Wharton Basin Research Articles

The tectonic stress field evolution of India since the Oligocene

A multitude of observations suggest neotectonic deformation in and around India, but its causes and history are unknown. We use a 2 dimensional finite element model with heterogeneous elastic strengths in continental regions to model the regional stress field orientation and relative magnitudes since the Oligocene. The large-scale geological structure of India is embedded in our model by using published outlines of cratons, fold belts and basins, associated with estimates of their relative strengths, enabling the modelling of stress field deflections along interfaces between relatively strong and weak tectonic elements through time. At 33Ma a roughly NNW–SSE oriented band of relatively high maximum horizontal compressive stress (SHmax) straddled India's west coast, while India's east coast and the adjacent Wharton Basin were characterized by relatively low intraplate stresses. Between 20Ma and the present growing collisional boundary forces combined with maturing mid-ocean ridge flanks result in the establishment of an arcuate belt with anomalously high intraplate stress that stretches from India to the Wharton Basin, intersecting the continental shelf roughly orthogonally and crossing the 85° East and Ninetyeast ridges. This results in a compressive tectonic regime favouring folding and inversion northeast of the Godavari Graben on India's east coast, as observed in seismic reflection data, whereas no tectonic reactivation is observed on the continental margin further north, closer to the Mahanadi Graben, or further south. Our stress models account for these differences via spatial variations in modelled horizontal stress magnitudes and intersection angles between margin-paralleling pre-existing basement structures and the evolving Neogene stress field. The models further account for fracture zone strike-slip reactivation offshore Sumatra and lithospheric folding along India's west and southeast coast and can be used to estimate the onset of these deformation episodes to at least the Oligocene and Miocene, respectively.

The April 2012 Indian Ocean earthquakes: Seismotectonic context and implications for their mechanisms

The 11 April 2012 earthquakes (M-w 8.6 and M-w 8.2) were sourced within the Northern Wharton Basin in the northeastern part of the Indo-Australian diffuse plate boundary. This unusually active oceanic intraplate region has generated many large earthquakes in the past, most of which are believed to have occurred by strike-slip motion, triggered by the NW-SE oriented compressional stresses acting across the Indian and Australian plates. In the aftermath of the 2004 megathrust earthquake along the nearby Sunda Trench, increased seismicity in the Northern Wharton Basin is attributed to the stress transfer from the Sumatra-Andaman plate boundary. Models proposed for the April 2012 earthquakes differ somewhat in details but partly attribute their complex rupture to the reactivation of pre-existing structures. These structures include previously mapped N-S trending fracture zones within the Northern Wharton Basin and E-W lineations across the Ninetyeast Ridge. In this paper, we review the regional tectonics and past seismicity on the Indo-Australian Plate in order to understand the seismotectonic setting of the April 2012 Indian Ocean earthquakes. (c) 2014 Elsevier B.V. All rights reserved.

Revisiting the structure, age, and evolution of the Wharton Basin to better understand subduction under Indonesia

AbstractUnderstanding the subduction processes along the Sunda Trench requires detailed constraints on the subducting lithosphere. We build a detailed tectonic map of the Wharton Basin based on reinterpretation of satellite‐derived gravity anomalies and marine magnetic anomalies. The Wharton Basin is characterized by a fossil ridge, dated ~36.5 Ma, offset by N‐S fracture zones. Magnetic anomalies 18 to 34 (38–84 Ma) are identified on both flanks, although a large part of the basin has been subducted. We analyze the past plate kinematic evolution of the Wharton Basin by two‐plate (India‐Australia) and three‐plate (India‐Australia‐Antarctica) reconstructions. Despite the diffuse plate boundaries within the Indo‐Australian plate for the last 20 Ma, we obtain finite rotation parameters that we apply to reconstruct the subducted Wharton Basin and constrain the thickness, buoyancy, and rheology of the subducting plate. The lower subductability of younger lithosphere off Sumatra has important consequences on the morphology, with a shallower trench, forearc islands, and a significant inward deviation of the subduction system. This deviation decreases in the youngest area, where the Wharton fossil spreading center enters subduction: The discontinuous magmatic crust and serpentinized upper mantle, consequences of the slow spreading rates at which this area was formed, weaken the mechanical resistance to subduction and facilitate the restoration of the accretionary prism. Deeper effects include the possible creation of asthenospheric windows beneath the Andaman Sea, in relation to the long‐offset fracture zones, and east of 105°E, as a result of subduction of the spreading center.

