Paper No. 70-12
Presentation Time: 11:00 AM
NEW AGE CONSTRAINTS FROM LAURENTIA’S MIDCONTINENT RIFT CALIBRATE THE TIMING OF RIFT CESSATION AND INVERSION, PROVIDING INSIGHTS INTO FAR-FIELD CONTROL ON THE GEODYNAMICS OF RIFT FAILURE
Despite being a prominent continental-scale feature on its way to lithospheric separation, the North American Midcontinent Rift failed to dismember Laurentia and subsequently underwent structural inversion. The record of syn-rift and post-rift tectonics is well-preserved, enabling significant insight into rifting, its cessation, and the compressional inversion that followed. However, the ages of the sedimentary rocks that record the interval of post-rift thermal subsidence and syn-compressional sedimentation are poorly constrained. Such constraints on the age of inversion are critical for constraining far-field effects of orogenesis and processes attributed to the rift’s duration and failure. The Keweenaw Fault in northern Michigan (USA) is a major structure associated with rift inversion; it thrusts ca. 1093 Ma rift volcanics and >4 km of overlying rift-related sedimentary rocks of the Oronto Group atop the post-rift Jacobsville Sandstone, which is folded in the footwall. To refine previous low-precision constraints on the timing of rift duration and cessation, we paired LA-ICP-MS and CA-ID-TIMS U-Pb detrital zircon dates on samples from the Oronto Group and from the Jacobsville Sandstone. In combination with these maximum depositional ages, we produced LA-ICP-MS U-Pb dates on vein calcite from the Keweenaw Fault. From the Jacobsville Sandstone, the youngest detrital zircon dated by CA-ID-TIMS at 992.51 ± 0.64 Ma (2σ) redefines its maximum depositional age and overlaps a U-Pb LA-ICP-MS date of 985.5 ± 35.8 Ma (2σ) on late-kinematic calcite veins within the brecciated Keweenaw Fault zone. These geochronological data constrain deposition of the Jacobsville Sandstone and final reverse faulting to have occurred during the ca. 1000 to 980 Ma Rigolet Phase of the Grenvillian orogeny, demonstrating that deformation propagated >500 km into the continental interior during the time of Grenville Front development. Maximum age constraints from the upper Oronto Group calibrate maximum age constraints on the timing of earlier inversion and a 35 Myr minimum duration related to Midcontinent Rift thermal subsidence prior to rift cessation and initial rift inversion. Through far-field effects that propagated far into the continental interior of Laurentia, the continent-continent collision of the Grenvillian orogeny played a critical role in intracratonic rift failure.