Collisional Plate Boundary Research Articles

Regional seismic stratigraphic framework and depositional history of the post-Valanginian passive margin sequences in the Northern Carnarvon Basin, North West Shelf of Australia

The post-rift succession of the Northern Carnarvon Basin (north-western Australia) has commonly been interpreted as a relatively simple and uniform sequence that records a transition from siliciclastic to carbonate sedimentation deposited on a passive margin. However, given the significant vertical and lateral variation in seismic facies visible on seismic data, this interpretation likely oversimplifies the depositional history of the margin. Regional composite seismic lines that cross most of the basin, integrated with lithological and biostratigraphic information from exploration wells, provide the context for a better understanding of the depositional processes and environments that characterize the post-rift continental margin succession. We show that the sedimentary sequences deposited above the Valanginian breakup unconformity contain a wide variety of seismic facies that can be linked to a number of different marine depositional environments, with the greatest lateral variation occurring in the Turonian – Rupelian and in the Tortonian – Present. The former interval consists of three dominant seismic facies, namely polygonally faulted, incised, and parallel bedded, which we interpret to indicate a lateral transition from an environment primarily dominated by fine-grained pelagic/hemipelagic deposition to one dominated by energetic bottom currents that created both depositional and erosional features, such as contourite drifts and associated moats. The latter interval is expressed by sigmoidal and continuous reflections which pass laterally into more chaotic reflection packages, which we interpret as clinoforms and mass-transport complexes (MTCs). The development of bottom currents may be connected to changes in circulation associated with the opening of oceans adjacent to the northwest margin of Australia, while the MTCs may indicate increased regional seismic activity and slope instability resulting from the development of collisional plate boundaries. Definition of these sequences highlights the significant changes that have occurred in the sedimentary processes that operated on the margin, and their potential link to its tectonic evolution.

Sunda subduction drives ongoing India-Asia convergence

The Himalaya and the Tibetan plateau, the highest mountain range on Earth, have been growing continuously for the last 55 Myr since India collided with Eurasia. The forces driving this protracted mountain building process are still not fully understood. Although subduction zones are considered the main driving force for plate tectonics, mantle flow and plate boundary migration, their role in driving the Indian indentation and the northward movement of the collisional plate boundary is yet to be tested with geodynamic models. Here, we use four-dimensional geodynamic physical models to show that active subduction of the Indo-Australian plate along the Sunda subduction zone is probably the main driver of the India-Asia convergence, Indian indentation, and the consequent growth of the Himalaya-Tibet mountains, and also the present-day eastward crustal displacement of southeast Asia. Our experiments show that at least 880 km of northward indentation of India would not have ensued in the absence of the lateral subduction zones. Our experiments with lateral subduction zones show that subduction of the Indian continental lithosphere is maximum close to the eastern and western syntaxes, which ranges between 450 and 500 km. Based on our model results we propose that the protracted growth of collisional mountains on Earth, like the Himalaya, is highly dependent on nearby active subduction zones.

Machine Learning Applied to a Modern‐Pleistocene Petrographic Data Set: The Global Prediction of Sand Modal Composition (GloPrSM) Model

AbstractPetrography has long been used as a tool to decipher the sedimentary provenance of sand and sandstone based on relative proportions of detrital grain types. However, there is not yet a predictive framework that constrains the relationship between sand grain composition and the factors that influence it, including bedrock lithology, elevation, slope, basin area, precipitation, and temperature. We used a globally extensive modal point‐count data set of 3,522 Pleistocene to modern sand samples to train an inline series of random forest meta‐estimators to predict relative abundances of quartz (Q), feldspar (F), and lithic (L) grain types and eight subcompositions. The trained random forest models were fit to a global data set of fluvial watersheds to produce the global prediction of sand modal composition (GloPrSM) model which predicts Q‐F‐L of fluvially transported sediment. GloPrSM predicts quartz enrichment in low latitudes (35°N to 35°S), feldspar enrichment near plutonic and metamorphic crystalline terranes in middle to high latitudes, and lithic enrichment near modern and ancient collisional plate boundaries and flood basalts. Our analyses reveal that slope, temperature, and the relative abundance of exposed metamorphic and felsic to intermediate plutonic rocks are the most important predictors of Q‐F‐L composition. In addition, temperatures higher than 15°C and large drainage areas appear to promote quartz enrichment, while steeply sloping environments favor lithic enrichment. The GloPrSM model represents the first global‐scale estimate of fluvial sand compositional modes and illustrates that the spatial distribution of Q‐F‐L at Earth's surface can be predicted from the first‐order factors that generate sediment.

