No abstract

[1] We investigate the respective roles of crustal tectonic shortening and asthenospheric processes on the topography of the High Atlas and surrounding areas (Morocco). The lithospheric structure is modeled with a direct trial-and-error algorithm taking into account gravity (Bouguer and free air), geoid, heat flow, and topography. Three parallel cross sections, crossing the High Atlas and Anti-Atlas ranges, show that the lithosphere is thinned to 60 km below these mountain ranges. An analysis of the effect of the lithospheric thinning allows us to conclude that the whole topography of the Anti-Atlas, which belongs to the Sahara domain, is due to asthenospheric processes. In the High Atlas the lithospheric thinning explains a third of the relief of the western High Atlas, 500 m for a mean altitude of 1500 m, and half of the relief of the central High Atlas, 1000 m for a mean altitude of 2000 m. At the scale of Morocco the domain affected by lithospheric thinning forms an elongated NE-SW strip crossing not only the main structural zones but also the Atlantic margin to the south and the Africa-Eurasia plate boundary to the north. This major lithospheric thinning is associated with Miocene to recent alkaline volcanism and seismicity. We propose that this thermal anomaly is related to a shallow mantle plume, emplaced during middle to late Miocene time, during a period of relative tectonic quiescence.

Recent measurements of surface vertical displacements of the European Alps show a correlation between vertical velocities and topographic features, with widespread uplift at rates of up to~2-2.5 mm/a in the NorthWestern and Central Alps, and~1 mm/a across a continuous region from the Eastern to the SouthWestern Alps. Such a rock uplift rate pattern is at odds with the horizontal velocity field, characterized by shortening and crustal thickening in the Eastern Alps and very limited deformation in the Central and Western Alps. Proposed mechanisms of rock uplift rate include isostatic response to the last deglaciation, long-term erosion, detachment of the Western Alpine slab, as well as lithospheric and surface deflection due to mantle convection. Here, we assess previous work and present new estimates of the contributions from these mechanisms. Given the large range of model estimates, the isostatic adjustment to deglaciation and erosion are sufficient to explain the full observed rate of uplift in the Eastern Alps, which, if correct, would preclude a contribution from horizontal shortening and crustal thickening. Alternatively, uplift is a partitioned response to a range of mechanisms. In the Central and Western Alps, the lithospheric adjustment to deglaciation and erosion likely accounts for roughly half of the rock uplift rate, which points to a noticeable contribution by mantle-related processes such as detachment of the European slab and/or asthenospheric upwelling. While it is difficult to independently constrain the patterns and magnitude of mantle contributions to ongoing Alpine vertical displacements at present, future data should provide additional insights. Regardless, interacting tectonic and surface mass redistribution processes, rather than an individual forcing, best explain ongoing Alpine elevation changes.

No abstract

[1] 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.

Summary The correspondence between the predicted brittle–plastic transition within the crust and the maximum depth of earthquakes is examined in the case of the Baikal rift, Siberia. Although little accurate information on depths is available through large‐ and moderate‐size earthquakes, there are frequent indications of foci at 20 km depth and more. We have relocated 632 events recorded at nearby stations that occurred between 1971 and 1997, with depth and epicentral uncertainties less than 5 km, over the eastern and southern parts of the Baikal rift. We have compared these results with other depth distributions obtained in previous studies from background seismicity in the NE rift (1365 events in the Kalar‐Chara zone and 704 events in the Muya region). The relative abundance of earthquakes is generally low at depths between 0 and 10 km (7–15 per cent) and high between 15 and 25 km (∼50 per cent). Earthquake activity is still significant between 25 and 30 km (9–15 per cent) and persists between 30 and 40 km (7–13 per cent). Very few earthquakes are below the Moho. We use empirical constitutive laws to obtain the yield‐stress limits of several layers made of dominant lithologies and to examine whether the observed distribution of earthquakes at depth (519 events controlled by a close station and located within the extensional domain of the Baikal rift system) can match the predicted crustal strength proportion with depth and the deeper brittle–ductile transition in the crust. A good fit is obtained by using a quartz rheology at 0–10 km depth and a diabase rheology at 10–45 km depth with a moderate temperature field which corresponds to a ∼100 Myr thermal lithosphere. No dioritic composition of the crust is found necessary. In any case, earthquakes occur at deep crustal levels, where the crust is supposed to be ductile, in a way very similar to what is found in the East African Rift System. From these results we conclude that the seismogenic thickness is ∼35–40 km in the Baikal rift system and that the depth distribution of earthquakes is at first order proportional to the strength profile found in a rheologically layered crust dominated by a mafic composition in the ∼10–45 km depth range. An upper mantle core with high strength, however, generally prevents it from reaching stress failure at greater depth.

No abstract

[1] 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-Hövsgöl 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.