RELATIVE TIMING AND METAMORPHISM OF LATE-STAGE, TRANSTENSIONAL SHEAR ZONES IN THE EASTERN ADIRONDACK HIGHLANDS

The eastern Adirondack Highlands consist of Mesoproterozoic anorthosite-mangerite-charnockite-granite (AMCG) metaigneous rocks that intruded older, mostly metasedimentary rocks during the mid to late Shawinigan Orogeny (~1160-1150 Ma). All rocks were subsequently metamorphosed to upper amphibolite to granulite facies conditions during the 1080-1050 Ma Ottawan Orogeny. New mapping reveals four periods of deformation: D1 produced rarely preserved isoclinal folds in the paragneiss and marble; D2 produced the dominant gneissic fabric and deformed all the Proterozoic units in the map area; D3 involved upright folding, doming, and pegmatite intrusions; and D4 is expressed by the development of km-wide, SE-striking, transtensional shear zones, boudinage, and pegmatite intrusions.

The Cheney Mountain shear zone is a newly mapped, 1-2 km-wide, 14 km-long, left-lateral, ductile shear zone that strikes SE from Lincoln Pond to Port Henry and was first recognized in new aeromagnetic and lidar data. The shear zone is steeply dipping (55°- 85° to the SW) and mineral lineations plunge around 30° to the SW. Transposed rocks in the hanging wall comprise marble, paragneiss, and Lyon Mountain Granite Gneiss, and the AMCG suite in the footwall. Within the shear zone, numerous slivers of mostly AMCG rocks are separated by anastomosing high-strain zones. Pegmatite commonly filled, outcrop-scale, conjugate shear zones.

Metamorphic conditions during D4 favored the stability of amphibole and biotite at the expense of pyroxene in metaigneous rocks. Garnet is deformed and elongated, and exhibit replacement by amphibole and biotite in gabbroic rocks. However, in more felsic rocks, garnet is deformed, but texturally appears more stable. In paragneiss, sillimanite and biotite rim garnet.

The development of the late-stage regional shear zones (D4) was likely due to the continuation of extensional doming and uplift from upper amphibolite facies conditions at the end of the Ottawan Orogeny. The Cheney Mountain shear zone is part of a regional set of transtensional conjugate shear zones. So far, the northwest trending shear zones appear consistently sinistral, while northeast trending shear zones are right lateral.