Deep seismic reflection images of the Wharton Basin oceanic crust and uppermost mantle offshore Northern Sumatra: Relation with active and past deformation

AbstractWe present deep seismic reflection images along two profiles collected in 2006 in the Wharton Basin offshore Northern Sumatra. The main profile is located subparallel to the Sumatran trench at a distance of 32–66 km. Faulting of the entire sedimentary section (strike‐slip deformation sometimes accompanied by a dip‐slip component) is imaged over two fracture zones of the extinct Wharton Spreading Center that prior studies have shown to be reactivated as left‐lateral faults. The western fracture zone is associated with a wide region of strong basement topography, a difference in crustal thickness of ~1.5 km, and an age offset of 9 Ma. The epicenters of the 11 April 2012 Mw 8.6 great strike‐slip earthquake, its Mw 7.2 foreshock, and Mw 8.2 aftershock align along this major structure > 100 km south of the profile intersection. Our high‐quality long‐offset seismic reflection data also reveal bright dipping reflections extending down to a maximum of ~24 km into the oceanic mantle (~37 km below sea level). Apparent dips are mostly 25–35°, corresponding to 30–55° along either N‐S to NNE‐SSW or E‐W to WNW‐ESE directions, which encompass the directions of plate fabric and nodal planes of the Mw 8.6 event. We suggest that these enigmatic reflections arise from presently inactive dip‐slip fault planes reaching for the deepest ones to the base of the brittle layer. Possible origins include extension related to plate bending or an episode of now inactive thrust‐type deformation reactivating paleonormal faults, similar to that taking place in the Central Indian Basin.

Active faulting on the Ninetyeast Ridge and its relation to deformation of the Indo‐Australian plate

The ~4500 km long Ninetyeast Ridge (NER) in the northeastern Indian Ocean crosses a broad zone of deformation where the Indo‐Australian plate is fracturing into three smaller plates (India, Capricorn, Australia) separated by diffuse boundaries whose extents are poorly defined. New multichannel seismic reflection profiles image active faults along the entire length of the NER and show spatial changes in the style of deformation along the ridge. The northern NER (0°N–5°N) displays transpressional motion along WNW‐ESE faults. Observed fault patterns confirm strike‐slip motion at the western extent of the April 2012 Wharton Basin earthquake swarm. In the central NER (5°S–8°S), deformation on WNW‐ESE‐trending thrust faults implies nearly N‐S compression. An abrupt change in fault style occurs between 8° and 11°S, with modest, extension characterizing the southern NER (11°S–27°S). Although extension is dominant, narrow zones of faults with strike‐slip or compressional character also occur in the southern NER, suggesting a complex combination of fault motions. At all sites, active faulting is controlled by the reactivation of original, spreading‐center formed, normal faults, implying that deformation is opportunistic and focused along existing zones of weakness, even when original fault trend is oblique to the direction of relative plate motion. Observed faulting can be interpreted as India‐Australia deformation in the northern NER and Capricorn‐Australia deformation in the southern NER. The India‐Capricorn boundary is directly adjacent to the northern NER and this juxtaposition combined with a different style of faulting to the east of the NER imply that the ridge is a tectonic boundary.