New Uppermost Mantle Pn Velocity and Anisotropy Model of the Eastern Caribbean

AbstractUsing the Pn tomography method and new ISC‐EHB data, we obtained a new Pn velocity and anisotropy model of the uppermost mantle in eastern Caribbean. The consistency between the Pn fast directions and SKS splitting fast wave directions indicates that the anisotropic structure of the uppermost mantle in the area of the collisional plate boundary is the main cause of shear‐wave splitting. The results show that the seismic velocity of uppermost mantle beneath the Aves Ridge is not anomalous, implying that this area is no longer affected by the upwelling of mantle material. We observe high Pn velocities in the uppermost mantle beneath eastern Caribbean Sea, which are higher than in the deeper parts (100–200 km in depth) of the upper mantle; comparing these results with similar findings in the Japan Sea at the northwestern Pacific subduction zone, we believe the uppermost mantle beneath the back‐arc region is not or little affected by hot material from the deeper mantle.

Evolution of the Alpine orogenic belts in the Western Mediterranean region as resolved by the kinematics of the Europe-Africa diffuse plate boundary

The West European collisional Alpine belts are the result of the inversion, initiated in the middle Cretaceous, of the complex western Neotethys and the Atlantic continental rift domains and closure of remnants of Tethys between the North Africa and European cratons. While the kinematics of Africa relative to Europe is well understood, the kinematics of microplates such as Iberia and Adria within the diffuse collisional plate boundary is still a matter of debate. We review geological and stratigraphic constraints in the peri-Iberia fold-thrust belts and basins to define the deformation history and crustal segmentation of the West European realm. These data are then implemented with other constraints from recently published kinematic and paleogeographic reconstructions to propose a new regional tectonic and kinematic model for Western Europe from the late Permian to recent times. Our model suggests that the pre-collisional extension between Europe and Africa plates was distributed and oblique, hence building discontinuous rift segments between the southern Alpine Tethys and the Central Atlantic. They were characterised by variably extended crust and narrow oceanic domains segmented across transfer structures and micro-continental blocks. The main tectonic structures inherited from the late Variscan orogeny localized deformation associated with rifting and orogenic belts. We show that continental blocks, including the Ebro-Sardinia-Corsica block, have been key in accommodating strike-slip, extension, and contraction in both Iberia and Adria. The definition of a new Ebro-Sardinia-Corsica block allows refining the tectonic relationships between Iberia, Europe and Adria in the Alps. By the Paleogene, the convergence of Africa closed the spatially distributed oceanic domains, except for the Ionian basin. From this time onwards, collision spread over the different continental blocks from Africa to Europe. The area was eventually affected by the West European Rift, in the late Eocene, which may have controlled the opening of the West Mediterranean. The low convergence associated with the collisional evolution of Western Europe permits to resolve the control of the inherited crustal architecture on the distribution of strain in the collision zone, that is otherwise lost in more mature collisional domain such as the Himalaya.

