Damien Delvaux

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.

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.

Palaeostress reconstructions and geodynamics of the Baikal region, Central Asia, Part I. Palaeozoic and Mesozoic pre-rift evolution

Abstract This paper presents the first palaeostress results obtained for the basement of the Baikal rift system, in southern Siberia (Russia). Large-scale structural analysis and palaeostress reconstructions show that the Palaeozoic-Mesozoic kinematic history, precursor of the Baikal Cenozoic rifting, is characterized by the succession of six regional palaeostress stages. Stress inversion of fault-slip data and earthquake focal mechanisms is performed using an improved right-dieder method, followed by rotational optimization (D. Delvaux, TENSOR program). The results are interpreted in the light of recent developments in the investigation of regional intraplate stress field, and used as additional constraints for palaeogeodynamic reconstruction of Central Asia. After the final Palaeozoic closure of the Palaeo-Asian ocean on the southern margin of the Siberian platform, the marginal suture with the Sayan-Baikal Caledonian belt was repeatedly and preferentially reactivated during the subsequent Palaeozoic and Mesozoic history. This suture zone also controlled the opening of the Baikal rift system in the Cenozoic. The progressive closure of the Palaeo-Asian and Mongol-Okhotsk oceans generated successive continental collisions, which were recorded in the Baikal area by brittle-ductile and brittle deformations. The first two palaeostress stages correspond to the successive collage of Precambrian microcontinents and Caledonian terranes along the southern margin of the Siberian platform: (1) Late Cambrian-Early Ordovician N-S compression; and (2) Late Silurian-Early Devonian NW-SE compression. The next two stages are related to the remote effects of the complex evolution of the western Palaeo-Asian ocean, southwest of the Siberian continent: (3) Late Devonian-Early Carboniferous N-S compression, recorded only in the Altai region; and (4) Late Carboniferous-Early Permian E-W compression, recorded both in the Altai and Baikal regions. The last stages are the consequences of the Mongol-Okhotsk oceanic closure: (5) Late Permian-Triassic NW-SE extension with development of Cordilleran-type metamorphic core complexes and volcanism along the active margin of the Mongol-Okhotsk ocean; (6) initial development of Early-Middle Jurassic 10–15-km-wide molassic basins in Trans-Baikal and large foredeeps along the southern margin of the Siberian platform in probable extensional context, but for which no palaeostress data are available; and (7) final closure of the Mongol-Okhotsk ocean in the Cretaceous. This last event results from the collision between the Mongol-China and the Siberian plates and is evidenced by the inversion of the Middle Mesozoic basins and by Late Jurassic-Early Cretaceous coal-bearing sedimentation in Trans-Baikal. This long tectonic history yielded a highly heterogeneous basement in the Baikal area, precursor of the Cenozoic rifting.

Present-day kinematics of the East African Rift

The East African Rift (EAR) is a type locale for investigating the processes that drive continental rifting and breakup. The current kinematics of this ~5000 km long divergent plate boundary between the Nubia and Somalia plates is starting to be unraveled thanks to a recent augmentation of space geodetic data in Africa. Here we use a new data set combining episodic GPS measurements with continuous measurements on the Nubian, Somalian, and Antarctic plates, together with earthquake slip vector directions and geologic indicators along the Southwest Indian Ridge to update the present-day kinematics of the EAR. We use geological and seismological data to determine the main rift faults and solve for rigid block rotations while accounting for elastic strain accumulation on locked active faults. We find that the data are best fit with a model that includes three microplates embedded within the EAR, between Nubia and Somalia (Victoria, Rovuma, and Lwandle), consistent with previous findings but with slower extension rates. We find that earthquake slip vectors provide information that is consistent with the GPS velocities and helps to significantly reduce uncertainties of plate angular velocity estimates. We also find that 3.16 Myr MORVEL average spreading rates along the Southwest Indian Ridge are systematically faster than prediction from GPS data alone. This likely indicates that outward displacement along the SWIR is larger than the default value used in the MORVEL plate motion model.

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.

Denudation history of the Malawi and Rukwa Rift flanks (East African Rift System) from apatite fission track thermochronology

Abstract Thirty apatite fission track ages and 22 track length measurements are presented from samples of basement rocks flanking the Malawi and Rukwa Rifts (East African Rift System) in order to elucidate the thermotectonic history of the rift flanks. The apatite fission track ages fall in the range 30 ± 15 to 296 ± 10 Ma. The relatively short (11.0–13.2 μm) mean track lengths and wide (1.3–2.3 μm) track length distributions suggest a protracted cooling history for the region, spanning Permian (Karoo) to Recent times. Thermal history reconstruction by inverse model calculations of the track length distribution suggests repeated phases of rapid cooling and denudation of the rift flanks at 250-200 Ma, ∼150 Ma and ≤40–50 Ma. These appear to be linked to the different rifting events in the area and can be correlated with deposition of the different sedimentary units within the basins. Erosion and isostatic rebound have modified the tectonically induced topography around the rifts: the elevation of the footwall flanks is augmented by flexural isostatic rebound, whereas the topography of the hanging wall flanks has been lowered by erosion. The footwall escarpments of the Malawi and Rukwa rifts are erosional features. The highly elevated plateaus flanking the Western Rift represent an erosional surface traditionally referred to as the “Gondwana surface”. The apatite fission track results of this study suggest that initial exhumation of the “Gondwana surface” to temperatures around 60–70°C took place during Karoo times, but that sub-aerial exposure of the surface did not take place until at least the Early Tertiary.

