa

A section through the Neoproterozoic Mozambique Belt of Tanzania exposes western foreland (Archaean Tanzania Craton and Palaeoproterozoic Usagaran Belt), marginal (Western Granulites) and eastern, internal (Eastern Granulites) portions of the orogen. The assembly of granulite nappes at ca. 620 Ma displays westward emplacement along an eastward deepening basal decollement and forward propagation of thrusts, climbing from the deep crust to the surface. This goes along with eastward increase of syntectonic temperatures, derived from prevalent deformation mechanisms, and eastward decrease of the kinematic vorticity number. Distinctly different pressure - temperature paths with a branch of isothermal decompression (ITD) in Western Granulites and isobaric cooling (IBC) in Eastern Granulites reflect residence times of rocks within lower crustal levels. Western Granulites, exhumed rapidly at the orogen margin, display ITD and non-coaxial fabrics. Eastern Granulites in the internal orogen portions escaped from rapid exhumation and show IBC and co-axial flow fabrics. The vertical variation of structural elements, i.e. basement — cover relations within the Eastern Granulites, shows decoupling between lower and middle crust with horizontal west — east stretching in the basement and horizontal west — east shortening in the cover. A model of hot fold nappes [Beaumont, C., Nguyen, M.H., Jamieson, R.A., Ellis, S., 2006. Crustal flow modes in large hot orogens. In: Law, R.D., Searle, M.P., Godin, L., (eds). Channel Flow, Ductile Extrusion and Exhumation in Continental Collision Zones. Geological Society, London, Special Publications. vol. 268, 91–145] is adopted to explain flow diversity in the deep crust. The lower crust represented by Eastern Granulite basement flowed coaxially outwards (westward) in response to thickened crust and elevated gravitational forces, supported by a melt-weakened, viscous channel at the crustal base. Horizontal flow with rates faster than thermal equilibration gave rise to isobaric cooling. Simultaneously the mid crust (Eastern Granulite cover) was shortened when hot fold nappes moved along upward climbing thrust planes. Western Granulites preserved isothermal decompression through exhumation by thrusting and coeval erosion at the orogen front. Two different styles define the Neoproterozoic East African Orogen between northern Egypt and southern Mozambique. The Arabian Nubian Shield in the north is classified as small and cold orogen in which thin — skinned thrusting was associated with lateral extrusion. The Central Mozambique Belt in Tanzania/Southern Kenya is classified as large and hot orogen characterized by thick-skinned thrusting and assembly of large granulite nappes.

Coupled and decoupled regimes of continental collision: Numerical modeling

Useful geodynamic distinction of continental collision zones can be based on the degree of rheological coupling of colliding plates. Coupled active collision zones (which can be either retreating or advancing) are characterized by a thick crustal wedge and compressive stresses (i.e. Himalaya and Western Alps), while decoupled end-members (which are always retreating) are defined by a thin crustal wedge and bi-modal distribution of stresses (i.e., compressional in the foreland and extensional in the inner part of the orogen, Northern Apennines). In order to understand physical controls defining these different geodynamic regimes we conducted a 2D numerical study based on finite-differences and marker-in-cell techniques. In our experiments we systematically varied several major parameters responsible for the degree of rheological coupling between plates during collision such as convergence rate, crustal rheology and effective velocity of upward propagation of aqueous fluids and melts in the mantle wedge. Low convergence rates and fluids/melts propagation velocities favor continuous coupling and convergence between the plates. Coupled collision zones are characterized by continuous accretion of the weak upper continental crust resulting in the development of a thick and broad crustal wedge, by hot temperature in the inner parts of the orogen due to radiogenic heating of the thickened crust, by compressive orogenic stresses and appearance of a double seismogenic (brittle) layer involving upper crust and sub-Moho mantle. In contrast high convergence rates and fluid/melt percolation velocities produce efficient weakening of the mantle wedge and of the subduction channel triggering complete decoupling of two plates, mantle wedging into the crustal wedge and retreating style of collision. The evolution of fully decoupled collision zones are characterized by the disruption of the accretionary wedge, formation of an extensional basin in the inner part of the orogen and delamination of the weak portion of the continental crust that is first thrusted toward the foreland and, subsequently, dissected by extensional tectonics. Transition from coupled to decoupled regime occurs always at the early stages of continental collision indicating that insertion of rheologically weak crustal material in the subduction channel is critical for the subsequent evolution of the collision zone. We found good correlations of our numerical results with some of the major collisional orogens. In particular, the decoupled retreating collision regime reproduces what is observed in the Northern Apennines.

