CONSTRAINTS ON TIMING OF DEFORMATION IN THE CABOT FAULT ZONE AND ON THE LITTLE GRAND LAKE FAULT IN WESTERN NEWFOUNDLAND

Tectonic interaction between the Laurentian margin (Humber Zone) and peri-Laurentian terrains present in the adjacent Dunnage Zone of western Newfoundland has been studied using structure and geochronology. Preliminary interpretation of data suggests the following chain of events. During Early to Middle Ordovician, the Little Grand Lake Fault accommodated north-directed thrusting within the Dunnage zone, emplacing amphibolite-facies arc plutonic and associated sedimentary rocks of the Dashwoods Subzone above greenschist-facies volcanic rocks of the Notre Dame Subzone. This process may have been coeval with initial loading of the Laurentian margin and initial emplacement of ophiolites during the Late Arenig. Subsequently during the Middle Ordovician, rocks of both Dunnage subzones are inferred to have been thrust westwards above rocks of the internal Humber Zone. This is consistent with the loading history of the Humber Zone and the presence of a thrust slice with Dashwoods-like rocks (Disappointment Hill Complex) juxtaposed with internal Humber zone rocks. The thrusting occurred prior to Caradoc-age oblique normal movements on the Cabot Fault, which brought Humber Zone rocks up relative to the adjacent Dashwoods Subzone. A late syn-deformational pegmatite dyke yielded a lower intercept U-Pb zircon age of 456 ± 12 Ma, constraining the earlier amphibolite-facies deformation in the Cabot Fault Zone, as well as the minimum age of movement on the Little Grand Lake Fault, which was truncated by the Cabot Fault Zone. This result agrees with a lower intercept U-Pb monazite age of 461 ± 2 Ma for a strongly deformed granite that occurs in the Cabot Fault Zone, but has not been observed elsewhere. Thrusting within the Humber Zone continued until at least the Late Silurian, during which thrust sheets were emplaced further westwards, including the Disappointment Hill Complex. During the Late Silurian – Devonian (?) the Cabot Fault Zone accommodated progressively more dextral strike-slip. This phase of deformation, which moved the Dunnage Zone down relative to the Humber Zone, cuts the earlier formed thrust stack and produced rootless outliers of Dunnage Zone rocks, such as the Disappointment Hill Complex, in the Humber Zone.