A Rapid Response Network to Record Aftershocks of the 2015 M 7.8 Gorkha Earthquake in Nepal

Abstract The Himalaya has experienced large damaging earthquakes over the past few centuries, most recently the damaging 25 April 2015 M 7.8 Gorkha earthquake in Nepal. Because of the continued earthquake risk presented by the continental collisional plate boundary at the Main Himalayan thrust and the high population densities in the region, collecting and processing data related to recent large earthquakes in this region is critically important for improving our understanding of the regional tectonics and earthquake hazard. Following the 2015 Gorkha earthquake, we deployed a National Science Foundation-funded rapid-response aftershock network known as the Nepal Array Measuring Aftershock Seismicity Trailing Earthquake network across the rupture area for 11 months beginning 7 weeks after the mainshock. The network consisted of 41 broadband and short-period seismometers, and 14 strong-motion sensors at 46 sites across eastern and central Nepal. The network spanned a region approximately 210 km along strike by 110 km across strike with a station spacing of 20–25 km. In this article, we report lessons learned from this deployment as well as details of the publicly accessible dataset including data recovery, data quality, and potential for future research.

From submarine continental accretion to arc-continent orogenic evolution: The thermal record in southern Taiwan

Constraining the timing of initial collision is critical for understanding how long-term plate convergence is accommodated at collisional plate boundaries. Reevaluation of the age of collision from orogenic thermal evolution requires distinguishing the onset of submarine continental accretion from earlier oceanic subduction and subsequent subaerial orogenic evolution. We present new thermochronological constraints from the first age-elevation relationship transect in Taiwan and zircon and apatite fission-track ages from sediments in the western foreland. Our data reveal the onset of cooling from at least 7.1 ± 1.3 Ma, at a minimum rate of 21 °C/m.y., in the submarine sedimentary wedge followed by a marked acceleration of subaerial exhumation after ca. 3.2 ± 0.6 Ma at an average rate of 1.7 km/m.y. Our data reflect the effect of margin architecture as Taiwan evolved from submarine accretion of the distal extremely thinned continental margin to crustal thickening of the proximal margin and orogenic development.

Tethyan collision forces and the stress field of the Eurasian Plate

Resistive forces along convergent plate boundaries have a major impact on surface deformation, most visibly at collisional plate boundaries. Although quantification of these forces is key to understanding the evolution and present state of mountain belts, they remain highly uncertain due to the complexity of plate boundary structures and rheologies. In previous studies of the Eurasian Plate, we have analysed the balance of plate boundary forces, tractions resulting from lithosphere–mantle coupling, and intraplate variations in topography and density structure. This yielded a range of acceptable force distributions. In this study, we investigate to which extent the observed present-day stress field provides further constraints on the distribution of forces. We address the dynamics of the Eurasian Plate as a whole. This enables us to base our analysis on mechanical equilibrium of a tectonic plate and to evaluate all forces as part of an internally consistent set of forces driving and deforming Eurasia. We incorporate tractions from convective mantle flow modelling in a lithospheric model in which edge and lithospheric body forces are modelled explicitly and compute resulting stresses in a homogeneous elastic thin shell. Intraplate stress observations used are from the World Stress Map project. Eurasia's stress field turns out to be particularly sensitive to the distribution of collision forces on the plate's southern margin and, to a much lesser extent, to lithospheric density structure and tractions from mantle flow. Stress observations require collision forces on the India–Eurasia boundary of 7.0–10.5 TN m−1 and on the Arabia–Eurasia boundary of 1.3–2.7 TN m−1. Implication of mechanical equilibrium of the plate is that forces on the contacts with the African and Australian plates amount to 1.0–2.5 and 0–1.3 TN m−1, respectively. We use our results to assess the validity of the classical view that the mean elevation of an orogenic plateau can be taken as a measure of the magnitude of the compressive (in this case: collision-related) forces involved. For both the Tibetan and the Iranian plateaus, two plateaus with significantly different average elevations, we find that the horizontal force derived from the excess gravitational potential energy (collapse force) is in balance with the collision force.

