Final inversion of the Midcontinent Rift during the Rigolet Phase of the Grenvillian Orogeny

Abstract

Despite being a prominent continental-scale feature, the late Mesoproterozoic North American Midcontinent Rift did not result in the break-up of Laurentia, and subsequently underwent structural inversion. The timing of inversion is critical for constraining far-field effects of orogenesis and processes associated with the rift’s failure. The Keweenaw fault in northern Michigan (USA) is a major thrust structure associated with rift inversion; it places ca. 1093 Ma rift volcanic rocks atop the post-rift Jacobsville Formation, which is folded in its footwall. Previous detrital zircon (DZ) U-Pb geochronology conducted by laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) assigned a ca. 950 Ma maximum age to the Jacobsville Formation and led researchers to interpret its deposition and deformation as postdating the ca. 1090–980 Ma Grenvillian Orogeny. In this study, we reproduced similar DZ dates using LA-ICP-MS and then dated 19 of the youngest DZ grains using high-precision chemical abrasion–isotope dilution–thermal ionization mass spectrometry (CA-ID-TIMS). The youngest DZ dated by CA-ID-TIMS at 992.51 ± 0.64 Ma (2σ) redefines the maximum depositional age of the Jacobsville Formation and overlaps with a U-Pb LA-ICP-MS date of 985.5 ± 35.8 Ma (2σ) for late-kinematic calcite veins within the brecciated Keweenaw fault zone. Collectively, these data are interpreted to constrain deposition of the Jacobsville Formation and final rift inversion to have occurred during the 1010–980 Ma Rigolet Phase of the Grenvillian Orogeny, following an earlier phase of Ottawan inversion. Far-field deformation propagated >500 km into the continental interior during the Ottawan and Rigolet phases of the Grenvillian Orogeny.

The late Mesoproterozoic to early Neoproterozoic Grenvillian orogeny and the assembly of Rodinia: Turning point in the tectonic evolution of Laurentia

“…The late Mesoproterozoic to early Neoproterozoic Grenvillian orogeny and the assembly of Rodinia: Turning point in the tectonic evolution of Laurentia 13 fields (Mueller, 2021). In summary, the data associated with the Keweenaw Fault relate the final contractional inversion of the Midcontinent Rift to the Rigolet phase of the Grenvillian orogeny (Hodgin et al, 2022), with data on other structures indicating earlier contraction during the Ottawan phase of the orogeny as well (Fig. 3).…”

“…Fluvial sandstones of the Jacobsville Formation were deposited unconformably atop these uplifted lithologies and were subsequently overthrust by Midcontinent Rift volcanic rocks along structures such as the Keweenaw Fault (Hinze et al, 1990;Cannon, 1994;Manson and Halls, 1994). A CA-ID-TIMS U-Pb date of 992.51 ± 0.64 Ma on detrital zircon from the Jacobsville Formation provides the maximum depositional age for the unit that, together with a low-precision U-Pb date of 985.5 ± 35.8 Ma on calcite from the Keweenaw Fault breccia, constrain this final stage of Midcontinent Rift inversion to have occurred during the Rigolet stage of the Grenvillian orogeny (Hodgin et al, 2022). The inferred WNW-ESE shortening direction associated with the Grenvillian orogeny (both Ottawan and Rigolet phases) aligns with the principal compressional stress direction inferred by Zhang et al (2016) from along-rift variations in seismically imaged crustal structure.…”

“…Post-rift sedimentary rocks record an interval of continued subsidence followed by convergent tectonism that uplifted portions of the Midcontinent Rift and underlying Paleoproterozoic rocks along major thrust faults, potentially during the later portion of the Ottawan phase of the Grenvillian orogeny (Cannon et al, 1993). Unconformably overlying these uplifted rocks are clastic sedimentary rocks that were likely deposited in a basin associated with lithospheric flexure during the Rigolet phase (Hodgin et al, 2022). Towards the end of the Rigolet phase at 980 Ma, as Grenvillian deformation propagated farther towards the interior of the continent, final shortening within the Midcontinent Rift occurred, as rift volcanic rocks were thrust over these sediments (Hodgin et al, 2022).…”

“…Unconformably overlying these uplifted rocks are clastic sedimentary rocks that were likely deposited in a basin associated with lithospheric flexure during the Rigolet phase (Hodgin et al, 2022). Towards the end of the Rigolet phase at 980 Ma, as Grenvillian deformation propagated farther towards the interior of the continent, final shortening within the Midcontinent Rift occurred, as rift volcanic rocks were thrust over these sediments (Hodgin et al, 2022).…”

“…The late Mesoproterozoic to early Neoproterozoic Grenvillian orogeny and the assembly of Rodinia: Turning point in the tectonic evolution of Laurentia 3 2022). This deformation was superimposed on shortening that occurred within the rift earlier during the Ottawan phase of the orogeny (Cannon et al, 1993;Hodgin et al, 2022). The last stage of shortening in the Midcontinent Rift resulted in the deposition of syn-orogenic sedimentary rocks that are now in the footwall of reverse faults that uplifted rift volcanic rocks during the Rigolet phase (Hodgin et al, 2022).…”