ABSTRACT: Structural Style and Morphology of Slide Complexes in Homoclinal Settings: Some Clues to Origin and Process

D. W. O'Leary

Many large submarine slides of homoclinal strata, particularly those in passive margin settings, have common structural and morphological characteristics. These include intrastratal deformation within the uppermost few hundred meters of sediment updip from the scarp and beneath the slide scar; laterally extended polygonal scarps and extensive normal faulting in the scarp area; and nearly complete disaggregation and removal of the detached mass and emplacement as laterally well

defined debris sheets on the rise. Displacement and emplacement occur on surfaces that differ in inclination by no more than 6° to 8°.

Homoclinal strata, a typically deep-water environment (>500 m head), and polygonal scarps indicate that these slides were not triggered by depositional loading on oversteepened slopes, but that they involve indurated, presumably drained sediment laid down in a stable environment. The very low slopes indicate that, without some additional condition, unrealistically low levels of effective stress are required to exceed factors of safety typical for this environment, or that failure is triggered by exceptionally large magnitude earthquakes. These observations, and especially the presence of intrastratal deformation, suggest that mass movement is preconditioned by creep that occurs in buried weak layers within the homoclinal section.

Long-term strength degradation and loss of frictional bonding along weak layer contacts caused by creep can make a homoclinal section increasingly (and continually) susceptible to earthquake shock with concomitant transient drops in effective stress. Indurated layers would thereupon fail in tension, and the mass of broken rock would be transported to the continental rise with little admixture of sea water.

AAPG Search and Discovery Article #90097©1990 Fifth Circum-Pacific Energy and Mineral Resources Conference, Honolulu, Hawaii, July 29-August 3, 1990