Extensional duplexing in the York Cliffs strike-slip fault system, southern coastal Maine

Abstract

A 150 m-long sinistral strike-slip fault system, consisting of left-stepping en échelon fault segments, cuts deformed Paleozoic metasediments and early Mesozoic dike intrusions in coastal terrace outcrops in southern Maine. The longer fault segments follow the vertical bedding in the metasediments and accommodate approximately 1 m of displacement. Detailed mapping at scales down to 1:6, indicates that overlap zones between offset layer-parallel fault segments consist of high-dilation angular breccias, rounded attrition breccias and extensional duplexes where the offset segments straddle upright quartzite layers. In one 10 m-long duplex, R- and R′-shears cut the layering in the quartzite at 28° and 69°, respectively, relative to the dominant layer-parallel fault segments, creating a doubly-tapered imbricate R-shear lens segmentation structure. Individual imbricate lenses accommodate additional layer-parallel extension through conjugate X- X′-shear fractures with average orientations of 109° and 60°, respectively, relative to the dominant fault segments. Extensional fractures oblique to the dominant fault segments are rare within this duplex but commonly form distinctive “horsetail’ arrays at fault terminations that average 44° to the layer-parallel faults. Strain analysis shows that approximately 60–80 cm of measured strike-slip displacement is accommodated internally within this duplex as layer-parallel extensional strain using the R-shear and X- X′-shear fracture mechanisms. The strain distribution and observed cross-cutting relations suggest an outward symmetric growth of the duplex structure from left-stepping, sinistral fault segments that developed sequences of tip-to-plane R-shear overlap linkages during continued displacements. Cyclic R- R′- to X- X′-shear fracturing may relate to stress reorientation following R-shear ruptures during episodic growth of the duplex and incremental slip of the fault system.