Modeling the 2012 Wharton basin earthquakes off‐Sumatra: Complete lithospheric failure

AbstractA sequence of large strike‐slip earthquakes occurred west of Sunda Trench beneath the Wharton Basin. First reports indicate that the main shock was extremely complex, involving three to four subevents (Mw > 8) with a maze of aftershocks. We investigate slip models of the two largest earthquakes by joint inversion of regional and teleseismic waveform data. Using the Mw7.2 foreshock, we developed hybrid Green's Functions for the regional stations to approximate the mixture of oceanic and continental paths. The main shock fault geometry is defined based on the back projection results, point‐source mechanisms, aftershock distribution, and fine tune of grid searches. The fault system contains three faults, labeled F1 (89°/289° for dip/strike), F2 (74°/20°), and F3 (60°/310°). The inversion indicates that the main rupture consisted of a cascade of high‐stress drop asperities (up to 30 MPa), extending as deep as 50 km. The rupture propagated smoothly from one fault to the next (F1, F2, and F3 in sequence) with rupture velocities of 2.0–2.5 km/s. The whole process lasted about 200 s, and the major moment release (>70%) took place on the N‐S oriented F2. The Mw8.2 aftershock happened about 2 h later on a N‐S oriented fault with a relatively short duration (~60 s) and also ruptured as deep as 50 km. The slip distributions suggest that the earthquake sequence was part of a broad left‐lateral shear zone between the Australian and Indian plates and ruptured the whole lithosphere. These earthquakes apparently reactivated existing fracture zones and were probably triggered by unclamping of the great Sumatran earthquake of 2004.

Estimating the age of volcanism in Seamount Provinces of the Northeast Indian Ocean

The breakup of Australian landmass from Greater India, part of the late dispersal of Gondwana, started around 136 Ma. Concurrent with this breakup was the eruption of significant volumes of volcanics, on both the continental margins of Australia and India, including the North-West shelf, and in the ocean basin separating the two. Later submarine volcanism occurred within the Christmas Island Seamount Province (ChrISP), or which Christmas and Cocos (Keeling) Islands are two subareal examples. These volcanic events significantly impacted the thermal evolution of these margins, but the ultimate cause for this disparate volcanism, and the relationship between margin volcanics and later submarine volcanic events, remains unclear. The study tries to establish a relation between the volcanic activities and look for any evidence of these volcanic episodes. The study uses gravity, magnetic, and subsidence modelling to attempt to constrain the structure and ages of seamount volcanism within the Christmas Island Seamount Province, including the Wharton Basin and Argo Abyssal Plain. Gravity modelling helps to determine the crustal structure and constrains the depth of limestone cap, which further helps in calculating when the seamount was sub aerially exposed and approximates the time since it was last exposure. The relationship between the volcanism observed in the ChrISP (Late Cretaceous to Eocene) and that recorded earlier on the NW shelf continental margin, remains ambiguous, and will be better constrained with forthcoming geochemical analysis. But the results presented here point to a rich and complex history of volcanism within the Indo-Australian plate.

Stress triggering of the great Indian Ocean strike‐slip earthquakes in a diffuse plate boundary zone

On April 11, 2012, two great magnitude 8+ earthquakes occurred within a two‐hour period off the west coast of northern Sumatra, Indonesia, in the broadly distributed India‐Australia plate boundary zone. The magnitude 8.6 mainshock holds the distinction of being both the largest instrumentally recorded strike‐slip earthquake and the largest earthquake away from a recognized plate boundary fault. The mainshock involved sequential ruptures of multiple fault planes oriented nearly perpendicular to each other. The adjacent 2004 megathrust earthquake statically loaded the northern Wharton Basin oceanic lithosphere on both of the 2012 mainshock fault plane orientations, and greatly enhanced the rate of earthquake activity in the region for a year. Viscoelastic relaxation of the asthenosphere following the 2004 and 2005 megathrust earthquakes continued to positively stress the offshore region, correlating with the locations of later strike‐slip earthquakes, including two magnitude 7+ and the 2012 magnitude 8+ earthquakes.

