Initiation of Cordilleran Miogeocline of Western North America
William J. Devlin, Gerard C. Bond, H. K. Brueckner
A better understanding of the events that initiated the Cordilleran miogeocline has been obtained by combining quantitative subsidence analyses, geologic field studies, and isotopic analyses of rift sequence volcanics. Tectonic subsidence of post-rift Cambrian-Ordovician strata is thermal in form and began 575 ± 25 Ma. The initial steep slopes of the subsidence curves indicate high cooling rates, and when considered in light of finite rift models, they suggest that rifting could not have lasted more than 10-20 m.y. prior to the onset of thermal cooling. Together with a preliminary age of 762 ± 44 Ma for volcanics near the base of the Windermere Supergroup, these data indicate that the Windermere Supergroup does not represent the rift deposits that led directly t the onset of thermal subsidence. Instead, Windermere sedimentation was initiated by an earlier rift event, probably of regional extent. This event was part of a protracted, episodic rift history that culminated with final rifting and the onset of thermal subsidence in the latest Proterozoic-earliest Cambrian.
Geologic field studies in the southern Canadian Cordillera uncovered evidence for younger rifting, which agreed with the subsidence analyses. The Hamill Group unconformably overlies the Windermere Supergroup (rift onset unconformity) and contains coarse-grained, feldspathic sandstones at its base that pass up-section into mature quartzarenites. Similar sedimentologic evidence within the base of the Gog Group is interpreted to represent uplift of basement sources, followed by a transition to more stable tectonic conditions associated with the initial stages of post-rift thermal subsidence. The Hamill Group also contains evidence of syndepositional tectonism that was accompanied by the extrusion of mafic volcanics.
The block that rifted from North America has yet to be identified. Subsidence analyses do not distinguish between a passive margin formed by the rifting of a continental block, a continental fragment, or an island arc. Also, the relationship of older Proterozoic miogeoclinal assemblages to the rift history of the early Paleozoic passive margin remains problematic.
AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.