A restraining bend in a young collisional margin: Mount Mundo Perdido, East Timor

Mount Mundo Perdido, a 1750 m-high, steep-sided massif situated in the Viqueque district of East Timor, comprises approximately 30 km2 of complexly juxtaposed rocks deriving from both sides of the collisional plate boundary between the Australian Plate and the Banda Arc. Lithologies include Triassic–Jurassic interior-rift basin deposits, Cretaceous–Oligocene pelagites of Australian passive margin origin, neritic Oligocene–Miocene limestones and volcanics of Asiatic affinity, and Pliocene–Pleistocene synorogenic deposits. Detailed structural mapping shows Mount Mundo Perdido to be dominated by recent, high angle, oblique-slip and strike-slip faults that have been active into the Pleistocene and control the present-day topography. The fault architecture and stratigraphic distribution in the study area are comparable to pop-up structures developed at restraining bends, in this case within an east–west oriented zone of sinistral strike-slip. Our observations, supported by comparisons to scaled sandbox models and to similar pop-up structures developed in strike-slip systems elsewhere in the world, suggest that plate boundary-parallel strike-slip deformation is an integral part of the kinematics within the collisional zone between the Australian and Eurasian/Pacific plates in the Timor region.

Lithospheric‐folding‐based understanding on the origin of the back‐arc basaltic magmatism beneath Jeju volcanic island

Compression near collisional plate boundaries causes lithospheric folding which results in the decrease of the pressure beneath the ridge of the fold while the pressure beneath the trough increases. The decompression beneath the lithosphere is likely to bring on basaltic magmatism along and below the ridge. We investigate the subsurface structure beneath Jeju (Cheju) volcanic island and its vicinity, and propose an alternative hypothesis that the basaltic magma beneath the island could be caused by (episodic) lithospheric folding. Unlike the prevailing hypothesis of the intraplate basaltic magmatism that requires extending lithosphere, our hypothesis can explain how the basaltic magma could be generated at the back‐arc regions without the extension.

Australian paleo-stress fields and tectonic reactivation over the past 100 Ma

Even though a multitude of observations suggest time-dependent regional tectonic reactivation of the Australian Plate, its large-scale intraplate stress field evolution remains largely unexplored. This arises because intraplate paleo-stress models are difficult to construct, and that observations of tectonic reactivation are often hard to date. However, because the Australian plate has undergone significant changes in plate boundary types and geometries since the Cretaceous, we argue that even simple models can provide some insights into the nature and timing of crustal reactivation through time. We present Australian intraplate stress models for key times from the Early Cretaceous to the present, and link them to geological observations for evaluating time-dependent fault reactivation. We focus on the effect time-dependent geometries of mid-ocean ridges, subduction zones and collisional plate boundaries around Australia have on basin evolution and fault reactivation through time by reconstructing tectonic plates, restoring plate boundary configurations, and modelling the effect of selected time-dependent plate driving forces on the intraplate stress field of a rheologically heterogeneous plate. We compare mapped fault reactivation histories with paleo-stress models via time-dependent fault slip tendency analysis employing Coulomb-Navier criteria to determine the likelihood of strain in a body of rock being accommodated by sliding along pre-existing planes of weakness. This allows us to reconstruct the dominant regional deformation regime (reverse, normal or strike-slip) through time. Our models illustrate how the complex interplay between juxtaposed weak and strong geological plate elements and changes in far-field plate boundary forces have caused intraplate orogenesis and/or tectonic reactivation in basins and fold belts throughout Australia.

