Fault reactivation in the central Indian Ocean and the rheology of oceanic lithosphere

The intraplate deformation in the central Indian Ocean basin is a well-known example of a deviation from an axiom of plate tectonics: that of rigid plates with deformation concentrated at plate boundaries. Here we present multichannel seismic reflection profiles which show that high-angle reverse faults in the sediments of the central Indian Ocean extend through the crust and possibly into the uppermost mantle. The dip of these faults, which we believe result from the reactivation of pre-existing faults formed at the spreading centre, is ˜40° in the basement, which is consistent with the distribution and focal mechanisms of earthquakes on faults now forming at spreading centres. This style of deformation, coupled with the observation of large earthquakes in the mantle lithosphere, indicates that brittle failure of the oceanic lithosphere may nucleate in the vicinity of the brittle/ductile transition and propagate through the crust

Sediment Velocities and Deep Structure from Wide-Angle Reflection Data around Leg 116 Sites

The reduction of six wide-angle reflection profiles shot within the two fault blocks visited by Ocean Drilling Program (ODP) Leg 116 in combination with the ODP sonic logs has produced a velocity-depth structure for this area. The sediment velocity increases from 1.6-1.7 km/s in the near surface to 3.4-3.5 km/s immediately above basement with a velocity gradient of 0.75/s. A depth converted seismic reflection profile suggests that the pre-deformational basement surface was similar to the abyssal hill topography developed in the Pacific Ocean. A velocity for the top of oceanic layer 2 of 4.1 km/s was identified as layer 2A. Assuming a velocity gradient of 0.7/s, an estimate of layer 2 thickness was obtained of 1.5 km. It is possible to interpret residual depth anomalies in terms of a layer 3 that may be thinner than for normal oceanic crust

Fault reactivation in the central Indian Ocean and the rheology of oceanic lithosphere

The intraplate deformation in the central Indian Ocean basin is a well-known example of a deviation from an axiom of plate tectonics: that of rigid plates with deformation concentrated at plate boundaries. Here we present multichannel seismic reflection profiles which show that high-angle reverse faults in the sediments of the central Indian Ocean extend through the crust and possibly into the uppermost mantle. The dip of these faults, which we believe result from the reactivation of pre-existing faults formed at the spreading centre, is ˜40° in the basement, which is consistent with the distribution and focal mechanisms of earthquakes on faults now forming at spreading centres. This style of deformation, coupled with the observation of large earthquakes in the mantle lithosphere, indicates that brittle failure of the oceanic lithosphere may nucleate in the vicinity of the brittle/ductile transition and propagate through the crust

Channels, echo character mapping and tectonics from 3.5kHz profiles, distal Bengal Fan

The distal parts of the Bengal Fan are spectacularly affected by tectonic deformation related to the diffuse plate boundary between the Indian and Australian plates. Here we use 3.5kHz and 12kHz echosounder profiles, seismic reflection profiles and piston core results to examine sedimentary processes and their relationships to tectonism within an area (78º-82ºE, 0º-6ºS) just to the south of Ocean Drilling Program Leg 116 sites.Echo character mapping was completed using echosounder data in conjunction with results from piston coring, and a total of five different echo types have been recognised. Four of these fall into the echo character classification scheme developed by Damuth (1980a), whilst the fifth is believed to represent hemiturbidite deposits. Several types of submarine channel were also identified from echosounder data and a correlation between echo type and channel location can be seen. Their abundance, erosional and/or depositional nature together with a complex meandering and bifurcation pattern across a wide region of average gradient around 1/km, are all features characteristic of a broad channel termination zone on a large elongate fan. It is clear, therefore that the Bengal Fan extends beyond 6ºS.Active faulting in the area has led to the development of an irregular topography of low rounded hillocks that interfere with incoming turbidity currents. This has resulted in ponding between highs rather than lobe construction, thinning and pinching out of turbidites against the flanks of local relief, flow lofting and hemiturbidite drape, and common small-scale slumping. In some cases uplifted channel segments can be seen abandoned and partially filled. <br/

Continental margin fault pattern mapped south-west of Ireland

A SIMPLE model for continental basement structures at rifted continental margins comprises large fault blocks which trend approximately parallel to, and step down towards, the continental–ocean boundary (for example, see ref. 1). These blocks may be cut by faults which strike across the margin, and, in many theoretical discussions, are shown as being separated from the true oceanic crust by an intermediate zone (see transitional crust of Fig. 3, ref. 2). On many rifted margins these features are deeply buried by young sediments and cannot be stutied in detail. On Goban Spur (Fig. 1), a marginal plateau south-west of Ireland, the young sediment cover is abnormally thin, however, and we have been able to map in detail a 150 km wide continental basement fracture pattern of horsts and grabens using a simple seismic reflection system (160 inch3 air-gun and two-channel hydrophone array). We also suggest a location for the continent–ocean boundary between the Spur and Porcupine Abyssal Plain. There are few previously published data from Goban Spur relevant to our study, although valuable sampling3 and geophysical3–5 results have been obtained north and south of the area

The photochemical mechanism of a B12-dependent photoreceptor protein

The coenzyme B(12)-dependent photoreceptor protein, CarH, is a bacterial transcriptional regulator that controls the biosynthesis of carotenoids in response to light. On binding of coenzyme B(12) the monomeric apoprotein forms tetramers in the dark, which bind operator DNA thus blocking transcription. Under illumination the CarH tetramer dissociates, weakening its affinity for DNA and allowing transcription. The mechanism by which this occurs is unknown. Here we describe the photochemistry in CarH that ultimately triggers tetramer dissociation; it proceeds via a cob(III)alamin intermediate, which then forms a stable adduct with the protein. This pathway is without precedent and our data suggest it is independent of the radical chemistry common to both coenzyme B(12) enzymology and its known photochemistry. It provides a mechanistic foundation for the emerging field of B(12) photobiology and will serve to inform the development of a new class of optogenetic tool for the control of gene expression