Old fractures caused rare 8.6 magnitude earthquake

The 8.6 magnitude earthquake that occurred on 11 April 2012 100 kilometers off the coast of Sumatra was unusual in that it originated within the plate rather than at a plate boundary. In fact, it is the largest such earthquake in observed human history. The quake originated under the Wharton Basin in the Indian Ocean, where hundreds of kilometers of rock were under crushing tension, causing the plate to deform at its base. However, this deforming zone also absorbed tension as two plates, the Indian and Australian plates, rotated toward each other.

April 2012 intra-oceanic seismicity off Sumatra boosted by the Banda-Aceh megathrust

Large earthquakes nucleate at tectonic plate boundaries, and their occurrence within a plate's interior remains rare and poorly documented, especially offshore. The two large earthquakes that struck the northeastern Indian Ocean on 11 April 2012 are an exception: they are the largest strike-slip events reported in historical times and triggered large aftershocks worldwide. Yet they occurred within an intra-oceanic setting along the fossil fabric of the extinct Wharton basin, rather than on a discrete plate boundary. Here we show that the 11 April 2012 twin earthquakes are part of a continuing boost of the intraplate deformation between India and Australia that followed the Aceh 2004 and Nias 2005 megathrust earthquakes, subsequent to a stress transfer process recognized at other subduction zones. Using Coulomb stress change calculations, we show that the coseismic slips of the Aceh and Nias earthquakes can promote oceanic left-lateral strike-slip earthquakes on pre-existing meridian-aligned fault planes. We further show that persistent viscous relaxation in the asthenospheric mantle several years after the Aceh megathrust explains the time lag between the 2004 megathrust and the 2012 intraplate events. On a short timescale, the 2012 events provide new evidence for the interplay between megathrusts at the subduction interface and intraplate deformation offshore. On a longer geological timescale, the Australian plate, driven by slab-pull forces at the Sunda trench, is detaching from the Indian plate, which is subjected to resisting forces at the Himalayan front.

Constraining the Jurassic extent of Greater India: Tectonic evolution of the West Australian margin

Alternative reconstructions of the Jurassic northern extent of Greater India differ by up to several thousand kilometers. We present a new model that is constrained by revised seafloor spreading anomalies, fracture zones and crustal ages based on drillsites/dredges from all the abyssal plains along the West Australian margin and the Wharton Basin, where an unexpected sliver of Jurassic seafloor (153 Ma) has been found embedded in Cretaceous (95 My old) seafloor. Based on fracture zone trajectories, this NeoTethyan sliver must have originally formed along a western extension of the spreading center that formed the Argo Abyssal Plain, separating a western extension of West Argoland/West Burma from Greater India as a ribbon terrane. The NeoTethyan sliver, Zenith and Wallaby plateaus moved as part of Greater India until westward ridge jumps isolated them. Following another spreading reorganization, the Jurassic crust resumed migrating with Greater India until it was re‐attached to the Australian plate ∼95 Ma. The new Wharton Basin data and kinematic model place strong constraints on the disputed northern Jurassic extent of Greater India. Late Jurassic seafloor spreading must have reached south to the Cuvier Abyssal Plain on the West Australian margin, connected to a spreading ridge wrapping around northern Greater India, but this Jurassic crust is no longer preserved there, having been entirely transferred to the conjugate plate by ridge propagations. This discovery constrains the major portion of Greater India to have been located south of the large‐offset Wallaby‐Zenith Fracture Zone, excluding much larger previously proposed shapes of Greater India.

