Drilling results from the Pacific margin of southern Mexico indicate that this region is characterized by a Neogene accretionary wedge, progressively emplaced against older, tectonically truncated continental crust. Accretion has occurred by both offscraping of sediments at the base of the trench slope and underplating of sediments at depth beneath the accretionary wedge and the continental crust. Mass-balance and incremental-uplift studies suggest that about one-third of the incoming sediment is subducted beneath the leading edge of the continental crust.
Piston and drill cores indicate that the trench is sand dominated and flanked by slopes covered principally by mud. A large submarine canyon bypasses sediment past the shelf and inner trench slope. The volume of sediment bypassed to the trench and adjacent lower and outer slope equals 5 to 6 times that deposited on the shelf, upper slope, and mid-slope. The muddy inner slope is characterized by foraminiferan-free mud below the calcite compensation depth (CCD) and foraminiferan-bearing mud above the CCD. The upper slope accumulates laminated mud within the oxygen minimum zone. The shelf is covered by sand and mud. Quartzofeldspathic sand compositions in the Leg 66 area reflect sources in the crystalline basement complex exposed along the coast.
Structural fabrics of Leg 66 cores from offscraped and overlying slope deposits show zones of inconsistent dip, stratal disruption, and scaly mudstone, characteristics of many mélange-wedges exposed on land. Deformation transgresses the boundary between the offscraped and slope deposits, demonstrating tectonic incorporation of the slope sediments into the accretionary wedge. The rate of deformation of the slope deposits diminishes rapidly landward from the trench. Deposits overlying the continental crust show dip patterns due to mesoscopic folding, as well as local spaced cleavage and faulting, but they show no stratal disruption or scaly mudstone Oblique-slip faulting predominates between the accreted wedge and the continental crust and may reflect decoupling of these two basement types.
Frozen sediment, probably bearing gas hydrate, was recovered above a bottom-simulating reflector at two sites.