Tectogenesis of the Hylas zone and eastern Piedmont near Richmond, Virginia

Abstract

Detailed structural and petrographic studies of the Hylas zone northwest of Richmond, Virginia, reveal that Late Paleozoic thrusting to the northwest produced a zone of protomylonite, mylonite and ultramylonite from felsic gneiss and amphibolite. The thrusting event, which initiated retrograde metamorphism to greenschist facies, carried an allochthonous plate of coarse-grained, locally porphyry tic Petersburg(?) Granite over a layered sequence of metamorphosed sedimentary and volcanic rocks now represented by biotite gneiss, biotite schist and amphibolite. Non-cataclastic rocks west of the Hylas zone have undergone at least two periods of deformation (DI,D2) prior to cataclasis. Prograde metamorphism (MI) was concurrent with deformation and is inferred to have occurred about 342 ± 70 m.y. ago. The Hylas zone deformation (D3) caused a pervasive cataclastic foliation (Sc) and a later widely spaced shear cleavage (Ss ) to be formed in rocks in the fault zone. Deformation was accompanied by retrograde metamorphism (m2) to greenschist facies mineral assemblages. Fracturing and high angle faulting (D4) was superimposed on the Hylas zone about 221 ± 6 m.y. ago under zeolite facies temperatures and pressures;laumontite, calcite and quartz crystallized in open spaces at this time. Sedimentation in the basin created by D4 faulting and downwarp occurred during Late Triassic time to form the Richmond Basin. Northwest-trending joints and faults appear to have formed post-221 m.y. ago and pre-Cretaceous time. Cenozoic reverse faulting along strike of the Hylas zone in the Coastal Plain of Maryland may indicate a continuation of this polytectonic zone to the north.