A possible cause of decreasing summer rainfall in northeast Australia

AbstractThe NCEP/NCAR reanalysis and precipitation data from the Australian Bureau of Meteorology are used to analyse variability in rainfall during the austral summer (December–March, DJFM) in northeast Australia (NEA). NEA rainfall shows a marked decrease over the past 50 years, mainly in the austral summer. Our analysis reveals that the summer rainfall decrease in NEA is generally an interdecadal phenomenon. The declining trend has an imprint in the tropical Australian summer monsoon (TASM). Not only does TASM have a phase‐to‐phase influence on NEA summer rainfall at the interannual scale, it is also closely linked with interdecadal variation in NEA summer rainfall. Thus, the decrease in NEA at the interdecadal scale could be attributed to corresponding variation in TASM. Moreover, the coupled linkage between TASM and NEA summer rainfall appears to be largely independent of El Niño–Southern Oscillation. One possible reason for the interdecadal weakening trend in TASM is a sustained interdecadal warming trend in sea surface temperature (SST) over the Wharton Basin (100°–130°E, 20°–5°S). When the Wharton Basin is in a cold state, anomalous westerlies occur in the lower troposphere in the TASM region, and cyclonic circulation anomalies and rising flows occur in the low and middle troposphere over NEA, which are associated with a strong TASM situation, consequently favouring enhanced rainfall over NEA; the opposite occurs in the case of a warm Wharton Basin. SST over the Wharton Basin has shown a continuous warming trend over the past 60 years, contributing to the weakening of TASM and, consequently, a decrease in NEA summer rainfall. Copyright © 2011 Royal Meteorological Society

Extremely thin crust in the Indian Ocean possibly resulting from Plume-Ridge Interaction

The thickness of the crust created at ocean spreading centres depends on the spreading rate and melt production in the mantle. It is ~5–8 km for a crust formed at slow and fast spreading centres and 2–4 km at ultra-slow spreading centres away from hotspots and mantle anomalies. The crust is generally thin at the fracture zones and thick beneath hotspots and large igneous provinces. Here we present results for the crust generated at the fast Wharton spreading centre 55–58 Ma ago using seismic reflection and refraction data. We find that the crust over a 200 km segment of the Wharton Basin is only 3.5–4.5 km thick, the thinnest crust ever observed in a fast spreading environment. A thin crust could be produced by the presence of depleted and/or cold mantle. Numerical simulations and recent laboratory experiments studying the impact of a hot plume under a lithosphere show that a curtain of weak cold downwellings of depleted mantle material is likely to develop around the edges of the hot plume pond. Because of a strongly temperature-dependent viscosity of lithospheric material, the hotter, therefore less viscous, bottom of the lithosphere can be mobilized by an impinging plume. If sampled by a spreading centre, the locally cold and depleted mantle should result in low production of melt. We suggest that the observed thin crust in the Wharton Basin is likely to have been formed by the interaction between the Kerguelen mantle plume and the Wharton spreading centre ~55 Ma ago.

Late Maastrichtian Volcanism in the Indian Ocean: Effects on Calcareous Nannofossils and Planktic Foraminifera

The biotic effects of volcanism have long been the unknown factors in creating biotic stress, and the contribution of the Deccan volcanism to the K–T mass extinction remains largely unknown. Detailed studies of the volcanic-rich sediments of Indian Ocean Ninetyeast Ridge Sites 216 and 217 and Wharton Basin Site 212 reveal that the biotic effects of late Maastrichtian volcanism on planktic foraminifera and calcareous nannofossils are locally as severe as those of the K–T mass extinction. The biotic expressions of these high stress environments are characterized by the Lilliput effect, which includes reduced diversity by eliminating most K-strategy species, and reduction in specimen size (dwarfing), frequently to less than half their normal adult size of both r-strategy and surviving K-strategy species. In planktic foraminifera, the most extreme biotic stress results are nearly monospecific assemblages dominated by the disaster opportunist Guembelitria, similar to the aftermath of the K–T mass extinction. The first stage of improving environmental conditions results in dominance of dwarfed low oxygen tolerant Heterohelix species and the presence of a few small r-strategy species ( Hedbergella, Globigerinelloides). Calcareous nannofossil assemblages show similar biotic stress signals with the dominance of Micula decussata, the disaster opportunist, and size reduction in the mean length of subordinate r-strategy species particularly in Arkhangelskiella cymbiformis and Watznaueria barnesiae. These impoverished and dwarfed late Maastrichtian assemblages appear to be the direct consequences of mantle plume volcanism and associated environmental changes, including high nutrient influx leading to eutrophic and mesotrophic waters, low oxygen in the water column and decreased watermass stratification.

