Abstract

The Ramapo fault system forms the northwestern boundary of the Newark Triassic basin in New York and adjacent northern New Jersey, and is commonly attributed to Mesozoic crustal fracturing. However, the detailed geology in the vicinity of this fault indicates an earlier, complex tectonic ancestry, perhaps dating from late Precambrian time. Intense cataclastic effects found in Precambrian to Middle Ordovician rocks on both sides of the Ramapo fault are absent from the bordering Triassic rocks of the footwall block. Significant post-Middle Ordovician right-lateral-transcurrent faulting is recorded by actual offset of distinctive units and by the megascopic fabric in mylonite zones. The total cumulative offset since the late Precambrian on these faults is unknown but could be large. Right-lateral displacement of 4 km is indicated for the Canopus Valley area. Intrusive relationships of probable late Precambrian to Late Ordovician diorite plutons and dikes, as well as pegmatites of probable Devonian age, help date the strike-slip movements as Paleozoic and older.

Block faulting along the Ramapo fault in the Lower Ordovician affected Middle Ordovician sedimentation and resulted in unconformable relationships between a Middle Ordovician gneiss-boulder conglomerate (Bucher, 1957) and Precambrian gneisses on the upthrown block west of the fault north of Peekskill, New York.

Repeated movements of the Highlands block during Late Triassic time produced the coarse clastic fanglomerates of the Hammer Creek Formation adjacent to the Ramapo fault. This syndepositional faulting was followed by post-Brunswick downdropping along the Ramapo fault south of the Hudson River. Recent seismic activity along this fault system reported by Page and others (1968) suggests that the fault may still be active. These observations suggest a possible tectonic longevity of some 700 m.y. for the “Triassic” border fault system here.

This fracture system had deep crustal connections along which basic magma rose toward the surface in Late Precambrian, Late Ordovician (Cortlandt), and Late Triassic time (Palisades sill and Triassic flows). The Ramapo fault also marks the boundary between tectonic blocks having different Paleozoic deformational histories. These observations, along with the remarkable longevity, suggest that this fault zone may be part of a fundamental crustal fracture system that was operative during formation of the Appalachian orogen.

The ideas presented here require a degree of tectonic permanence not commonly reported in orogenic belts. Activity along this fracture system in pre-Middle Ordovician time as documented here suggests strongly that the northern end of the Reading Prong is not allochthonous in the sense proposed by Isachsen (1964).

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