Cataclastic deformation in accretionary wedges: Deep Sea Drilling Project Leg 66, southern Mexico, and on-land examples from Barbados and Kodiak Islands
Stephen E. Lucas, J. Casey Moore, 1986. "Cataclastic deformation in accretionary wedges: Deep Sea Drilling Project Leg 66, southern Mexico, and on-land examples from Barbados and Kodiak Islands", Structural Fabrics in Deep Sea Drilling Project Cores From Forearcs, J. Casey Moore
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Microstructural studies of DSDP cores from the lower slope off southern Mexico (Sites 488 and 492) reveal intense cataclastic grain breakage (as much as 37% of grains) in structurally disrupted zones penetrating offscraped trench and slope deposits. In contrast, minimal grain breakage occurs in sands overlying the upper slope basement of southern Mexico and in sands from a reference site in the California Borderland. Grain breakage in the latter is consistent with that developed elsewhere due to burial. The enhanced cataclasis at Sites 488 and 492 correlates with their steep average bedding dips. A percentage of broken grains occurs in zones of coarse grain size at these two sites due to increased stress concentration and loading of grain boundaries. Experimental results and a mechanical analysis indicate that these cataclastic fabrics are not due to gravity sliding and are probably tectonic in origin.
Microstructural studies of rocks from the Barbados and Kodiak accretionary complexes show both cataclastic shear zones, defining and permeating sandstone boudins, and distributed grain breakage similar to that observed in DSDP cores. Early-forming concretions cross-cut the cataclastic shear zones, suggesting that they developed in partially lithified sediment.
Our observations suggest that the sands of accretionary complexes initially deform by particulate flow; individual grains are cushioned by elevated pore fluid pressures. With increased effective confining stress, distributive cataclasis occurs, producing a dense, poorly sorted sediment with an increased coefficient of friction. Continuing deformation and further increases in effective confining stress allows failure along discrete shear zones, which ultimately results in boudinage of sandstone along an anastomosing network of cataclastic shear zones.