Fluids in high- to ultrahigh-temperature metamorphism along collisional sutures: Record from fluid inclusions

Petrographic studies and microthermometric investigations on fluid inclusions associated with high- to ultrahigh-temperature metamorphic rocks in three major Precambrian suture zones on the globe demonstrate the dominant occurrence of CO 2-rich fluids. These rocks form part of hot orogens developed along collisional plate boundaries. The sapphirine-quartz-bearing Mg–Al-rich rock from the Palghat-Cauvery Suture Zone, a trace of the Cambrian Gondwana suture zone in southern India, preserves evidence for a prograde high-pressure event and subsequent peak ultrahigh-temperature metamorphism along a clockwise path, and contains abundant CO 2-rich inclusions in corundum, garnet, and sapphirine. Most of the fluid inclusions are either primary or secondary and preserve low-density CO 2-rich fluids (0.569–0.807 g/cm 3). Similar low-density CO 2-rich fluid inclusions (0.853–0.953 g/cm 3) are also present in pelitic granulites from the Limpopo Complex of southern Africa, a Neoarchean granulite-facies orogen formed by continent–continent collision. In contrast, the garnet–orthopyroxene granulite from Tonagh Island in the Neoarchean Napier Complex in East Antarctica contains very high-density primary (1.095–1.129 g/cm 3) and secondary (0.960–1.179 g/cm 3) carbonic inclusions in garnet and quartz. The calculated isochores for the fluid inclusions from the Palghat-Cauvery Suture Zone and the Limpopo Complex yield significantly lower-pressure estimates than those predicted from peak metamorphic conditions. We interpret this as a result of significant density decrease due to rapid decompression along a clockwise P– T trajectory. In contrast, the estimated isochores for primary inclusions in garnet–orthopyroxene granulites from the Napier Complex are consistent with the peak P– T conditions estimated from mineral phase equilibria for the Tonagh Island rocks, suggesting that most of the fluid inclusions in these rocks did not undergo any marked effect of volume change and density decrease. The contrasting fluid densities among the localities investigated in this study are probably related to the nature of the P– T trajectory; the Tonagh Island rocks had a near-isochoric exhumation history whereas the metamorphic orogens in the other two sutures witnessed rapid decompression. Our results suggest that whereas the composition of the syn-metamorphic fluids are preserved in all cases, density reversal occurs within inclusions as a function of the tectonic history and exhumation style.

Three-dimensional mantle lithosphere deformation at collisional plate boundaries: A subduction scissor across the South Island of New Zealand

The continental plate collision across the South Island of New Zealand is highly oblique (dextral) and bounded by oppositely verging ocean plate subduction zones. As such, the region can be considered as a type of ‘subduction scissor’. Within this tectonic context, we use three-dimensional computational geodynamic models to consider how convergent mantle lithosphere can be modified by scissor and strike–slip effects. Bounding subduction at both ends of the continental collision causes flow of the descending mantle lithosphere in the direction along strike of the model plate boundary, with thinning in the centre and thickening towards the subduction zones that bifurcates the continental mantle lithosphere root. With dipping bounding subduction, the mantle lithosphere root takes on a more complex morphology that folds over from one subduction polarity to the other, but remains as a continuous feature as it folds under the collision zone. In the absence of bounding subduction, the plate convergence causes a linear (along strike) mantle lithosphere root to develop. A rapid strike–slip motion between the converging plates transfers material in the plate boundary-parallel direction and tends to blur out features that develop in this direction—such as descending viscous instabilities. The along-strike variations in the morphology of the mantle lithosphere root that develop in the models—viz., thickening of the root towards the subduction edges, thinning in the centre—are consistent with recent, albeit poorly constrained, geophysical interpretations of the large-scale lithospheric structure of the South Island. We speculate that this reflects the nature of the evolution of the South Island collision as a limited continental segment of the plate boundary that it is dominated and guided by adjacent well-developed/developing ocean plate subduction.

