Structural analysis of progressive deformation within a complex strike-slip fault system : southern Narragansett Basin, Rhode Island
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Complex progressive ductile deformation on Dutch Island, Narragansett Basin, Rhode Island, is localized along a system of non-parallel strike-slip faults with opposite senses of motion. The structural intensity and complexity of this area can be attributed in part to the proximity of NE- and NNE-trending, preexisting basement faults bounding early horsts and grabens. Left-lateral faults in NNE to NE orientations correspond to R' riedel shears and right lateral faults in NE to ENE orientations correspond to right-lateral primary shears within an E-trending right-lateral transform system. Structural superposition of mesoscopic folds and incremental crenulation cleavages, formed due to episodic movement along these faults, indicates that left-lateral motion preceded right-lateral motion and that many NE-trending faults underwent first left-lateral and then right-lateral movement. As many as 8 sets of discrete superposed crenulation cleavages and 7 sets of discrete mesoscopic folds were developed at fault intersections on the island. Preservation of these structures can be attributed to the presence of a strong preexisting structural and metamorphic fabric which formed as a result of initial basin closure associated with the collision of the African and North American continents in the late Paleozoic. Study of geometry and overprinting relations of the fold and crenulation sets within adjacent shear zones served to track movement along individual faults through time. Up to 32 degrees of anticlockwise internal rotation on left lateral faults and clockwise rotation on right-lateral faults has occurred. Discrete crenulations form due to flexural slip at an angle to the shear plane and are subsequently tightened and rotated toward the principal extensional strain direction until flexural slip can no longer occur. A younger set then forms and overprints the preexisting, and presently locked, crenulation cleavage at an angle varying from 4 to 22 degrees. Structural analysis of the complex deformation on Dutch Island indicates that accurate tracking of fault motion within complex shear systems is feasible in areas in which overprinting relations between progressive deformations are preserved. In addition to the regional significance of the study, the structural styles and deformation patterns described on Dutch Island should be analogous to what might be expected in a larger block caught within a system of non-parallel, successively forming strike-slip faults.