The Black Lake shear zone: A boundary between terranes in the Adirondack Lowlands, Grenville Province

Abstract

The Black Lake shear zone has been proposed as a potentially important tectonic boundary between the Frontenac and Adirondack Lowlands terranes based on geologic and isotopic evidence. We present new structural, geochronologic and anisotropy of magnetic susceptibility (AMS) results from the Black Lake area that constrain the location, kinematics and timing of deformation in the region. Both structural and AMS results document a northeast trending zone of high strain between Black Lake and the Saint Lawrence River. Within this zone, strain is heterogeneous and includes a strongly developed sub-vertical northeast trending foliation. Less commonly, two orthogonal lineation sets are present; one that is sub-horizontal and another that is down dip. The foliation and granitic dikes are locally folded forming tight to isoclinal northeast trending folds. Kinematic indicators show thrust deformation in areas of down-dip lineation but are mixed in areas of sub-horizontal lineation with a slight majority suggesting left-lateral shear. These results suggest major northwest directed shortening accommodated by flattening and ductile thrust faults, possibly with a component of sub-horizontal shear. The two lineations may have formed during discrete episodes or synchronously during transpression. U–Pb SHRIMP geochronology on igneous titanite from deformed dikes, in combination with previous geochronology, show that significant deformation occurred from ca. 1170–1100 Ma. This deformation likely occurred during the late stages of the Shawinigan orogeny and may record accretion of the Adirondacks to the margin of Laurentia during terrane assembly.