Carole Petit

PALEOSTRESS RECONSTRUCTIONS AND GEODYNAMICS OF THE BAIKAL REGION, CENTRAL ASIA, PART 2. CENOZOIC RIFTING

Abstract Investigations on the kinematics of rift opening and the associated stress field present a renewed interest since it has recently been shown that the control of the origin and evolution of sedimentary basins depends to a large extent on the interplay between lithospheric strength and applied stresses. It appears that changes of stress field with time are an important factor that either controls or results from the rifting process. The object of this paper is to study the changes of fault kinematics and paleostress field with time in the Baikal Rift System during the Cenozoic. Reduced paleostress tensors were determined by inversion from fault-slip data measured in the central part of the rift and its southwestern termination, between 1991 and 1995. Results show that the stress field varies as well in time as in space. Two major paleostress stages are determined, corresponding broadly to the classical stages of rift evolution: Late Oligocene-Early Pliocene and Late Pliocene-Quaternary. The first paleostress stage is related to the rift initiation and the second to the major stage of rift development. Similarities between the recent paleostress field and the present-day stress field inverted from focal mechanisms indicate that the second paleostress stage is still active. Therefore, we propose to use ‘proto rift’ for the Late Oligocene-Early Pliocene stage and ‘active rift’ for the Late Pliocene-Quaternary stage of rift development. During the ‘proto rift’ stage, the stress field was characterized by a compressional to strike-slip regime. A progressive change from transpression to transtension is suspected for the central part of the rift (Baikal and Barguzin basins) during this period. In the western termination of the rift (Sayan Massif, Tunka depression), a strongly compressional stress field with oblique thrusting kinematics is well constrained in the Late Miocene-Early Pliocene interval. The ‘active rift’ stage was initiated by a marked change in fault kinematics and stress regime in the Late Pliocene. In the central part of the rift, the stress regime changed into pure extension, while in the southwestern extremity, it changed into pure strike-slip. Fault kinematics suggests that rifting was initiated by an extrusion mechanism due to the interaction of far-field compressional stress on a mechanically heterogeneous crust, with the southwards-pointing wedge of the Siberian Craton acting as a passive indentor. The Cenozoic time-space evolution of the stress field is believed to reflect the increasing influence of locally generated buoyancy extensional stresses associated with density anomalies of the lithosphere, on intraplate stresses generated by the India-Eurasia convergence and the West-Pacific subduction.

Arabia-Somalia plate kinematics, evolution of the Aden-Owen-Carlsberg triple junction, and opening of the Gulf of Aden

New geophysical data collected at the Aden‐Owen‐Carlsberg (AOC) triple junction between the Arabia, India, and Somalia plates are combined with all available magnetic data across the Gulf of Aden to determine the detailed Arabia‐Somalia plate kinematics over the past 20 Myr. We reconstruct the history of opening of the Gulf of Aden, including the penetration of the Sheba Ridge into the African continent and the evolution of the triple junction since its formation. Magnetic data evidence three stages of ridge propagation from east to west. Seafloor spreading initiated ∼20 Myr ago along a 200 kmlong ridge portion located immediately west of the Owen fracture zone. A second 500 kmlong ridge portion developed westward up to the Alula‐Fartak transform fault before Chron 5D (17.5 Ma). Before Chron 5C (16.0 Ma), a third 700 km‐long ridge portion was emplaced between the Alula‐Fartak transform fault and the western end of the Gulf of Aden (45°E). Between 20 and 16 Ma, the Sheba Ridge propagated over a distance of 1400 km at an extremely fast average rate of 35 cm yr−1. The ridge propagation resulted from the Arabia‐Somalia rigid plate rotation about a stationary pole. Since Chron 5C (16.0 Ma), the spreading rate of the Sheba Ridge decreased first rapidly until 10 Ma and then more slowly. The evolution of the AOC triple junction is marked by a change of configuration around 10 Ma, with the formation of a new Arabia‐India plate boundary. Part of the Arabian plate was then transferred to the Indian plate.

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.

Flexure and mechanical behavior of cratonic lithosphere: Gravity models of the East African and Baikal rifts

The effective elastic thickness of the lithosphere T e has been investigated by inverse and forward models based on the assumption that flexure of the surface and crust-mantle boundaries produces gravity field variations which are commonly expressed in free air or Bouguer gravity anomalies. The aim of this study is to examine T e variations across the East African and Baikal rifts that have developed within or adjacent to early Proterozoic-Archean cratons in order to investigate the possibility that the crust and upper mantle beneath the cratons retain considerable strength in extension. We use forward models of five 1800-km-long Bouguer gravity profiles in the East African rift and one profile previously modeled in the Baikal rift. The forward modeling method uses a multilayered lithospheric rheology to compute the mechanical plate deflection in response to vertical loads. We show that the origin of the rift topography may be highly variable: uplifted rift flanks are few in the Baikal rift, where mountain ranges have generally deep crustal roots, while flexural rebound accounts for most of the short-wavelength topography encountered in the East African rift. Reasonable fits of models and observed Bouguer gravity anomalies can only be obtained if we allow large rheological contrasts between the cratons and surrounding areas; T e values reach 60 km beneath the cratons and range between 20 and 45 km beneath the surrounding orogenic belts. The major part of the rifts develops close to the craton margins, where rheological contrasts may cause stress concentrations. These results agree well with previous Bouguer cohe- rence analyses but are significantly different from previous free-air coherence models and forward models of topography alone, which yielded T e estimates <10 km beneath the rifts and the surrounding areas. We believe that these differences largely stem from model assumptions of compensation depth and lithospheric mechanical properties and from the fact that free-air anomalies contain short-wavelength noise that correlates with topography, which biases free-air based methods toward low T e estimates.