Storm frequency and magnitude in response to Holocene climate variability, Lake Tutira, North-Eastern New Zealand

A mid to late Holocene record of storm events is preserved in the sediments of Lake Tutira, located on the eastern North Island of New Zealand. Previous studies of a 6 m sediment core established a storm chronology for the last 2250 years (Eden, D.N., Page, M.J., 1998. Palaeoclimatic implications of a storm erosion record from late Holocene lake sediments, North Island, New Zealand. Palaeogeogr. Palaeoclimatol. Palaeoecol. 139, 37–58). Here we extend the record, using a 27 m sediment core, to lake formation ∼ 7200 cal. yr ago. Outside the last ∼ 500 yr of human influence, the core records landscape response to natural climate variability and to tectonism/volcanism associated with the active collisional plate boundary setting. Based on sedimentological and biogeochemical analyses, 1400 layers of dominantly terrigenous, storm-derived sediment have been identified. A storm event chronology, supported by twelve tephra and three 14C ages, indicates that storm magnitude and frequency has varied over the last 7200 yr. Twenty-five periods with an increased frequency of large storms occur, typically of decadal to centennial duration. Periods of major storm activity occur at about 500–700, 1100–1250, 1850–2100, 2850–3200, 3600–4000, 4300–4500, 4700–4900, 5700–5900, and 6850–6900 cal. yr BP. Several other local and regional climate proxies record conditions that are consistent with the timing and periodicity of major storms in the Tutira record. A period centred on 2000 yr ago has the highest incidence of storms, with a recurrence of 1 storm/2.9 yr. This is identified as a time of major environmental change by several other records. By comparison, average storm frequency throughout the record is ∼ 1 storm/5 yr, and while this is consistent with modern El Niño-Southern Oscillation (ENSO) periodicity, the record shows no increase in frequency around 4000 years at the time of ENSO intensification. This likely reflects New Zealand's location between tropical and polar latitudes, where climate shifts are driven by the interplay between ENSO, Interdecadal Pacific Oscillation (IPO), and the Southern Annular Mode (SAM). The Lake Tutira record provides support for the growing evidence that climate variability during the Holocene has involved periods of rapid change. Correlation of this detailed storm event and sediment flux record with other geomorphic and sedimentary records in the nearby Waipaoa Source-to-Sink sedimentary system, improves understanding of the timing and rates of climatically-induced landscape change, and its downstream effects.

Pore water profiles and authigenic mineralization in shallow marine sediments above the methane-charged system on Umitaka Spur, Japan Sea

Umitaka Spur, situated on an unusual collisional plate boundary along the eastern margin of the Japan Sea, hosts gas seeps, pock-marks, collapse structures, and gas hydrates. Piston cores were recovered from this ridge to understand carbon cycling, pore fluid gradients and authigenic mineralization above a methane-charged system. We present the chemistry of fluids and solids from three cores adjacent to seep locations. High fluxes of CH4 and alkalinity transport carbon from a deep zone of methanogenesis toward the seafloor. Methane, however, reacts with SO42- across a shallow sulfate–methane transition (SMT), which generates additional alkalinity and HS−. A fraction of these CH4 oxidation products form authigenic carbonate and pyrite. These minerals are not readily apparent from visual inspection of split cores, because they exist as micritic coatings on microfossils or as framboidal pyrite. They are, however, readily observed in chemical analyses as peaks of “labile” Ca, Sr, Ba or S in sediment at or near the SMT. Carbon inputs and outputs nicely balance across the SMT in all three cores if one considers four relevant fluxes: loss of alkalinity to the seafloor, addition of methane from below, addition of alkalinity from below, and carbonate precipitation. Importantly, in all cores, the magnitude of the fluxes decreases in this order. Although some carbon rising from depth forms authigenic carbonate, most (>80%) escapes to the ocean as alkalinity. Nonetheless, authigenic fronts in sediment on Umitaka Spur are a significant reservoir of inorganic carbon. Given calculated pore fluid fluxes for Ca and Sr, the fronts require tens of thousands of years to form, suggesting that the present state and loss of carbon represent long-lived processes.

