Oleg V. Levchenko

The structure and tectonics of the intraplate deformation area in the Indian Ocean

Abstract Intense tectonic deformations of sediments and basement unusual for the interior of the oceanic lithosphere plates can be seen on seismic reflection profiles from the northern Central Indian Basin. 10,000-mile long CSP profiles collected by the P.P. Shirshov Institute of Oceanology of the USSR Academy of Sciences allow delineation of a NE-trending area of these deformations. The intraplate deformation area has a complicated tectonic framework, formed by a mosaic of crustal blocks which have been severely deformed or tilted alternating with less deformed parts of the sea floor. The results of a detailed CSP grid survey reveal that these uplifted faulted blocks are bounded by tectonic faults of two genetic types: old nearly meridional fracture zones, and young NE-striking faults. The seismic refraction results indicate an anomalous structure of the crust and upper mantle within these blocks. Seismological observations from Ocean Bottom Seismographs have proved that there is high-level intraplate seismicity in the northern Central Indian Basin. The intraplate deformation area is supposed to correspond to a large-scale zone of shearing strains that formed as a result of the stress difference in the Indo-Australian plate due to the continued collision of the continents of India and Asia along with spreading in the Central Indian Ridge and normal subduction in the Sunda Island Arc. This continental collision led to an increase in compressional stress in the northernmost part of the plate, while its southern part continuously subducted beneath the Sunda Trench. In the complicated transitional zone between these parts of the plate, NE-SW trending shearing stress abated, apparently in the Late Miocene, as a result of folding and faulting of the sediments and basement, observed on seismic reflection profiles. NE-SW trending wrench-fault tectonics affected the system of ancient failures in the spreading oceanic crust (near-meridional transform faults and, perhaps, near-latitudinal scarps parallel to the mid-oceanic ridge axis). The mosaic fault-fold framework of the intraplate deformation area might be partly a result of this interaction.

Tectonic aspects of intraplate seismicity in the northeastern Indian Ocean

Abstract Since the Late Cenozoic, the northeastern Indian Ocean region of the Indo-Australian plate has been characterized by tectonic activity that is unusual in a mid-plate setting. This activity, including deformation of the basement and the sediments and a high level of seismicity, is typical of plate boundaries. The characteristics of the seismicity and other phenomena there bears evidence to the initial stage of diffuse, intraplate deformation of oceanic lithosphere. It would be reasonable to suggest that the intraplate deformation in the northeastern Indian Ocean is episodic in behaviour. The previous pulse of deformation seems to have occurred in the Late Miocene, when the recent deformation of the sediments and the basement observed on the seismic reflection profiles was generated. Modern tectonic activity, manifesting itself in high seismicity, can be related to a new pulse of intraplate deformation. A near-latitudinal equatorial seismic zone, comprising the northern segment of the Ninetyeast Ridge and northern Central Indian and Cocos Basins, was suggested on the basis of analysis of seismological data for the eastern Indian Ocean from 1907 to 1983. The intraplate earthquake epicenters are concentrated in this broad band, without distinct physiographic boundaries, from nearly 80°E in the Central Indian Basin to the Sunda Trench. Northeastern trends of earthquake epicenters, that seem to be related to reactivation of some old faults, can be observed within this band. The orientation of the intraplate seismic zone is approximately similar to that of the continental collision zone at the northern Indo-Australian plate margin, the Himalaya. The possible cause of this high intraplate seismicity might be the southward migration or “jumping” of the Himalayan subduction zone after the subduction there was stopped by the Indian continental block due to continental collision. The episodic intraplate deformation in the northeastern Indian Ocean region may manifest a future site of subduction along the southern margin of the Indian subcontinent.

Detailed survey of the ocean bottom structure in the Central Indian Ocean Intense Deformation Zone: Tectonic implications

Abstract New bathymetric and seismic reflection data on the bottom structure of a small area in the Intense Deformation Zone (IDZ) of the Central Indian Ocean show highly deformed crustal blocks. The blocks seem to be controlled by E-W trending faults reflected in the ocean bottom topography. The data also suggest N-S trending fractures. Structural analysis of the data sets confirm a complex mosaic-block framework of the deformation zone. Tectonic fragmentation occurs on a smaller scale than that suggested earlier. The presence of secondary strongly-offset localised rises suggests an advanced stage of deformation in this segment of the IDZ.

Some new observations on the intra-plate deformation in the Central Indian Basin (CIB)

Abstract Single channel airgun seismic reflection data from equatorial region of the Central Indian Basin (CIB) revealed some new information on the structure and stratigraphy of the deformed blocks in this area. The data from five (N-S and E-W) traverses and from a detailed grid over a 45 × 45 km block, covering the area between 2°N and 2°S, and 81° and 84°E, indicate (1) a NE-SW trend for the faulted deformed blocks; (2) presence of fracture zones older than and unrelated to the tectonics associated with the process of deformation; (3) an older age (17 Ma) for the base of the Bengal Fan turbidites; and (4) a thin layer of transparent probably pre-fan (pelagic) sediments immediately above the acoustic basement.

Radiocarbon dates of plant detritus from sedimentary deposits of drill hole Bav-480, Pechora Sea

Data obtained during cruises 7, 11, 13, and 14 of R/V Akademik Sergei Vavilov (1990, 1997, and 1998) contributed a lot to reconstruction of paleogeographic environments that existed in the Barents Sea during the last glaciation maximum. This was possible owing to extensive acoustic profiling with Parasound equipment and use of a new approach to study of the upper part of the sedimentary cover. It is established that during the last glaciation maximum the Barents Sea was substantially smaller. It was almost completely surrounded by ice caps that descended to the shelf from glaciation centers on the land. The sea was connected with the World Ocean only by a narrow strait corresponding to the Bear Trough. Sea areas free of bottom glaciers were covered with floating ice: either by old pack ice or by plates of widespread shelf glaciers. Particular glaciomarine sediments accumulated under the floating ice. Destruction of the shelf glaciers during deglaciation resulted in formation of anomalously thick bottom sediment layers. The Pechora Sea shelf appeared to be above the sea level and was eroded by rivers. During entire Late Quaternary and, probably, the earlier epoch, the South Novaya Zemlya Trough represented an area of continuous marine and glaciomarine sedimentation.