Jacques Déverchère

Strain accommodation by slow slip and dyking in a youthful continental rift, East Africa

Continental rifts begin and develop through repeated episodes of faulting and magmatism, but strain partitioning between faulting and magmatism during discrete rifting episodes remains poorly documented. In highly evolved rifts, tensile stresses from far-field plate motions accumulate over decades before being released during relatively short time intervals by faulting and magmatic intrusions. These rifting crises are rarely observed in thick lithosphere during the initial stages of rifting. Here we show that most of the strain during the July–August 2007 seismic crisis in the weakly extended Natron rift, Tanzania, was released aseismically. Deformation was achieved by slow slip on a normal fault that promoted subsequent dyke intrusion by stress unclamping. This event provides compelling evidence for strain accommodation by magma intrusion, in addition to slip along normal faults, during the initial stages of continental rifting and before significant crustal thinning.

Back arc extension, tectonic inheritance, and volcanism in the Ligurian Sea, Western Mediterranean

[1] The Ligurian basin, western Mediterranean Sea, has opened from late Oligocene to early Miocene times, behind the Apulian subduction zone and partly within the western Alpine belt. We analyze the deep structures of the basin and its conjugate margins in order to describe the tectonic styles of opening and to investigate the possible contributions of forces responsible for the basin formation, especially the pulling force induced by the retreating subduction hinge and the gravitational body force from the Alpine wedge. To undertake this analysis, we combine new multichannel seismic reflection data (Malis cruise, 1995) with other geophysical data (previous multichannel and monochannel seismic sections, magnetic anomalies) and constrain them by geological sampling from two recent cruises (dredges from Marco cruise, 1995, and submersible dives from Cylice cruise, 1997). From an analysis of basement morphology and seismic facies, we refine the extent of the different domains in the Ligurian Sea: (1) the continental thinned margins, with strong changes in width and structure along strike and on both sides of the ocean; (2) the transitional domain to the basin; and (3) a narrow, atypical oceanic domain. Margin structures are characterized by few tilted blocks along the narrow margins, where inherited structures seem to control synrift sedimentation and margin segmentation. On the NW Corsican margin, extension is distributed over more than 120 km, including offshore Alpine Corsica, and several oceanward faults sole on a relatively flat reflector. We interpret them as previous Alpine thrusts reactivated during rifting as normal faults soling on a normal ductile shear zone. Using correlations between magnetic data, seismic facies, and sampling, we propose a new map of the distribution of magmatism. The oceanic domain depicts narrow, isolated magnetic anomalies and is interpreted as tholeitic volcanics settled within an unroofed upper mantle, whereas calcalkaline volcanism appears to be discontinuous but massive and has jumped in space and time, from the beginning of rifting on the Ligurian margin (� 30 Ma), toward the Corsican margin at the end of the Corsica-Sardinia block rotation (� 16 Ma). This space and time shift reveals the importance of the rollback of the Apulian slab and of the migration of the Alpine-Apennines belt front toward the E-SE for driving basin formation. We also state that initial rheological conditions and inherited crustal fabric induce important changes in the styles of deformation observed along margins and between conjugate margins. In the NE Ligurian basin the prerift Alpine crustal thickening together with slow rollback velocity likely contribute to distribute strain across the whole NW Corsican margin, whereas farther south the inherited Hercynian structural pattern combined with a faster rollback of the subducting plate tend to focus the extension at the foot of the margin, up to the Sardinian rift which ends within the SW Corsican margin. Therefore the mode of opening and the margin structures mainly depend on the balance between intrinsic, inherited crustal heterogeneity (fabric and rheological changes) and external conditions imposed by rollback of the subducting lithosphere. INDEX TERMS: 3040 Marine Geology and Geophysics: Plate tectonics (8150, 8155, 8157, 8158); 3025 Marine Geology and Geophysics: Marine seismics (0935); 8109 Tectonophysics: Continental tectonics—extensional (0905); 8159 Tectonophysics: Rheology—crust and lithosphere;

