A shear zone origin for Alleghanian (Permian) multiple deformation in eastern Massachusetts

摘要

The area around Worcester, Massachusetts, has been used to determine which deformational and metamorphic features are due to the Alleghanian (Permian) orogeny and which are pre‐Alleghanian. Isolated, fault‐bounded inliers of Carboniferous rocks display evidence of a single metamorphism and the formation of two prominent cleavages. The first cleavage formed synchronously with metamorphism. Pre‐Carboniferous metasedimentary rocks have been affected by two metamorphisms and contain three prominent cleavages. The initial cleavage formed during the first metamorphism and the second cleavage formed during the second, retrogressive metamorphism. Thus the second two cleavages and the second metamorphism in pre‐Carboniferous rocks are interpreted as Alleghanian. Normal displacements on two distinct faults are bracketed by the formation of the first and second Alleghanian cleavages. The initial faulting, along the newly defined Wachusett mylonite zone (WMZ), formed a wide zone of ductile mylonites which dips moderately to the northwest and contains elongation lineations which trend northwest. The second faulting, along the Clinton‐Newbury fault (CNF) occurred along a steeply inclined plane which cuts the WMZ mylonites and contains a thin zone of phyllonites and mylonites which have elongation lineations which trend west. Alleghanian cleavages and metamorphism are confined to a mappable zone which is approximately 15 km wide where well defined. This zone is interpreted as a ductile shear zone which moved twice, forming the first and second Alleghanian cleavages. Early‐formed, pre‐Alleghanian metamorphic minerals are retrograded to hydrous phyllosilicates, and new hydrous minerals formed in the shear zone, indicating that it was a pathway for fluid flow. The initial motion was left‐lateral with a thrust component. The second cleavage formed during top‐to‐the‐northwest normal displacements. Thus the history of Alleghanian tectonism in this area began with sinistral faulting and then included three displacements along normal faults or shear zones. This suggests that the strain history included two distinct episodes, with the second experiencing slightly different strain orientations at different times, resulting in first WMZ normal motion, then CNF normal motion and finally normal displacements related to the second Alleghanian cleavages. This history agrees well with other work on the nature of the Alleghanian orogeny in the no