Late Paleozoic strike-slip faults and related vein arrays of Cape Elizabeth, Maine

摘要

Strike-slip faults and related quartz vein arrays of Late Paleozoic-age cut gently-dipping metasedimentary rocks at Cape Elizabeth in southern coastal Maine and formed in response to regional dextral shearing along the Norumbega fault system. Vertical quartz veins up to 20 m wide and 10s of meters long were emplaced orthogonal to the local shear zone-parallel elongation fabric, reflecting strain partitioning during transpression. Earlier veins were reoriented by clockwise rotation toward this NE-trending regional shear direction. The later brittle strike-slip faults are oblique to the regional shear direction and interpreted as a 10-km-scale R-shear array on the southeast flank of the Norumbega fault system. These left-stepping en echelon fault zones consist of the three Two Lights fault zones (～ 200 m lengths and up to ～ 5 m displacements) and the Richmond Island fault zone (～ 1.6 km length and ～40 m displacement). Displacements on these fault zones have developed fine-grained silicified, obliquely-foliated and laminated cataclasites and locally, millimeter-thin pseudotachylyte fault and injection veins. Individual fault core zones are up to 10s of centimeters thick as part of several complex anastamosing zones of faulting 10s of meters wide. Initial segments within each fault zone are typically terminated with oblique extension fractures in horsetail configurations. The left-stepping en echelon relationships between these segments led to dominantly contractional step-over zones where P-shear linkages created a through-going fault that truncated the ends of the earlier-formed terminated segments. This linkage-growth model for fault zone evolution works toward larger scales and longer fault lengths as displacement accumulates, within a limiting maximum displacement/length ratio characteristic of the host lithologies. Length-frequency data for fault segments within these zones suggest a transition to linkage-dominated growth once fault segments were longer than ? 15 m. Continued displacement was accommodated along the P-shear linked en echelon faults through imbrication, contractional duplexing and adhesive wear on the outcrop-scale. Core zone processes on the micro-scale reflect cataclasis and frictional sliding during coseismic slip as well as cataclastic flow and pressure solution during post-seismic creep. The development of foliated-to-laminated cataclasite was accompanied by pore volume collapse, pressure solution and fluid expulsion that, in turn, triggered the development of the late fault-related quartz vein arrays.