Michel Diament

Mentawai fault zone off Sumatra: A new key to the geodynamics of western Indonesia

The geodynamic evolution of the western part of the Sunda arc is controlled by the change from frontal subduction of the Indo-Australian plate along Java to oblique subduction along Sumatra. This obliquity gives rise to the Sumatra fault zone that links the accretionary zone of the Andaman Sea to the Sunda Strait. Previous studies have shown a decrease of displacement rate of the movement along the fault zone from north to south. Consequently, it has been proposed that the area between the subduction zone and the fault zone—i.e., the Sumatra sliver platelet—was deformed. An oceanographic cruise on the Indonesian ship R/V Baruna Jaya III was designed to study this area. Seismic reflection data reveal the existence of a major strike-slip fault, parallel to the Sumatra fault zone, that we called the Mentawai fault zone, located in the fore-arc area just east of the Mentawai Islands; it is at least 600 km long. Thus, the Sumatra sliver plate appears to be composed of several strips that move toward the northwest to accommodate the oblique subduction.

Direct evidence of active deformation in the eastern Indian oceanic plate

Conventional plate tectonics theory postulates that plates only deform on their boundaries. To the contrary, there is ample evidence of intraplate deformation in the equatorial Indian Ocean, west of the Ninetyeast aseismic ridge. Prior to this study, no direct evidence of deforma- tion east of the Ninetyeast Ridge was available. We present the results of a multipurpose geo- physical cruise showing that intraplate deformation also occurs in this area. Long, at least 1000 km, left-lateral north-south strike-slip faults are active and reactivate fossil fracture zones. This style of deformation is strikingly different from the east-west folds and reverse faults that affect the region west of the Ninetyeast Ridge. Contrasting processes of convergence at the northern plate boundaries can account for the two styles of deformation. West of the Ninetyeast Ridge there is a continent-continent collision, and east of the ridge oceanic lithosphere subducts along the Sumatra trench. The Ninetyeast aseismic ridge therefore appears to be a mechanical border separating two distinct deformed areas.

Deep structure of the Baikal rift zone revealed by joint inversion of gravity and seismology

[1] The question of plate boundary forces and deep versus shallow asthenospheric uplift has long been debated in intracontinental rift areas, particularly in the Baikal rift zone, Asia, which is colder than other continental rifts. As previous gravity and teleseismic studies support the dominance of opposing mechanisms in the Baikal rift, we reconsidered both data sets and jointly inverted them. This more effective approach brings insight into location of the perturbing bodies related to the extension in this region. Our new joint inversion method allows for inverting the velocity-density relationship with independent model parametrization. We obtain velocity and density models that consistently show (1) crustal heterogeneities that coincide with the main tectonic features at the surface, (2) a faster and denser cratonic mantle NW of Lake Baikal that we relate to the thermal contrast between old and depleted Archean (Siberian platform) and Paleozoic orogenic belt (Sayan-Baikal belt), (3) three-dimensional topographic variations of the crust-mantle boundary with well-located upwarpings, and (4) the lithosphere-asthenosphere boundary uplift up to 70 km depth with a NW dip. Our resulting velocity and density models support the idea of a combined influence of lithospheric extension and inherited lithospheric heterogeneities for the origin of the Baikal rift zone. INDEX TERMS: 1234 Geodesy and Gravity: Regional and global gravity anomalies and Earth structure; 7218 Seismology: Lithosphere and upper mantle; 8122 Citation: Tiberi, C., M. Diament, J. Deverchere, C. Petit-Mariani, V. Mikhailov, S. Tikhotsky, and U. Achauer, Deep structure of the Baikal rift zone revealed by joint inversion of gravity and seismology,

Characterization of geological boundaries using 1-D wavelet transform on gravity data: theory and application to the Himalayas

We investigate the use of the continuous wavelet transform for gravity inversion. The wavelet transform operator has recently been introduced in the domain of potential fields both as a filtering and a source-analysis tool. Here we develop an inverse scheme in the wavelet domain, designed to recover the geometric characteristics of density heterogeneities described by simple-shaped sources. The 1-D analyzing wavelet we use associates the upward continuation operator and linear combinations of derivatives of any order. In the gravity case, we first demonstrate how to localize causative sources using simple geometric constructions. Both the upper part of the source and the whole source can be studied when considering low or high altitudes, respectively. The homogeneity degree of the source is deduced without prior information and allows us to infer its shape. Introducing complex wavelets, we derive analytically the scaling behavior of the wavelet coefficients for the dyke and the step sources. The modulus term is used in an inversion procedure to recover the thickness of the source. The phase term provides its dip. This analysis is performed on gravity data we measured along a profile across the Himalayas in Nepal. Good agreement of our results with well-documented thrusting structures demonstrates the applicability of the method to real data. Also, deeper, less constrained structures are characterized.

