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

Isolating the respective roles of factors responsible for the initiation an d localization of fault zones remains one of the more important goals of re search in neotectonics, The frontal decollement zone of the northern Barbad os accretionary prism provides several important clues as to how the interw oven variables of clay mineralogy, fluid flow, chemical interactions, and s ediment physical properties affect strain localization. This plate-boundary fault is centered at a lithologic contact between Miocene claystone and fi ne-grained Oligocene turbidites, The fault zone is nearly 40 m thick (at Oc ean 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 decollement. There is a consistent increase in percent smectite with distance above the base of the decollement, but the top of the decollement is poorly defined by clay mineralogy, The intrinsic mechanical weakness of strata with abundant smectite-group clays probably i nfluences where the fault tip propagates into the undeformed stratigraphy o f the Atlantic abyssal plain. A second inherited parameter is the local abu ndance of radiolarians, which contribute to higher than normal porosities. Sediment shear strength also decreases because pore pressure within the fau lt zone is significantly greater than hydrostatic, The principal cause of e xcess pore pressure seems to be updip fluid advection; in theory, however, decreases in pare-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 z ones of deeper seated dehydration reactions. If fresher pore water migrates to the propagating tip of the decollement, its arrival should increase sme ctite swelling and reduce the shear strength of the mudstone even more. The location and evolution of the decollement, 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 c hanging fluid chemistry.