Early Cretaceous denudation related to convergent tectonics in the Baikal region, SE Siberia

We present 24 new apatite fission track (AFT) ages and 18 track length measurements from the Baikal region, SE Siberia. Most samples have AFT ages between 140 and 100 Ma, with relatively high mean track lengths (c. 13.2 μm). The relationship between AFT ages, elevation and mean track lengths indicate that the samples record rapid cooling during the Early Cretaceous (140–120 Ma), as also shown by thermal history inversion of track length distributions. Cooling took place during a Late Jurassic-Early Cretaceous orogenic phase, related to closure of the Mongol-Okhotsk ocean and reflected in the exhumation of metamorphic core complexes followed by thrusting and reverse faulting, basin inversion and large vertical motions. The variation in AFT ages throughout the study area can be partly explained by differences in geothermal structure but differential denudation also played a role. Minimum amounts of Early Cretaceous denudation are estimated at 2–3 km.

LATE QUATERNARY TECTONIC ACTIVITY AND LAKE LEVEL CHANGE IN THE RUKWA RIFT BASIN

Interpretation of remotely sensed images and air photographs, compilation of geological and topographical maps, morphostructural and fault kinematic observations and 14C dating reveal that, besides obvious climatic influences, the lake water extent and sedimentation in the closed hydrological system of Lake Rukwa is strongly influenced by tectonic processes. A series of sandy ridges, palaeolacustrine terraces and palaeounderwater delta fans are related to an Early Holocene high lake level and subsequent progressive lowering. The maximum lake level was controlled by the altitude of the watershed between the Rukwa and Tanganyika hydrological systems. Taking as reference the present elevation of the palaeolacustrine terraces around Lake Rukwa, two orders of vertical tectonic movement are evidenced: i) a general uplift centred on the Rungwe Volcanic Province between the Rukwa and Malawi Rift Basins; and ii) a tectonic northeastward tilting of the entire Rukwa Rift Basin, including the depression and rift shoulders. This is supported by the observed hydromorphological evolution. Local uplift is also induced by the development of an active fault zone in the central part of the depression, in a prolongation of the Mbeya Range-Galula Fault system. The Ufipa and Lupa Border Faults, bounding the Rukwa depression on the southwestern and northeastern sides, respectively, exert passive sedimentation control only. They appear inactive or at least less active in the Late Quaternary than during the previous rifting stage. The main Late Quaternary tectonic activity is represented by dextral strike-slip movement along the Mbeya Range-Galula Fault system, in the middle of the Rukwa Rift Basin, and by normal dip-slip movements along the Kanda Fault, in the western rift shoulder.

Geophysical investigations of seismically induced surface effects: Case study of a landslide in the Suusamyr Valley, Kyrgyzstan

In summer 1998, a geophysical survey including seismic profilesand electrical tomography has been carried out in the Suusamyr valley, Kyrgyzstan. The scope wasto investigate surface effects induced by the Ms = 7.3 Suusamyr earthquake, the 19th of August, 1992. Inthis paper, special attention is paid to the case study of a debris slide triggered by the earthquake.Seismic data are analysed by P-wave refraction technique and by surface wave inversion. Electrical tomographicprofiles are processed by 2D-inversion.Using geotechnical and geological information, P-velocity modelsand resistivity sections are interpreted in terms of geological materials, in order to build a geological3D model. On the basis of the latter, we carried out static finite element computations as well as staticand pseudo-static calculations with Janbu’s method. Newmark displacement was computed, considering or notthe influence of the shallow soft deposits. The results are compared to the real displacementobserved in the field and conclusions are drawn about the mechanism of the landslide.

Spatial variation of present-day stress field and tectonic regime in Tunisia and surroundings from formal inversion of focal mechanisms: Geodynamic implications for central Mediterranean

We compiled 123 focal mechanisms from various sources for Tunisia and adjacent regions up to Sicily, to image the current stress field in the Maghrebides chain (from Tunisia to Sicily) and its foreland. Stress inversion of all the available data provides a first-order stress field with a N150°E horizontal compression (SHmax) and a transpressional tectonic regime, but the obtained stress tensor poorly fit to the data set. We separated them into regional subsets (boxes) in function of their geographical proximity, kinematic regime, homogeneity of kinematic orientations, and tectonic setting. Their respective inversion evidences second- and third-order spatial variations in tectonic regime and horizontal stress directions. The stress field gradually changes from compression in the Maghrebides thrust belt to transpression and strike slip in the Atlassic and Pelagian foreland, respectively, where preexisting NW-SE to E-W deep faults system are reactivated. This spatial variation of the sismotectonic stress field and tectonic regime is consistent with the neotectonic stress field determined by others from fault slip data. The major Slab Transfer Edge Propagator faults (i.e., North-South Axis-Hammamet relay and Malte Escarpment), which laterally delimit the subducting slabs, play an active role in second- and third-order lateral variations of the tectonic regime and stress field orientations over the Tunisian/Sicilian domain. The past and current tectonic deformations and kinematics of the central Mediterranean are subordinately guided by the plate convergence (i.e., Africa-Eurasia), controlled or influenced by lateral slab migration/segmentation and by deep dynamics such as lithosphere-mantle interaction.