DEFORMATION WITHIN PLATE INTERIORS: EXAMPLES FROM EASTERN AFRICA AND THE MIDCONTINENT OF THE UNITED STATES (Invited Presentation)

The documentation of the thermal history of rocks provides important constraints on the timing, intensity and tempo of major events that shaped the earth's surface such as mountain building, volcanism, and basin formation. In this talk, I will present current research we are conducting focusing on the application of mid- and low-temperature thermochronometers (particularly apatite and zircon U-Th/He) aimed at constraining:

(1) the exhumation history from the time of the Pan-African orogeny to the formation of the Paleozoic-Mesozoic Mekele intracontinental basin that unconformably overlie the Neoproterozoic successions in northern Ethiopia. The basin presents a unique opportunity in which sedimentary formations and structures associated with intracontinental basin are exposed on the surface. Intracontinental basins often provide uninterrupted geologic record that is otherwise missing in regions characterized by active tectonic processes. There is extensive clastic deposition throughout northern and NE Africa following post-orogenic erosion and exhumation of the Neoproterozoic mobile belts. Thermochronologic data can be used to test whether this exhumation happened early right after the Pan-African orogen or later during Ordovician glaciation.

(2) the timing of exhumation associated with inversion of the Midcontinent Rift along the Montreal River monocline in the Lake Superior region. The Midcontinent Rift developed in the late Mesoproterozoic and is hypothesized to have failed due to compression associated with the Grenvillian orogeny. Deformation led to exhumation of a ~25 km thick crustal cross-section known as the Montreal River monocline. Rb-Sr dates on biotite from previous study suggest that this exhumation occurred ca. 1060 Ma (Ottawan Phase), much earlier than the latest stages of the orogeny ca. 980 Ma (Rigolet Phase). The enormity of the exhumation and lack of comprehensive data makes constraining the timing quite important both for understanding the Grenvillian orogeny and the geodynamics associated with failure of the Midcontinent Rift. Also, proximity of the locality to the "great unconformity" that is the unconformity with Cambrian sediments atop the crystalline basement, makes it interesting for understanding the dynamics of exhumation prior to the Sauk transgression.