Glacial/interglacial control of terrigenous and biogenic fluxes in the deep ocean off a high input, collisional margin: A 139 kyr-record from New Zealand

Sediment input to the deep ocean, off the New Zealand collisional plate boundary, is large and variable as revealed by a 139 kyr-old record from the 34.92 m-long core, MD97 2121. Mass accumulation rates (MARs) were derived for terrigenous and biogenic (carbonate and silica) sedimentary components, with temporal control from a stable isotope age model verified by tephra and radiocarbon ages. Terrigenous MARs changed in phase with glacio-eustatic fluctuations of sea level. Highest rates (≥ 30 g/cm 2/kyr) coincided with the late regressive-lowstand-early transgressive phase of marine isotope stage (MIS) 2 when rivers discharged at or near the shelf edge. Lesser, but still high terrigenous rates (20–30 g/cm 2/kyr) characterized peak warm phases of early MIS 5 and 1, when a strengthened Subtropical Inflow probably introduced sediment from fluvial and seabed sources to the north of MD97 2121. However, during prolonged highstands the flux declined as more sediment was retained on a tectonically subsiding shelf with a shore-parallel current. Paradoxically, the lowest MARs (10–5 g/cm 2/kyr) occurred in MIS 4; either sea level was too high to allow much sediment to escape off-shelf, or the fluvial input was modest, or both these factors. Despite the terrigenous dominance, biogenic MARs are high, and are 2–4 times larger than SW Pacific Ocean rates. MARs were lowest during MIS 4 and 3, possibly due to reduced marine production under a lowered input of fluvial micronutrients. In contrast, MARs increased irregularly though glacial maxima to peak at 5–6 g/cm 2/kyr carbonate and 2–2.5 g/cm 2/kyr silica in early-mid MIS 5 and 1. Such rates reflected the interaction of macronutrient-rich subantarctic waters, sourced from the south, with local, micronutrient-rich subtropical waters. Production also responded to warmer temperatures, elevated nutrient runoff and a more stratified surface ocean. Later in MIS 5 and 1, fluxes reduced as less productive subtropical waters prevailed. That biogenic and aeolian MAR profiles are out of phase, suggests Fe-fertilization by aerosolic dust was not a major influence.

Basement reactivation and inversion mechanisms in the Timor and Norwegian seas

The North East Atlantic Margin and Australian North West Shelf share similar tectonic evolutions. Both are passive margins of similar age and size, both experienced significant periods of reactivation, inversion and volcanism, and both host significant oil and gas resources. The main differences between these two margins lie in their modern tectonic settings and hydrocarbon resources. The North West Shelf occurs at a present-day collisional plate boundary, the dynamics of which strongly control the location of oil and gas reserves, whereas the North East Atlantic has not yet entered a collisional phase. The North West Shelf tends to be gas prone, in contrast to its European counterpart. Recent mapping of Neogene to Recent modification across the North West Shelf highlights the influence of inherited basement architecture, long-wavelength strain partitioning and continuous flexural deformation. Similarities in the structural evolution of these two margins, especially in terms of the role of the basement structure, episodic modification, the location and extent of Cenozoic inversions and the role of inversion in controlling hydrocarbon distribution, illustrate the importance of these issues in passive margin development. Lessons learned from the Australian example include that structural inheritance has an overwhelming control on later reactivation, and that even small amounts of shortening strain (1–2%) can lead to significant modifications of structural traps and petroleum habitats.