State and origin of present-day stress fields in sedimentary basins

The present-day stress field provides fundamental insight into the forces driving plate tectonics and intra-plate deformation. Furthermore, Knowledge of the present-day stress field is essential in petroleum, geothermal and mining geomechanics applications such as the stability of boreholes and tunnels, and improving production through natural and induced fractures. The World Stress Map (WSM) Project has, for over 20 years, compiled a public global database of present-day tectonic stress information to determine and understand the state of stress in the Earth?s lithosphere. The WSM database has revealed that plate-scale stress fields are controlled by forces exerted at plate boundaries (e.g. mid-ocean ridges, continental collision zones), commonly resulting in regional stress orientations sub-parallel to plate motion. However, the state and origin of present-day stress fields at smaller scales, such as within sedimentary basins, remains poorly understood in comparison. Detailed analysis of present-day stresses from within 70 sedimentary basins commonly reveals significant and complex variations in the present-day stress orientation, both across basins and within fields. For example, borehole breakouts in the North German Basin, Nile Delta and the Baram Delta province of northwest Borneo indicate broad regional rotations in the horizontal stress orientation. The present-day maximum horizontal stress orientations in the Gulf of Thailand are approximately north-south at the basin-scale (perpendicular to plate motion) and are perturbed locally to be sub-parallel to fault strike. The North Sea and Permian Basin of Texas display widely varying stress orientations between fields, with some neighbouring fields exhibiting perpendicular stress orientations. Basin- and field-scale stress fields result from the complex combination of numerous factors acting at different scales, including far-field forces (e.g. plate boundary forces), basin geometry (e.g. the shape of deltaic wedges), geological structures (e.g. diapirs, faults), mechanical contrasts (e.g. evaporites, overpressured shales, detachment zones), topography and deglaciation.

MORB mantle hosts the missing Eu (Sr, Nb, Ta and Ti) in the continental crust: New perspectives on crustal growth, crust–mantle differentiation and chemical structure of oceanic upper mantle

We have examined the high quality data of 306 mid-ocean ridge basalt (MORB) glass samples from the East Pacific Rise (EPR), near-EPR seamounts, Pacific Antarctic Ridge (PAR), near-PAR seamounts, Mid-Atlantic Ridge (MAR), and near-MAR seamounts. The data show a correlated variation between Eu/Eu⁎ and Sr/Sr⁎, and both decrease with decreasing MgO, pointing to the effect of plagioclase crystallization. The observation that samples with MgO>9.5 wt.% (before plagioclase on the liquidus) show Eu/Eu⁎>1 and Sr/Sr⁎>1 and that none of the major phases (i.e., olivine, orthopyroxene, clinopyroxene, spinel and garnet) in the sub-ridge mantle melting region can effectively fractionate Eu and Sr from otherwise similarly incompatible elements indicates that the depleted MORB mantle (DMM) possesses excess Sr and Eu, i.e., [Sr/Sr⁎]DMM>1 and [Eu/Eu⁎]DMM>1. Furthermore, the well-established observation that DNb≈DTh, DTa≈DU and DTi≈DSm during MORB mantle melting, yet primitive MORB melts all have [Nb/Th]PMMORB>1, [Ta/U]PMMORB>1 and [Ti/Sm]PMMORB>1 (where PM indicates primitive mantle normalized), also points to the presence of excess Nb, Ta and Ti in the DMM, i.e., [Nb/Th]PMDMM>1, [Ta/U]PMDMM>1 and [Ti/Sm]PMDMM>1. The excesses of Eu, Sr, Nb, Ta and Ti in the DMM complement the well-known deficiencies of these elements in the bulk continental crust (BCC). These new observations, which support the notion that the DMM and BCC are complementary in terms of the overall abundances of incompatible elements, offer new insights into the crust–mantle differentiation. These observations are best explained by partial melting of amphibolite of MORB protolith during continental collision, which produces andesitic melts with a remarkable compositional (major and trace element abundances as well as key elemental ratios) similarity to the BCC, as revealed by andesites in southern Tibet produced during the India–Asia continental collision. An average amphibolite of MORB protolith consists of ~66.4% amphibole, ~29.2% plagioclase and 4.4% ilmenite. In terms of simple modal melting models, the bulk distribution coefficient ratios D2Eu/(Sm+Gd)=1.21, D2Sr/(Pr+Nd)=1.04, DNb/Th=44, DTa/U=57, DTi/Sm=3.39 and DNb/Ta=1.30 readily explains the small but significant negative Eu and Sr anomalies, moderate negative Ti anomaly and huge negative Nb and Ta anomalies as well as the more sub-chondritic Nb/Ta ratio in the syncollisional andesitic melt that is characteristic of and contributes to the continental crust mass. These results support the hypothesis that continental collision zones are primary sites of net continental crust growth, whereas the standard “island arc” model has many more difficulties than certainties. That is, it is the continental collision (vs. "island arc magmatism" or "episodic super mantle avalanche events") that produces and preserves the juvenile crust, and hence maintains net continental growth. The data also allow us to establish the robust composition of depleted and most primitive (or “primary”) MORB melt with 13% MgO. This, together with the estimated positive Eu and Sr anomalies in the DMM, further permits estimation that the DMM may occupy the uppermost ~680 km of the convective mantle following the tradition that the DMM lies in the shallowest mantle. However, the tradition may be in error. The seismic low velocity zone (LVZ) may be compositionally stratified with small melt fractions concentrated towards the interface with the growing lithosphere because of buoyancy. Such small melt fractions, enriched in volatiles and incompatible elements, continue to metasomatize the growing lithosphere before it reaches the full thickness after ~70 Myrs. Hence, the oceanic mantle lithosphere is a huge enriched geochemical reservoir. On the other hand, deep portions of the LVZ, which are thus relatively depleted, become the primary source feeding the ridge because of ridge-suction-driven lateral material supply to form the crust and much of the lithosphere at and in the vicinity of the ridge.

