A STRUCTURAL, GEOCHRONOLOGIC, AND ANISOTROPY OF MAGNETIC SUSCEPTIBILITY (AMS) STUDY OF THE BLACK LAKE SHEAR ZONE, ADIRONDACK LOWLANDS (GRENVILLE PROVINCE, NEW YORK)

The Grenville Province of northeastern North America is interpreted to consist of a complex collage of crustal blocks that were assembled from 1.4-0.9 Ga. However, granulite facies metamorphism and polyphase deformation has frequently obscured the boundaries between these zones, thus hindering our ability to reconstruct the tectonic evolution of the region.

The Black Lake shear zone has been proposed as a potentially important tectonic boundary in the Adirondack Lowlands 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 data and anisotropy of magnetic susceptibility (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, rocks in this zone are lineated. Regionally, two 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 indicate 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 shearing. 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 occurred during the late stages of the Shawinigan orogeny and likely records accretion of the Adirondacks to the margin of Laurentia during terrane assembly.