Lower slope morphology of the Sumatra trench system

ABSTRACTAt convergent margins, the structure of the subducting oceanic plate is one of the key factors controlling the morphology of the upper plate. We use high‐resolution seafloor mapping and multichannel seismic reflection data along the accretionary Sumatra trench system to investigate the morphotectonic response of the upper plate to the subduction of lower plate fabric. Upper plate segmentation is reflected in varying modes of mass transfer. The deformation front in the southern Enggano segment is characterized by neotectonic formation of a broad and shallow fold‐and‐thrust belt consistent with the resumption of frontal sediment accretion in the wake of oceanic relief subduction. Conversely, surface erosion increasingly shapes the morphology of the lower slope and accretionary prism towards the north where significant oceanic relief is subducted. Subduction of the Investigator Fracture Zone and the fossil Wharton spreading centre in the Siberut segment exemplifies this. Such features also correlate with an irregularly trending deformation front suggesting active frontal erosion of the upper plate. Lower plate fabric extensively modulates upper plate morphology and the large‐scale morphotectonic segmentation of the Sumatra trench system is linked to the subduction of reactivated fracture zones and aseismic ridges of the Wharton Basin. In general, increasing intensity of mass‐wasting processes, from south to north, correlates with the extent of oversteepening of the lower slope (lower slope angle of 3.8° in the south compared with 7.6° in the north), probably in response to alternating phases of frontal accretion and sediment underthrusting. Accretionary mechanics thus pose a second‐order factor in shaping upper plate morphology near the trench.

Cenozoic tectonic and depth/age evolution of the Indonesian gateway and associated back-arc basins

We reconstruct the tectonic and paleo-age and -depth history of the Indonesian seaway and associated southeast Asian back-arc basins to create a comprehensive paleogeographic framework for the region during the Cenozoic. We combine published tectonic models with revised models of the evolution of back-arc basins based on available marine magnetic anomaly data and a recently published high-resolution (1 min) marine gravity anomaly grid which reveals many fine-scale tectonic features previously not resolved. Reconstructions of the now-subducted Ceno-Tethys ocean floor are integrated with a revised absolute plate motion reference frame for the Indian Ocean based on moving hotspots. All marginal seas north of Australia formed in a back-arc setting, with the Caroline and Celebes seas opening north of a northward dipping subduction zone, and the Solomon Sea south of a southward dipping subduction zone north of Australia. We suggest that the evolution of the two sub-basins of the Caroline Sea from 36 to 25 Ma followed different seafloor spreading patterns, with the western sub-basin being influenced by the Manus hotspot. Our model suggests that collision between the Melanesian and the Caroline arcs led to cessation of oceanic crustal production in the Caroline Basin shortly before 25 Ma. Combined geophysical and geological observations suggest that several major tectonic events occurred north of Australia at around 45 Ma, roughly at the time when Australia started moving northwards at fast rates similar to the present. We suggest that this event is related to a relocation of the subduction zone NW of Australia under the Philippine Sea plate due to a collision and subsequent accretion of old Pacific plate material to the northward subducting Australian plate. Our gridded oceanic paleo-depth maps capture the history of marginal seas north of Australia, and the stepwise destruction of the Ceno-Tethys ocean floor in the northern Wharton Basin and the west Pacific, providing boundary conditions for both paleoceanograpic and geodynamic models. A regional analysis of basement depth anomalies reveals that median negative anomalous depth of several back-arc basins is of the order of 650–800 m, with a range of 300–1100 m, accompanied by negative regional heatflow anomalies, suggesting that mantle-driven dynamic topography plays a larger role in modulating regional basement depth than previously suggested. Regional shear wave mantle tomography cross sections highlight that all marginal basins that exhibit large negative dynamic topography are underlain by massive buried slab material in the lower portion of the upper mantle and/or below the transition zone, supporting the notion that the observed paired negative dynamic topography and heatflow anomalies are due to basin formation above slab burial grounds.

