Reprocessing of Multichannel Seismic Data off Guatemala for an IPOD Transect (Final Report to Joint Oceanographic Institutions, Inc.)
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Slope deposits drilled during Leg 67 were detailed in redisplayed seismic records after the leg. These deposits are of significantly lower seismic velocity and probably lower density than the underlying basement to indicate a contact between rocks of differing consolidation and not a continuous sedimentary sequence. The slope deposits cover basement terranes of 3 different topographies. The shelf edge is an arch whose seaward flank forms a steep (up to 15°) upper slope. The mid-slope area has a rugged topography covered by thick slope deposits. The lower slope is relatively smooth except where broken locally by benches. The upper and middle slope areas are associated with strong magnetic anomalies and rare landward dipping reflections truncated by the rough surface. We explain the rough mid-slope topography by subareal erosion succeeding the first uplift of this area in the Paleocene and prior to major subsidence in the early Miocene. The present slope deposits then covered the trench landward slope, perhaps coincident with the increased arc volcanism indicated by ash layers, and thus the present period of subduction. The subducted ocean crust has a distinct linear topography of hundreds of meters relief that seems to disappear beneath the landward slope of the trench along with most of the ocean basin and trench sediment. The almost passive assimilation of oceanic material without significant accretion in the late Neogene argues for significant decoupling at the front of the subduction zone. A base of gas hydrate reflections can be identified in many of the redisplayed seismic records off Guatemala. Base of hydrate reflections are most common where slope deposits are thick and the reflections have not been identified in the underlying acoustic basement. This is consistant with the geochemical evidence that gas hydrate has its source in the organic rich slope sediment. The hydrate depth and temperature measurements in drill holes indicate a temperature gradient similar to that measured across the Japan Trench.