Isostasy, equivalent elastic thickness, and inelastic rheology of continents and oceans

The equivalent elastic thickness (EET) is used to estimate lithospheric strength expressed in response to loading by topography and subsurface loads. The data on EET allow comparisons between different plates and detection of thermal events. In oceans, the EET corresponds to the mechanical “core” of the lithosphere, i.e., a geotherm (400–600 °C). In continents, the EET has no relation to any depth. This has led to doubts in applicability of a unique approach to the continents and oceans, and in the utility of estimates of EET for continents. Rheological data suggest that most rocks are inelastic in the long term (>0.1 m.y.). This requires interpretation of the EET in terms of real rheology. We propose an analytical model that gives rheological interpretation of both the oceanic and continental EET. It also allows estimates of the mechanical thickness of the lithosphere. The EET depends upon three parameters: geotherm age, crustal thickness, and flexural plate curvature. Any one of these values can be estimated if the others are known. Comparisons of model predictions with the observed EET suggest that most continental plates have a weak lower crust, allowing mechanical decoupling between the upper crust and the mantle lithosphere. Such decoupling leads to strong reduction in the EET and thus can be easily detected. Flow of rocks in the weak lower crust may have a significant influence on the temporal evolution of relief (mountain building, erosion). Differences in the mechanical behavior of oceans and continents can be explained by domination of different parameters: geotherm age has a major control in the oceans, whereas in the continents crustal thickness is equally important. Additional local variations of the EET result from weakening by flexural stresses.

Evidence for a seismogenic upper mantle and lower crust in the Baikal Rift

The high level seismicity of the Baikal rift zone and its spatial distribution in dense swarms and belts provide an opportunity to study the seismogenic behavior of a continental lithosphere submitted to extension in an early stage. Using data from a regional seismological network, the authors analyze a significantly large set of events from an earthquake swarm located east of the nearly aseismic northern Baikal lake. They find that at least 10% of the well-constrained events are located in the lower crust or the uppermost mantle. The fault plane solutions of earthquakes within the crust define a NW-SE extensional stress regime perpendicular to the rift axis. Results confirm the idea that zones of continental extension may exhibit significant rigidity. The authors propose to infer a migration of deformation from the northern Baikal lake to an initially stronger part of the lithosphere, i.e. the Barguzin rift and its extension to the east.

Micro-gravity investigations for the LNE watt balance project

We report on a micro-gravity survey of the laboratories where the LNEs watt balance experiment is being conducted, including the characterization of the Scintrex CG-5 relative gravimeter used for this study. The results of the survey are compared with a model of the gravity field generated by the local mass distribution. The ultimate goal is to transfer an absolute measurement of g from one room to another with minimal uncertainty.

Evidence of earthquake triggering by the solid earth tides

article i nfo Clearevidence forearthquake triggeringbythe earth tides hasremained elusive for morethan a century.Using the largest global earthquake catalog available (the NEIC catalog with 442412 events), we observe a clear correlation (with ∼99% confidence) between the phase of the solid Earth tide and the timing of seismic events: earthquakes occur slightly more often at the time of ground uplift by the Earth tide, i.e. when normal stresses are reduced within the lithosphere. We observe that this phase distribution anomaly is larger for smaller and shallowerearthquakes.Although earthquakes inregionswithdominantlynormalandstrike-slip faulting seem to exhibit more tidal triggering than regions dominated by thrust faulting, there is no statistically significant evidence for a focal mechanism-dependence on earthquake triggering. Finally, we show here that it is highly probablethat the observedtriggeringiscaused bythe solid Earth tide, ratherthan byloading fromthe oceanor atmospheric tides. Although an additional impact due to loading from ocean tides is possible and probable, we cannot detect it here because the earthquake database is not sufficiently complete and homogeneous (more smallmagnitudeearthquakesinoceanicareasareneeded).Ourresultsareconsistentwiththeideaofadamped

Application of artificial intelligence for Euler solutions clustering

Results of Euler deconvolution strongly depend on the selection of viable solutions. Synthetic calculations using multiple causative sources show that Euler solutions cluster in the vicinity of causative bodies even when they do not group densely about the perimeter of the bodies. We have developed a clustering technique to serve as a tool for selecting appropriate solutions. The clustering technique uses a methodology based on artificial intelligence, and it was originally designed to classify large data sets. It is based on a geometrical approach to study object concentration in a finite metric space of any dimension. The method uses a formal definition of cluster and includes free parameters that search for clusters of given properties. Tests on synthetic and real data showed that the clustering technique successfully outlines causative bodies more accurately than other methods used to discriminate Euler solutions. In complex field cases, such as the magnetic field in the Gulf of Saint Malo region (Brittany, France), the method provides dense clusters, which more clearly outline possible causative sources. In particular, it allows one to trace offshore the main inland tectonic structures and to study their interrelationships in the Gulf of Saint Malo. The clusters provide solutions associated with particular bodies, or parts of bodies, allowing the analysis of different clusters of Euler solutions separately. This may allow computation of average parameters for individual causative bodies. Those measurements of the anomalous field that yield clusters also form dense clusters themselves. Application of this clustering technique thus outlines areas where the influence of different causative sources is more prominent. This allows one to focus on these areas for more detailed study, using different window sizes, structural indices, etc.

A broken plate beneath the North Baikal Rift Zone revealed by gravity modelling

We modelled a 1200 km long gravimetric profile in the North Baikal rift to assess the mechanical behaviour of the lithosphere, using a numerical model that accounts for realistic brittle-elasto-ductile rheology. We use published seismicity and refraction data, a new 5′ × 7.5′ free-air/Bouguer gravity and topography data set, and a detailed map of faults obtained from high resolution SPOT imagery. Analysis of the gravity field over the North Baikal rift zone indicates significant asymmetry of the mechanical processes governing the deformation of the diverging sides of the rift. These anomalies cannot be explained by a conventional continuous plate undergoing extension beneath the rift zone, whereas a strong mechanical discontinuity (wedge shaped detachment zone beneath the rift axis) is able to reproduce observations. Such a discontinuous model provides a good fit to the gravity and crustal thickness data and explains the deep seismicity reported there.