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Abundance of smectite and the location of a plate-boundary fault, Barbados accretionary prism

Xinhua Deng^*,1 and Michael B. Underwood^,1

¹ Department of Geological Sciences, University of Missouri, Columbia, Missouri 65211, USA

Isolating the respective roles of factors responsible for the initiation and localization of fault zones remains one of the more important goals of research in neotectonics. The frontal décollement zone of the northern Barbados accretionary prism provides several important clues as to how the interwoven variables of clay mineralogy, fluid flow, chemical interactions, and sediment physical properties affect strain localization. This plate-boundary fault is centered at a lithologic contact between Miocene claystone and fine-grained Oligocene turbidites. The fault zone is nearly 40 m thick (at Ocean Drilling Program Site 948), and its upper part passes through smectite-rich deposits. A sharp minimum in percent smectite and a maximum in percent illite mark the base of the décollement. There is a consistent increase in percent smectite with distance above the base of the décollement, but the top of the décollement is poorly defined by clay mineralogy. The intrinsic mechanical weakness of strata with abundant smectite-group clays probably influences where the fault tip propagates into the undeformed stratigraphy of the Atlantic abyssal plain. A second inherited parameter is the local abundance of radiolarians, which contribute to higher than normal porosities. Sediment shear strength also decreases because pore pressure within the fault zone is significantly greater than hydrostatic. The principal cause of excess pore pressure seems to be updip fluid advection; in theory, however, decreases in pore-fluid salinity and porosity collapse should increase the amount of physicochemical stress generated by expandable clay minerals. The imported fluid is unusually low in salinity because it has migrated from zones of deeper seated dehydration reactions. If fresher pore water migrates to the propagating tip of the décollement, its arrival should increase smectite swelling and reduce the shear strength of the mudstone even more. The location and evolution of the décollement, therefore, are controlled by a complicated interplay of static factors inherited from the abyssal Atlantic stratigraphy and dynamic factors associated with episodic fluid flow and changing fluid chemistry.

Key Words: Barbados • décollement • Ocean Drilling Program • shear strength • smectite

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