Instantaneous deformation and kinematics of the India-Australia Plate

SUMMARY Active intraplate deformation of the India‐Australia Plate is now being captured by far-field global positioning system (GPS) measurements as well as measurements on a few islands located within the deforming zone itself. In this paper, we combine global and regional geodetic solutions with focal mechanisms of earthquakes to derive the present-day strain field of the India‐Australia Plate. We first compile an updated catalogue of 131 Indian intraplate earthquakes (M > 5) spanning the period between the two Asian mega earthquakes of Assam 1897 and Sumatra 2004. Using Haines and Holt’s numerical approach applied to a fully deformable India‐Australia Plate, we show that the use of GPS data only or earthquakes data only has severe drawbacks, related, respectively, to the small number of stations and the incompleteness of the earthquakes catalogue. The combined solution avoids underestimation of the strain inherent to the Kostrov summation of seismic moments and provides details that cannot be reached by pure GPS modelling. We further explore the role of heterogeneity of the India‐Australia Plate and find that the best model, in terms of geodetic vectors fit, relative distribution of strain, style and direction of principal strain from earthquakes, is obtained using the surface heat-flow as a proxy for rheological weakness of the oceanic lithosphere. The present-day deformation is distributed around the Afanasy Nikitin Chain in the Central Indian Basin (CIB)—where it is almost pure shortening—and within the Wharton Basin (WB) off Sumatra—where it is almost pure lateral strike-slip. The northern portion of NinetyEast ridge (NyR) appears as a major discontinuity for both strain and velocity. The new velocity field gives an India/Australia rotation pole located at 11.3 ◦ S, 72.8 ◦ E( −0.301 ◦ Myr −1 )o verlapping with previous solutions, with continental India moving eastward at rates ranging from 13 mm yr −1 (southern India) to 26 mm yr −1 (northern India) with respect to Australia. Taking into account the intraplate velocity field in the vicinity of the Sumatra trench, we obtain a convergence rate of 46 mm yr −1 towards N18 ◦ Ea tthe epicentre of the 2004 Aceh megaearthquake. The predicted instantaneous shortening in the CIB and WB and extension near Chagos-Laccadive are in good agreement with the finite deformation measured from plate reconstructions and seismic profiles, suggesting a continuum of deformation since the onset of intraplate deformation around 7.5‐8 Ma. Since no significant change in India convergence is detected at that time, we suggest that the intraplate deformation started with the trenchward acceleration of Australia detaching from India along a wide left-lateral oceanic shear band activating the NyR line of weakness as well as north‐south fracture zones east of it. The predicted total amount of left lateral finite strain along these faults is in the range 110‐140 km.

Nd and Pb isotope evolution of deep water masses in the eastern Indian Ocean during the past 33 Myr

New time series of Nd and Pb isotopes were generated from two hydrogenous ferromanganese crusts from the eastern Indian Ocean, which were dated by 10Be / 9Be profiles and Co chronometry. The goal was to gain information on the nature and timing of variations of the deep water radiogenic isotope composition as a function of the evolution of the Indonesian Island Arcs since the Early Miocene, changes in ocean circulation related to the different stages of the closing of the Indonesian seaway for deep and intermediate water mass exchange since the Mid-Miocene, and enhanced Himalayan weathering since the Late Oligocene. A crust from 4119 m water depth in the Wharton Basin adjacent to Sumatra and the Java Trench (DODO 232D) has recorded a small variability in Nd isotope, but pronounced variations in Pb isotope composition over the past 17 Myr. Patterns and absolute values of the Nd and Pb isotope time series closely resemble the record of previously published crust SS663 from the central Indian Ocean, located some 2500 km SW of crust DODO 232D in the northern central Indian Ocean. In accordance with the interpretation derived from crust SS663, the Pb isotope composition of deep water in the Wharton Basin has apparently been mainly controlled by products of Himalayan erosion and weathering and to a lesser extent by the evolution of the Indonesian Island Arcs. The location of a second crust (VA16 13KD-1) from 2100 m water depth on the Scott Plateau off NW Australia is beneath the present day outflow of Pacific-derived surface and thermocline water masses into the eastern Indian Ocean. The pattern of the Pb isotope time series of this crust and the observed changes in Pb isotope mixing relationships, which occurred at ∼ 11.5 and ∼ 3.5 Ma, reflect the combined influence of advection and mixing of water masses through the Indonesian Seaway and weathering of volcanic source rocks within the emerging Indonesian Island Arcs over the past 33 Myr. The Nd isotope time series shows a pattern similar to central and southwest Pacific crusts and is best explained by a mixture between Nd from water masses advected from the Indian and Southern Ocean and Nd released by weathering from the emerging Indonesian Island Arcs.

