Synthesis of the Barbados décollement seismic reflection response from drilling-based geophysical observations and physical properties

Abstract

A three-dimensional seismic reflection experiment has revealed a distinctivedistribution of waveforms and amplitudes associated with the decollementreflection in the Barbados Trench. One objective of Ocean DrillingProgram Leg 156 was to calibrate this seismic reflection response,which was thought to be related to variations in fault zone properties.Site 948 is characterized by a normal polarity reflection that iscorrelated to the decollement. Sampling and logging reveal a 40-m-thickfault zone, with density that increases across the zone as the claymineralogy changes from smectite rich to illite rich downhole. Asynthetic seismogram, derived from the logging data, shows that thelithology-driven density shift is responsible for generating thenormal polarity reflection even though the log, packer, and long-termpressure measurements confirm the presence of abnormally high pressures.The objective of Site 947, to characterize the more common, high-amplitudenegative polarity waveform, was not completed due to technical difficulties.However, a density log, run at the beginning of the cruise, did reveala 14-m-thick 1.6 g/cm3 layer 540-554 m below seafloor (mbsf). Thebit reached within 60 m of the decollement, and the density log wentto 565.5 mbsf. The low-density layer correlates with a fault splayoff the decollement, having similar seismic characteristics to thatof the negative polarity decollement and can serve as an analogue.Although it is unsatisfying not to have directly calibrated the decollementat Site 947, the fault splay revealed characteristics that are consistentwith modeling done before the cruise that required a 12- to 14-m-thicklow impedance layer to account for the observed waveform. These verylow densities are probably created by introduction of additionalfluids and fluidized sediments during hydrofracturing, which aremaintained by lithostatic pressure. The requirement for extremelylow density indicates that a >12-m-thick lithostatic pressured hydrofracturedzone occurs within the thicker decollement. The variations in theamplitude of the negative polarity waveform relate mainly to theamount of enhanced fluid content. The mapped pattern of amplitudessuggests a broad northeast-trending channel that possibly focusedfluid transport along the decollement. However, numerous fault splaysintersect the decollement, which may cause barriers that preventrapid fluid flow along the decollement.