Present‐day stress field changes along the Baikal rift and tectonic implications

Intraplate extension, in a frame of a global compressional stress field, seems linked to local lithospheric perturbations (lithospheric thinning or thickening) able to modify the resulting state of stress [Zoback, 1992]. The Baikal Rift Zone (BRZ), Siberia, is located north of the India-Asia collision zone and exhibits no direct communication with any oceanic domain. It can thus be fully considered as an area of continental extension, dominated by the “global compressional intraplate stress field” resulting from plate driving forces. In order to address the problem of its dynamics and kinematics and their links with the India-Asia collision, a comprehensive stress tensor analysis is presented, based on 319 focal mechanisms of earthquakes located along the whole Baikal rift. The stress field is varying at different scales of observation: when looking at central Asia (several thousands kilometers), the maximum horizontal stress SHmax directions remain rather constant (with a fan-shape geometry) when the tectonic regime goes from compressional (Himalayas) to extensional (Baikal). When observing the Baikal rift (about 1000 km long), clear variations of the stress regime are observed, from an extensional regime in the central part of the rift to wrench ones in its northern and southern ends. Finally, at the scale of 100 km, systematic SHmax reorientations occur close to major rift faults. We thus infer that the interaction between collisional processes and inherited structures may have a strong influence on rift dynamics. We then use computed stress tensors to predict slip vectors on major rift faults. Deformation patterns show two distinct parts of the rift: the South Baikal Rift (SBR) is characterized by a constant trending (around N100°E) slip vector, meanwhile the North Baikal Rift (NBR) exhibits a complex block rotation behavior involving at least three crustal blocks. We propose to interpret these surficial structures and motions as the result of an interaction between the regional compression coming from the India-Asia collision and the geometry of the hardly deformable Siberian platform. This particular setting can explain most of the surficial deformation patterns, which suggest a large-scale cracking of the lithosphere in the Baikal region. Other possible sources of stress could also be considered, like deep mantellic upwelling, or trench suction linked to the Pacific subduction.

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,

Seismicity, active faults and stress field of the North Muya Region, Baikal Rift: New insights on the rheology of extended continental lithosphere

The eastern Baikal rift is characterized by a succession of young en echelon half-grabens distributed in a 300-km-wide zone of high seismic activity. In this study we present the overall seismotectonic setting of the north Muya region, which is an along-strike transfer zone between two en echelon dip-slip fault systems. Using both regional and local seismological networks, we refine the hypocenters and the single-event focal mechanisms of 704 earthquakes recorded during 3 years, including the main period of activity of a very dense cluster, the Angarakan swarm. Our final hypocenter locations, obtained after determining a region-specific velocity model and applying a master event technique, allow us to observe a widespread and unusual seismic deformation throughout the upper 30 km of the continental crust. Deep earthquakes (15–30 km) define a planar surface related to a large north-dipping basement fault (Upper Muya), and also occur in a small cluster at 23 km depth at the intersection of two major structural trends. Along more than 150 km of strike length from west to east, maximum focal depths continuously increase from 20 to 30 km, whereas fault dip concomitantly increases from 30° to 55°. On the other hand, most activity of the Angarakan swarm is confined in the “classical” depth range of 0–15 km and depicts a narrow and steeply south-dipping seismic band related to a very recent fault scarp at the surface (eastern Kovokta). This fault geometry, which is observed at depth and in the field, and the opposite tilting of regularly 30-km-spaced horsts and grabens, reveal an asymmetrical rift system. The Agreement of fault lengths estimated from field studies with rupture lengths estimated from large historical earthquakes strengthens the inference that large earthquakes may occur near the base of the seismogenic layer of the crust, i.e., 30 km. Most of the 39 focal mechanisms determined in the study region show a dominant normal dip-slip displacement on nodal planes striking in the main inherited structural direction WSW-ENE, whereas some limited pure strike-slip faulting mainly occurred in the highly fractured Angarakan zone. From inversion of 37 focal mechanisms, we deduce a tensional stress tensor with a σ3 axis striking N160° and a σ2 axis slightly compressional (shape factor R of 0.4). Comparing these results with stress tensors computed along the eastern Baikal rift and with the focal mechanisms of three strong earthquakes that occurred in the Muya region during this century shows that small-magnitude earthquake data give consistent and accurate constraints on the regional stress regime. Whether this present stress tensor remained stable since the beginning of opening of the Baikal rift and during Quaternary is unknown. Our results confirm that continental lithosphere submitted to rapid extension in an early stage of rifting may retain significant rigidity.

Structure and evolution of the Baikal rift: A synthesis

Active continental rifts are spectacular manifestations of the deformation of continents but are not very numerous at the surface of the Earth. Among them, the Baikal rift has been extensively studied during the last decades. Yet no simple scenario explains its origin and development because the style of rifting has changed throughout its ∼30 Myr history. In this paper, we use forward and inverse models of gravity data to map the Moho and lithosphere-asthenosphere boundary in three dimensions. We then integrate these new results with existing geophysical and geological data on the Baikal rift structure and dynamics, and propose a scenario of its evolution. Earthquake depths, mantle xenoliths, heat flow, and seismic and gravity models advocate for a normal to moderately thinned continental lithosphere and crust, except beneath the Siberian craton, which exhibits a >100-km-thick lithosphere. Relatively thin lithosphere (70–80 km) is found east and south of the rift system and is in spatial connection with the Hangai-Hovsgol region of anomalous mantle in Mongolia. From top to bottom, the rift structure is asymmetric and appears strongly controlled by the geometry of the suture zone bounding the Siberian craton. Moreover, the mode of topography support changes significantly along the length of the rift: mountain ranges south and north of the rift are underlain by negative Bouguer anomalies, suggesting deep crustal roots and/or anomalous mantle; rift shoulders in the center of the rift seem to result from flexural uplift. The commonly assumed “two-stage” rift evolution is not corroborated by all stratigraphic and seismic data; however, it seems clear that during the Oligocene, an “early stage,” which might be dominated by strike-slip tectonics instead of pure extension, created primitive basins much different from the present ones. Most of the “true” rift basins seem to have initiated later, during the Late Miocene or Pliocene. This kinematic change from strike-slip to extensional tectonics in the Baikal rift is part of a more general kinematic reorganization of Asia and can be associated with the rapid growth of the Tibetan plateau and the end of marginal basins opening along the Pacific boundary.

