Tectonics of an extinct ridge-transform intersection, Drake Passage (Antarctica)

New swath bathymetric, multichannel seismic and magnetic data reveal the co mplexity of the intersection between the extinct West Scotia Ridge (WSR) an d the Shackleton Fracture Zone (SFZ), a first-order NW-SE trending high-rel ief ridge cutting across the Drake Passage. The SFZ is composed of shallow, ridge segments and depressions, largely parallel to the fracture zone with an 'en echelon' pattern in plan view. These features are bounded by tecton ic lineaments, interpreted as faults. The axial valley of the spreading cen ter intersects the fracture zone in a complex area of deformation, where N1 20 degrees E lineaments and E-W faults anastomose on both sides of the inte rsection. The fracture zone developed within an extensional regime, which f acilitated the formation of oceanic transverse ridges parallel to the fract ure zone and depressions attributed to pull-apart basins, bounded by normal and strike-slip faults. On the multichannel seismic (MCS) profiles, the igneous crust is well strat ified, with numerous discontinuous high-amplitude reflectors and many irreg ular diffractions at the top, and a thicker layer below. The latter has spa rse and weak reflectors, although it locally contains strong, dipping refle ctions. A bright, slightly undulating reflector observed below the spreadin g center axial valley at about 0.75 s (twt) depth in the igneous crust is i nterpreted as an indication of the relict axial magma chamber. Deep, high-a mplitude subhorizontal and slightly dipping reflections are observed betwee n 1.8 and 3.2 s (twt) below sea floor, but are preferentially located at ab out 2.8-3.0 s (twt) depth. Where these reflections are more continuous they may represent the Mohorovicic seismic discontinuity. More locally, short ( 2-3 km long), very high-amplitude reflections observed at 3.6 and 4.3 s (tw t) depth below sea floor are attributed to an interlayered upper mantle tra nsition zone. The MCS profiles also show a pattern of regularly spaced, ste ep-inclined reflectors, which cut across layers 2 and 3 of the oceanic crus t. These reflectors are attributed to deformation under a transpressional r egime that developed along the SFZ, shortly after spreading ceased at the W SR. Magnetic anomalies 5 to 5 E may be confidently identified on the flanks of the WSR. Our spreading model assumes slow rates (ca. 10-20 mm/yr), with slight asymmetries favoring the southeastern flank between 5C and 5, and t he northwestern flank between 5 and extinction. The spreading rate asymmetr y means that accretion was slower during formation of the steeper, shallowe r, southeastern flank than of the northwestern flank.