Paleostress field evolution of the Australian continent since the Eocene

Although the low‐order present stress field of most continents is fairly well established, information on paleostress fields is generally sparse. Knowledge of paleostresses is crucial for understanding brittle tectonic reactivation through time. The Indian‐Australian plate lends itself well to a reconstruction of paleostresses, as it has undergone enormous changes in plate‐driving forces through the Tertiary, and there is a rich record of fault reactivation from sedimentary basins. We reconstruct the plate boundary configuration and age‐area distribution of ocean crust around Australia through time to obtain estimates for ridge push, slab pull, and collisional forces acting on the Indian‐Australian plate since the Eocene. Other model parameters we explore are the effects of the Australian‐Antarctic discordance and the mechanical strength of the Australian continental margin. We apply these constraints to model the orientation of the maximum horizontal compressive stress (SHmax) regime for the present, early Miocene, and early Eocene using the commercial software ABAQUS™ along with the optimization software Nimrod/O. We use an elastic two‐dimensional plane stress finite element model with a resolution of ∼0.2° in both longitude and latitude. Realistic elastic parameters representing different rock types and geologic provinces for the Australian continent have been included to model the stress field of a heterogeneous plate. We show that spatially significant rotations of SHmax directions can be modeled as a consequence of perturbations of SHmax in areas of juxtaposed rigid and compliant rheologies. The absence of the collisional Papua New Guinea boundary in the Miocene and reduced ridge push force from the south result in stress directions considerably different from the present. Stress directions over the northern Australian continent in the early Miocene in particular show large disparity with present stress directions. Stress orientations for the Australian plate during the early Eocene are controlled predominantly by ridge push forces arising from spreading in the Wharton Basin in the Indian Ocean and vary substantially with stress directions in the early Miocene and the present because of the drastically different plate geometry and boundary configurations. Fault reactivation histories observed on the northwest shelf of Australia and in the Bass Strait region are consistent with modeled changes in stress directions through time.

Palynology of the Baratang Formation, Andaman-Nicobar Islands and the significance of reworked palynomorphs

Spores-pollen and dinoflagellate cysts data from five sections of the Baratang Formation (Baratang Island) are analysed. The record of stratigraphic potential forms indicates an Early to Late Eocene age for the recovered assemblages. The palynoassemblages from this Formation are also associated with the reworked palynofossils belonging to Permian, Triassic and Jurassic-Cretaceous ages. The reworked taxa dominate over the poorly represented Eocene palynomorphs and have Gondwanic affinity. A comparison of Tertiary palynoflora of Baratang with Assam and Myanmar demonstrate the presence of some palynotaxa in all the areas but common occurrence of significant taxa such as Retitrisyncolpites, Baculimonocolpites and Lanagiopollis (regularis) shows more close relationship between Andaman and Myanmar flora than Assam. The recycled palynomorphs of Gondwanic affinity have provided valuable clue to locate the source and direction of sediments. Various views relating to provenance of reworked palynomorphs have been analysed on the basis of present data which suggest that Chindwin Basin of Myanmar mainly supplied reworked palynomorphs containing sediments. The palynological, palaeocurrent and lithological evidences do not support the origin of reworked elements from Wharton Basin, Assam Basin or autochthonous (Triassic) sediments.