ROLE OF LYON MOUNTAIN SERIES GRANITOIDS AS STRAIN SOFTENING AGENTS DURING LATE OTTAWAN-AGED EXTENSION ALONG THE CARTHAGE COLTON SHEAR ZONE

The Carthage Colton Shear Zone (CCSZ) is a major Grenville-aged tectonic boundary separating the Adirondack Highlands (ADKH) and Lowlands (ADKL) terranes. The CCSZ has a complex history recording early transpressional and late extensional deformation. During late Ottawan the CCSZ developed into a folded (corregated) detachment surface. Folding associated with transtensional movements along the CCSZ can be traced in both the hangingwall and footwall. These folds rotate the earlier shear fabric and are well expressed as regionally developed folds and as second order folds within the shear zone. Stretching lineations across these second order folds parallel fold hinge orientations suggesting that folding and shearing where contemporaneous. Syn- to post Ottawan (1045Ma) granitoids and associated pegmatites of the Lyon Mountain Series (LMG) intrude into the CCSZ along its exposed length. Selleck et al. 2005 demonstrates that these magmas intrude during active extension along the CCSZ. These granitoids make up a substantial volume fraction (10-25%) of the CCSZ footwall (ADKH) with lesser but poorly constrained volumes intruding into the adjacent hangingwall (ADKL). Intrusion of relatively large volumes of LMG magma into the CCSZ represents a substantial heat source and an effective strain softening agent that would facilitate slip along the detachment zone. Many LMG bodies occupy F-4 fold noses which are kineamatically related to transtensional movements along the CCSZ. Based on these observations, it seems likely that these granitoids preferentially intruded into developing folds during Ottawan extension. The intrusion of syn-extensional granite bodies where present should act to promote deformation along the CCSZ. The observed heterogeneous distribution of LMG, therefore, should drive differential slip along the length of the CCSZ leading to the formation of transfer faults. Several nearly orthogonal strike-to oblique- slip faults that cut the CCSZ have been mapped in a region marked by an abrupt change in the abundance of LMG and may represent a syn-extensional transfer zone. Reference: Selleck, B.W. McLelland J. Bickford M.E. (2005) Granite emplacement during tectonic exhumation; the Adirondack example, Geology: 33, 10, pp.781-784.