Reconstruction of the evolution of the Alpine-Himalayan orogen - an introduction

The Alpine-Himalayan belt is a young orogen suturing Gondwana-derived fragments with Eurasian continental crust. Its general structure, as a continuous belt of Mesozoic and Cenozoic deformation from Spain to Southeast Asia is described in the pioneering work of Argand (1924)(Figure 1). Further southeast, in the southwest Pacific, the orogenic belt is partly submerged but probably stretched as far as New Zealand (Figure 2). The advent of plate tectonics thirty-five years ago provided an explanation for the large-scale plate kinematics associated with Alpine-Himalayan orogeny. However, when it comes to details, plate tectonics has proved to be insufficient to reconstruct this complex system. In reality, the Alpine-Himalayan orogen has been subjected to numerous processes involving motions of smaller continental ribbons, retreating subduction systems, back-arc extension and accretion of magmatic arcs. In order to reconstruct the Alpine-Himalayan system, we need to analyse data derived from multidisciplinary sources and develop sets of spatio-temporal constraints. We can then test these constraints against a set of possible alternative reconstructions. Examples are the works of Dercourt et al. (1986; 1993; 2000) and Stampfli et al. (2001), which are extremely valuable to the reconstruction community. This volume includes ten contributions on the reconstruction of the tectonic evolution of the AlpineHimalayan orogen (see locations of study areas in Figure 3). The first paper by Martinez-Martinez & Azanon presents results of their study in the westernmost part of the Alpine-Himalayan orogen, the Gibraltar Arc. The authors provide evidence for two episodes of orthogonal extension in this region, and they discuss the significance of such events in terms of the regional kinematics and the evolution of the Alboran Sea. In the same region, Sanchez-Go mez et al. present structural and metamorphic analysis related to the emplacement of mantle peridotites within the Rif-Betic orogen. The authors argue that these peridotites were emplaced as a single slab during an Oligocene/Early Miocene suturing. Two contributions deal with the Pyrenean Mountains that formed a collisional plate boundary between Iberia and Europe until the Oligocene (Srivastava et al., 1990). Schellart presents results of a structural and sedimentological study of the western termination of the axial zone in the southern Pyrenees. These results provide the means for the construction of balanced cross sections and for crustal shortening calculations. Verge s et al. provide a comprehensive review of the tectonism in the Pyrenees since the Cretaceous. The authors analyse stratigraphical cross sections and lithospheric-scale transects, and discuss transitions between crustal shortening and extensional regimes. A plate reconstruction of western Tethys is presented by Stampfli et al., with an emphasis on orogeny in the western Alps. The latter is probably the most studied segment of the Alpine-Himalayan belt, but is still controversial as far as tectonic reconstruction is concerned.

Surface rupture and behavior of thrust faults probed in Taiwan

Taiwan's destructive Chi‐chi earthquake of September 21, 1999, was a dramatic expression of active tectonic processes at a complex collisional plate boundary. It resulted in more than 2,400 causalities and tens of billions of dollars in property loss. During the earthquake, an 80‐km stretch of the country's mountainous backbone moved upward and westward along the range‐bounding Chelungpu thrust fault (Figure la). A team of earthquake geologists from the United States, in collaboration with geoscientists from Academia Sinica, National Taiwan University and the Central Geological Survey of Taiwan, worked together to address questions concerning the recurrence of large‐magnitude earthquakes along reverse faults in Taiwan.

The Pamir‐Hindu Kush seismic zone as a strain marker for flow in the upper mantle

We introduce a new hypothesis to explain the unusual pattern of intermediate‐depth earthquakes in the Pamir‐Hindu Kush region of central Asia. In our model the seismicity of the zone is defined by a remnant piece of oceanic crust that became neutrally buoyant and now hangs in the mantle underneath the center of the active mountain belt. We interpret an abrupt cutoff in seismicity shallower than 70 km under the Pamir as evidence that the slab has become decoupled from the surface deformation, and we suggest that the remnant slab has acted as a strain marker for flow in the upper mantle. The contortion of the slab can be explained by superimposed vertical and lateral simple shear related to the western edge of the Indian Plate. This model requires relatively low strain rates of the order of 10−15 s−1, consistent with strain rates estimated from earthquake moment release in Benioff zones. In contrast, the double‐facing subduction zone model applied by a number of previous authors requires strain rates of the order of 10−13 s−1, which are much higher than inferred strain rates in subducting slabs. The results are important for understanding mantle dynamics, providing a unique way to measure strain in the upper mantle. The implied flow field also has significant implications for the kinematics of the entire collisional plate boundary in Tibet and the Tien Shan.