Buruanga peninsula and Antique Range: Two contrasting terranes in Northwest Panay, Philippines featuring an arc–continent collision zone

AbstractAlternating chert–clastic sequences juxtaposed with limestone blocks, which are units typical of accretionary complexes, constitute the Buruanga peninsula. New lithostratigraphic units are proposed in this study: the Unidos Formation (Jurassic chert sequence), the Saboncogon Formation (Jurassic siliceous mudstone–terrigenous mudstone and quartz‐rich sandstone), the Gibon Formation (Jurassic(?) bedded pelagic limestone), the Libertad Metamorphics (Jurassic–Cretaceous slate, phyllite, and schist) and the Buruanga Formation (Pliocene–Pleistocene reefal limestone). The first three sedimentary sequences in the Buruanga peninsula show close affinity with the ocean plate stratigraphy of the North Palawan terrane in Busuanga Island: Lower–Middle Jurassic chert sequences overlain by Middle–Upper Jurassic clastics, juxtaposed with pelagic limestone. Moreover, the JR5–JR6 (Callovian to Oxfordian) siliceous mudstone of the Saboncogon Formation in the Buruanga peninsula correlates with the JR5–JR6 siliceous mudstone of the Guinlo Formation in the Middle Busuanga Belt. These findings suggest that the Buruanga peninsula may be part of the North Palawan terrane. The rocks of the Buruanga peninsula completely differ from the Middle Miocene basaltic to andesitic pyroclastic and lava flow deposits with reefal limestone and arkosic sandstone of the Antique Range. Thus, the previously suggested boundary between the Palawan microcontinental block and the Philippine Mobile Belt in the central Philippines, which is the suture zone between the Buruanga peninsula and the Antique Range, is confirmed. This boundary is similarly considered as the collision zone between them.

Tectonics of the Simplon massif and Lepontine gneiss dome: deformation structures due to collision between the underthrusting European plate and the Adriatic indenter

This study presents a review of published geological data, combined with original observations on the tectonics of the Simplon massif and the Lepontine gneiss dome in the Western Alps. New observations concern the geometry of the Oligocene Vanzone back fold, formed under amphibolite facies conditions, and of its root between Domodossola and Locarno, which is cut at an acute angle by the Miocene, epi- to anchizonal, dextral Centovalli strike-slip fault. The structures of the Simplon massif result from collision over 50 Ma between two plate boundaries with a different geometry: the underthrusted European plate and the Adriatic indenter. Detailed mapping and analysis of a complex structural interference pattern, combined with observations on the metamorphic grade of the superimposed structures and radiometric data, allow a kinematic model to be developed for this zone of oblique continental collision. The following main Alpine tectonic phases and structures may be distinguished: 1. NW-directed nappe emplacement, starting in the Early Eocene (~50 Ma); 2. W, SW and S-verging transverse folds; 3. transpressional movements on the dextral Simplon ductile shear zone since ~32 Ma; 4. formation of the Bergell – Vanzone backfolds and of the southern steep belt during the Oligocene, emplacement of the mantle derived 31–29 Ma Bergell and Biella granodiorites and porphyritic andesites as well as intrusions of 29–25 Ma crustal aplites and pegmatites; 5. formation of the dextral discrete Rhone-Simplon line and the Centovalli line during the Miocene, accompanied by the pull-apart development of the Lepontine gneiss dome – Dent Blanche (Valpelline) depression. It is suggested that movements of shortening in fan shaped NW, W and SW directions accompanied the more regular NW- to WNW-directed displacement of the Adriatic indenter during continental collision.