Crustal deformation in the Baikal Rift from GPS measurements

Three years and four campaigns of Global Positioning System (GPS) measurements (1994–1997) in the Baikal rift zone, largest active continental rift system in Eurasia, show crustal extension at a rate of 4.5±1.2 mm/yr in a WNW-ESE direction. A comparison with moment release of large historical earthquakes suggests that elastic strain is currently accumulating in the Baikal rift zone along active faults that currently have the potential for a M=7.5 earthquake. The GPS-derived extension rate in the Baikal rift zone is at least two times greater than the prediction of most deformation models of Asia. This result could reflect the dynamic contribution of the Pacific-Eurasia subduction to intracontinental deformation in Asia, in addition to the effect of the India-Eurasia collision.

Deep structure and mechanical behavior of the lithosphere in the Hangai–Hövsgöl region, Mongolia: new constraints from gravity modeling

We investigate the deep structure and mechanical behavior of the lithosphere beneath the Hangai^Ho « vsgo « l region, central Mongolia, Asia, in order to explain the origin and support of large-scale doming in this deforming area. We propose a gravity- and topography-based model which accounts for constraints provided by other independent results from xenolith and tomography studies. Deviations of the measured gravity from the theoretical Airy-compensation model are examined. A long-wavelength low-gravity anomaly is spatially correlated with low pressure and shear velocity anomalies in the mantle, and with the extent of Cenozoic volcanic outcrops. We interpret it as a deep-seated low-density asthenosphere and model its effect on the Bouguer gravity signal using a 600 km wide light asthenospheric body (density reduction 310 kg m 33 ) located between 100 and 200 km. North and south of the Hangai^Ho « vsgo « l dome, short-wavelength highs and lows in the Bouguer gravity field are clearly correlated with fault activity. They seem to reflect opposite senses of flexure of a rigid lithosphere across two major active faults, the Sayan and Bogd transpressional systems, and are modeled by Moho deflections of 10 and 5 km, respectively. Finally, a shortwavelength (200 km), high-amplitude (350 mGal) gravity residual remains beneath the highest part of the mountain bulge, namely the Hangai dome. Based on previously published xenolith analyses, we interpret it as an anomalous, low-density body which may represent underplated cumulates or mafic granulites at the uppermost mantle. We conclude that upper mantle dynamics necessarily play an important role in the origin and evolution of the Hangai^ Ho « vsgo « l dome, but without requiring significant thinning of the lithosphere. fl 2002 Elsevier Science B.V. All rights reserved.

VELOCITY STRUCTURE AROUND THE BAIKAL RIFT ZONE FROM TELESEISMIC AND LOCAL EARTHQUAKE TRAVELTIMES AND GEODYNAMIC IMPLICATIONS

Abstract We present new results on the velocity structure of the Baikal rift zone, Asia, deduced from a comparative teleseismic and local tomography analysis. The aim of this paper is to better identify the role of deep mantle processes versus that of far-field tectonic effects on the occurrence of extensional tectonics within a continental plate. We use 36000 traveltimes of P-refracted waves from the ISC catalogues and Pg and Pn traveltimes of 578 earthquakes recorded by the Russian regional network to determine a velocity model by the use of local and teleseismic inversion procedures. The models show that some velocity patterns are continuous from the surface down to at least 400 km. Among them, a narrow negative anomaly goes through Mongolia and follows the southern and eastern margins of the Siberian craton: this structure is interpreted as a thin mantle plume rising beneath the rift axis. However, our results do not evidence any wide asthenospheric upwarp at this place. Other velocity anomalies observed near the surface are not deeply rooted. In particular, a negative anomaly is observed at shallow levels (48 km) beneath the northern third of Lake Baikal, which is disconnected from deeper structures. It may be explained by the existence of underplated magmatic material at the bottom of the crust. By comparing the geometry of deep-rooted anomalies to the present-day stress field patterns, we conclude that the sub-lithospheric mantle dynamics is not the main factor controlling extensional processes in the Baikal rift. However, it does contribute to a thermal weakening of the lithosphere along a mechanical discontinuity bounding the Siberian shield. We finally conclude that three favourable conditions are gathered in the Baikal area to generate extension: far-field extensional stress field, mechanical inherited lithospheric weakness and heat supply. Further studies should help to precise the genetic link between these three factors.

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.