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.

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.

How old is the Baikal Rift Zone? Insight from apatite fission track thermochronology

Apatite fission track analysis (AFTA) data are used to bring new light on the long-term and recent history of the Baikal rift region (Siberia). We describe the evolution of the topography along a NW-SE profile from the Siberian platform to the Barguzin range across the Baikal-southern Patom range and the northern termination of Lake Baikal. Our results show that the Baikal-Patom range started to form in the Early Carboniferous and was reactivated in Middle Jurassic-Lower Cretaceous times during the orogenic collapse of the Mongol-Okhotsk belt. Samples located in the Siberian platform recorded a continuous sedimentation up to the early Carboniferous but remain unaffected by later tectonic episodes. The Barguzin basin probably started to form as early as Late Cretaceous, suggesting a continuum of deformation between the postorogenic collapse and the opening of the Baikal Rift System (BRS). The initial driving mechanism for the opening of the BRS is thus independent from the India-Asia collision. AFTA show a late Miocene-early Pliocene increase in tectonic extension in the BRS that confirms previous thoughts and might reflect the first significant effect of the stress field generated by the India-Asia collision.

On the structure and mechanical behaviour of the extending lithosphere in the Baikal Rift from gravity modelling

Abstract The crustal structure and lithospheric flexure of the Baikal Rift Zone, Siberia, are examined by means of gravity modelling. We model the Bouguer Anomaly (BA) along five 1200 km long gravity profiles. We first evidence that continuous elastic plate flexure due to surface loading cannot explain the observed BA. Then we introduce plate discontinuities coupled with a realistic brittle-elasto-ductile plate rheology, which allow us to model of external tectonic forces acting on the plates and determination of the Moho geometry. We show that the clearest expression of extensional processes occurs in the central part of the rift, which exhibits the highest crustal thinning. It evolves southwards to a rapidly increasing compression, resulting in an overthickening of the southern plates crust and in the long-wavelength flexure of the Siberian plate. North of the central rift, crustal thinning (which is always less than 7 km) gives way to a more diffuse zone of deformation inside the Sayan-Baikal folded belt. Based on plate flexure models, we propose that the rift shoulders surrounding the central and north Baikal basins are not supported by upward bending plate, but have a deep crustal root caused by a downward flexure. The other parts of the rift depict two adjacent plates with antithetic flexures. We also infer that the axial mantle material upwarping is not related to a large-scale asthenospheric upwelling, since the lithosphere rheological interfaces are not significantly disturbed. Our results favour the role of horizontal forces and motions, resulting from the India-Asia collision, combined with the effect of inherited tectonic structures (and especially the Paleozoic suture bounding the Siberian craton) for explaining the crustal structure and plate flexure modeled.

Tectonic and climatic controls on rift escarpments: Erosion and flexural rebound of the Dhofar passive margin (Gulf of Aden, Oman)

We investigate the respective roles of climatic parameters and the flexural rigidity of the lithosphere in the erosion history and behavior of two adjacent rift escarpments along the northern coast of the Gulf of Aden, in Oman. At this 25 Myr old passive margin, we define a type 1 scarp, which is high, sharp-crested and has retreated 25-30 km inland from its master fault, and a type 2 scarp, which exhibits a more rounded profile, lower relief, and still coincides with its mapped normal fault trace. Since about 15 Ma, the margin has been seasonally affected by monsoon precipitation but with contrasting effects at the type 1 and type 2 escarpments depending on the position of the Intertropical Convergence Zone in the geologic past: during peak monsoon conditions, both scarps experienced heavy rainfall and runoff, whereas during monsoon-starved conditions (such as today), the type 2 scarp experienced a foggy, moist climate while the type 1 scarp remained much drier. In order to assess the relative effects of climate and flexural parameters on the present-day morphology of the Dhofar margin, we present onedimensional numerical models of erosion and flexure along two profiles representative of the type 1 and type 2 scarps. Unlike most surface process models previously published, where present-day topography is the only criterion by which to evaluate the quality of model outputs, model behavior here is additionally constrained by independent estimates of denudation provided by geological cross sections, well-defined fault traces, and other stratigraphic markers. The best fitting models indicate that the type 1 escarpment formed under relatively arid climatic conditions and was affected by significant erosion, recession and flexural uplift due to a low (7 km) effective elastic thickness. In contrast, the morphology of the type 2 fault scarp was smoothed by a more humid climate, but a high effective elastic thickness ( 15 km) prevented it from uplifting or receding. In addition, we show that the sedimentary load acting at the foot of the escarpments exerts significant influence on their morphological evolution, though this parameter is often neglected in other scarp evolution models.