Tectonic significance of the geochemistry and petrology of ophiolites in southeast Anatolia, Turkey

The southeast Anatolian ophiolites form discontinuous linear belts of oceanic fragments immediately north of the Bitlis–Zagros suture, which marks a continental collision zone between the Arabian platform to the south and the Taurides to the north. The Late Cretaceous ophiolites in the southeast Anatolia are represented by the Kızıldağ in the Hatay area, the Göksun and the Berit to the north of Kahramanmaraş, İspendere in the Malatya area and Kömürhan–Guleman in the Elazığ area. The Göksun, Berit, İspendere, Kömürhan and Guleman ophiolites were attached to the base of the Tauride platform (i.e. the Malatya–Keban) along its northern active margin and intruded by the I-type calc-alkaline granitoids of Late Cretaceous age. In contrast, the Kızıldağ (Hatay) ophiolite was thrust over the Arabian passive margin in the south. The cumulate rocks in the ophiolites are represented by dunite, wehrlite, lherzolite, olivine clinopyroxenite, olivine gabbronorite, olivine gabbro, gabbronorite and gabbro. Highly magnesian olivines (Fo 88 to 74) and pyroxenes (Mg# 95 to 60) as well as highly calcic plagioclases (An 95 to 68) from the cumulate rocks differ from oceanic equivalents, which mainly formed from a MOR basaltic melt. The order of crystallization in mineral phases, whole rock and mineral chemistry data from the cumulates suggest that the primary magma is compositionaly similar to that observed in modern island arc tholeiitic sequences. The volcanic and subvolcanic rocks exhibit tholeiitic compositions. Chondrite normalized REE, N-MORB normalized multi-element patterns and tectonic discrimination diagrams suggest the existence of two main types of parental basic magmas in the crustal rocks of the SE Anatolian ophiolites. These are (i) IAT series that characterize the Kızıldağ, Göksun, İspendere, Kömürhan and Guleman ophiolites and (ii) Low-Ti boninitic series known only in the Kızıldağ ophiolite. The geochemistry of the crustal rocks suggests that they formed in a suprasubduction zone tectonic setting, including arc–forearc environments in the southern Neotethys. These well-preserved plutonic oceanic crustal remnants mainly exhibit an intact ophiolite pseudostratigraphy and are overlain by a volcanic-sedimentary unit made up of alternations of basic to acidic extrusive rocks, debris flows, volcaniclastic sandstones and pelagic limestones, interpreted as a tholeiitic ensimatic island arc assemblage built on the suprasubduction zone type crust. The metamorphism of some of the ophiolites (i.e. Berit) in the SE Anatolia might be related to later stages of intraoceanic subduction during the evolution of the southern Neotethys.

Active tectonics of an apparently aseismic region: distributed active strike-slip faulting in the Hangay Mountains of central Mongolia

SUMMARY We identify and describe a series of east–west left-lateral strike-slip faults (named the SonginoMargats, the Hag Nuur, the Uliastay and the South Hangay fault systems) in the Hangay mountains of central Mongolia: an area that has little in the way of recorded seismicity and which is often considered as a rigid block within the India–Eurasia collision zone. The strikeslip faults of central Mongolia constitute a previously unrecognized hazard in this part of Mongolia. Each of the strike-slip faults show indications of late Quaternary activity in the form of aligned sequences of sag-ponds and pressure-ridges developed in alluvial deposits. Total bed-rock displacements of ∼3 km are measured on both the Songino-Margats and South Hangay fault systems. Bed-rock displacements of 11 km are observed across the Hag Nuur fault. Cumulative offset across the Uliastay fault systems are unknown but are unlikely to be large. We have no quantitative constraint on the age of faulting in the Hangay. The ≤20 km of cumulative slip on the Hangay faults might, at least in part, be inherited from earlier tectonic movements. Our observations show that, despite the almost complete absence of instrumentally recorded seismicity in the Hangay, this part of Mongolia is cut through by numerous distributed strike-slip faults that accommodate regional left-lateral shear between Siberia and China. Central Mongolia is thus an important component of the India–Eurasia collision that would be overlooked in models of the active tectonics based on the distribution of seismicity. We suggest that active faults such as those identified in the Hangay of Mongolia might exist in other, apparently aseismic, regions within continental collision zones.