No abstract

International audienceEarthquake scarps associated with recent historical events have been found on the floor of the Sea of Marmara, along the North Anatolian Fault (NAF). The MARMARASCARPS cruise using an unmanned submersible (ROV) provides direct observations to study the fine-scale morphology and geology of those scarps, their distribution, and geometry. The observations are consistent with the diversity of fault mechanisms and the fault segmentation within the north Marmara extensional step-over, between the strike-slip Ganos and Izmit faults. Smaller strike-slip segments and pull-apart basins alternate within the main step-over, commonly combining strike-slip and extension. Rapid sedimentation rates of 1?3 mm/yr appear to compete with normal faulting components of up to 6 mm/yr at the pull-apart margins. In spite of the fast sedimentation rates the submarine scarps are preserved and accumulate relief. Sets of youthful earthquake scarps extend offshore from the Ganos and Izmit faults on land into the Sea of Marmara. Our observations suggest that they correspond to the submarine ruptures of the 1999 Izmit (Mw 7.4) and the 1912 Ganos (Ms 7.4) earthquakes. While the 1999 rupture ends at the immediate eastern entrance of the extensional Cinarcik Basin, the 1912 rupture appears to have crossed the Ganos restraining bend into the Sea of Marmara floor for 60 km with a right-lateral slip of 5 m, ending in the Central Basin step-over. From the Gulf of Saros to Marmara the total 1912 rupture length is probably about 140 km, not 50 km as previously thought. The direct observations of submarine scarps in Marmara are critical to defining barriers that have arrested past earthquakes as well as defining a possible segmentation of the contemporary state of loading. Incorporating the submarine scarp evidence modifies substantially our understanding of the current state of loading along the NAF next to Istanbul. Coulomb stress modeling shows a zone of maximum loading with at least 4?5 m of slip deficit encompassing the strike-slip segment 70 km long between the Cinarcik and Central Basins. That segment alone would be capable of generating a large-magnitude earthquake (Mw 7.2). Other segments in Marmara appear less loaded

Dedicated with gratitude, respect, and affection to Warren Bell Hamilton on the occasion of his retirement from active service to the United States Geological Survey and in recognition of his remarkable pioneering work on the tectonics of the Altaids, on which the research reported here is firmly based.

No abstract

The Junggar, Turfan and Alakol basins in northwestern China and Kazakhstan formed as Late Permian to ?Early Triassic extensional structures in a broad sinistral shear zone between large strike-slip faults that separate two main domains of the Altaid orogenic collage. This extension was in response to an inferred large (> l000 km) sinistral motion of the East European craton with respect to the Angaran craton during this time. Deformation associated with the formation of the basins was taken up in part by counter-clockwise rotations of crustal blocks with respect to the Altaid orogenic collage and to the Angaran craton. This event is the only important phase of extension in a region otherwise dominated by compressional tectonics throughout the Phanerozoic. The basement rocks of these basins formed by Altaid subduction–accretion through the latter half of the Palaeozoic and the region was subsequently thrown into compression again during the Mesozoic Cimmeride and Cenozoic Alpide evolution.

The history of Mesozoic accretion and growth of the Asia eastern margin, occupied by Southeastern Russia, includes five main events; two main tectonic regimes were responsible for the growth of the continent. In the Triassic-Jurassic, Early Cretaceous and Late Cretaceous-Paleogene, the subduction of the oceanic lithosphere resulted in the formation of wide accretionary wedges of the Mongol-Okhotsk, Khingan-Okhotsk and Eastern Sikhote-Alin active continental margins, respectively.These stages of the comparacively slow growth of the continent were broken by stages of rapid growth and drastic changes in the shape of the continent, since at these stages large terranes of various tectonic nature collided with active continental margins. A t the end of the Early-Middle Jurassic, the Bureya terranes collided with the Mongol-Okhotsk active margin, and at the beginning of the Late Cretaceous there was collision of the Central and Southern Sikhote-Ah terranes with the Khingan-Okhotsk active margin.Collision-related structural styles in all cases are indicative of oblique collision and great strike-slip motions along the main sutures. The peculiarities of the terrane's geological structure show that prior to collision with the Mongol-Okhotsk and Khingan-Okhotsk active margins, they had already accreted to Asia and then migrated along its margins along the strike-slip faults. The Bureya terranes were squeezed out of the compression zone between Siberia and North China. This compression zone originated after the Paleozoic oceans which divided these cratons had closed. The Khanka terranes and Mesozoic accretionary wedge terranes of the Sikhote-Alin shifted along the strike-slip faults subparallel to the Asia Pacific margin. Strike-slip motions resulted in duplication of the primary tectonic zonation.

The submerged section of the North Anatolian fault within the Marmara Sea was investigated using acoustic techniques and submersible dives. Most gas emissions in the water column were found near the surface expression of known active faults. Gas emissions are unevenly distributed. The linear fault segment crossing the Central High and forming a seismic gap-as it has not ruptured since 1766, based on historical seismicity, exhibits relatively less gas emissions than the adjacent segments. In the eastern Sea of Marmara, active gas emissions are also found above a buried transtensional fault zone, which displayed micro-seismic activity after the 1999 events. Remarkably, this zone of gas emission extends westward all along the southern edge of Cinarcik basin, well beyond the zone where 1999 aftershocks were observed. The long term monitoring of gas seeps could hence be highly valuable for the understanding of the evolution of the fluid-fault coupling processes during the earthquake cycle within the Marmara Sea.

No abstract