Using multi-scale structural and petrological analysis coupled with zircon and monazite SIMS and in-situ EPMA geochronology to document the evolution of a mid-crustal transpression system: a case study from the Northern Appalachians, U.S.A.

Abstract:

Three-dimensional transpressional strain is commonly associated with zones of oblique convergence, rather than ideal 2D simple shear or pure shear. Consequently, a considerable body of modeling has been aimed at understanding the progressive evolution of transpression, which has been used to explain an assortment of structures observed in natural settings. The basic tenants of most models involve simultaneous strike-slip and shortening, which provide the underlying mechanism for a constantly evolving finite strain geometry and magnitude. Despite the obvious temporal-dependence, very few studies have evaluated timescales of transpression. In the Northern Appalachians, the Bronson Hill arc and Central Maine basin of southern New England largely reflect highly oblique dextral transpression. Fabrics were initially characterized by strong foliations, subhorizontal lineations, and dextral kinematics, all of which are present in 360-354 Ma tonalite, diorite, and granite intrusions, the youngest placing a maximum age on transpression. As strains accumulated, fabrics began to reflect the increasing manipulation of the shortening component, marked by tightening of foliations, closed to isoclinal folding, and reverse high strain zones; stretching lineations changed in orientation to steeply plunging parallel to dip, while older pre-existing subhorizontal lineations were rotated. Syntectonic monazite and metamorphic zircon nucleated episodically throughout this time. Y-enriched monazite nucleated at 330 Ma along with fabric-forming biotite and sillimanite, and place a minimum age on the development of dip-parallel lineations. Mineral assemblages and associated ages document retrograde cooling attending deformation from partial melting at 355-350 Ma, to sillimanite grade at 330 Ma, below the Ar closure temperature for amphibole of 500°C at 326-314 Ma, and into biotite grade deformation as young as 295 Ma. Collectively, structures, fabrics, mineral assemblages, and petrochronology reveal that mid-crustal transpression in the Northern Appalachians was sustained for 60 million years, while the fabric geometry evolved only over the course of the first 25 million years from ~355 Ma to ~330 Ma at relatively high temperatures. The common assumption of extrusion is also valid even in the middle crust.