North America’s Midcontinent Rift: When rift met LIP

Abstract

Rifts are segmented linear depressions that are filled with sedimentary and igneous rocks; they form by extension and often evolve into plate boundaries. Flood basalts, a class of large igneous provinces (LIPs), are broad regions of extensive volcanism formed by sublithospheric processes. Typical rifts are not filled with flood basalts, and typical flood basalts are not associated with significant crustal extension and faulting. North America's Midcontinent Rift (MCR) is an unusual combination, because its 3000-km length formed during a continental breakup event 1.1 Ga, but it contains an enormous volume of igneous rocks that are mostly flood basalt. We show that MCR volcanic rocks are significantly thicker than other flood basalts, due to their deposition in a narrow rift rather than across a broad region, giving the MCR a rift's geom etry but a LIP's magma volume. Structural modeling of seismic-reflection data shows that LIP volcanics were deposited during two phases-an initial rift phase where flood basalts filled a fault-controlled extending basin and a postrift phase where LIP volcanics and sediments were deposited in a thermally subsiding sag basin without associated faulting. The crust thinned during the initial rifting phase and then rethickened during the postrift phase and later compression, yielding the present thicker crust observed seismologically. The restriction of extension to a single normal fault in each rift segment, steeply inward-dipping rift shoulders with sharp hinges, and persistence of volcanism after rifting ended gave rise to a deep flood basalt-filled rift geometry not observed in other presently active or ancient rifts. The unusual coincidence of a rift and LIP arose when a new rift associated with continental breakup interacted with a mantle plume or overrode anomalously hot or fertile upper mantle.

The End of Midcontinent Rift Magmatism and the Paleogeography of Laurentia

“…A recent structural reanalysis of the 1986 GLIMPCE seismic data posited that rift-related faulting within the Midcontinent Rift ended prior to the eruption of the Portage Lake Volcanics at ca. 1096 Ma, with all subsequent accommodation space arising from thermal subsidence (Stein et al, 2015). Our new U-Pb dates of 1084.35 ± 0.20 Ma and 1083.52 ± 0.23 Ma for Michipicoten Island Formation volcanics indicate that this proposed timing of the end of active structures would predate the end of rift-related volcanism in the Lake Superior region by more than 12 million years.…”

“…Rifting has been attributed to: (1) far-field stresses with different proposed tectonic drivers (e.g., Gordon and Hempton, 1986;Stein et al, 2014) that initiated extension into which plume-derived melts erupted (Green, 1983;Stein et al, 2015); and (2) an upwelling mantle plume that drove crustal thinning and rifting (Burke and Dewey, 1973;Cannon and Hinze, 1992). The new paleomagnetic pole from the Michipicoten Island Formation implies that, from the start to end of rift magmatism, Laurentia had traveled more than 45° of latitude, 20° of which were covered from the main stage of rift volcanism at ca.…”

Instability of the southern Canadian Shield during the late Proterozoic

“…Nicholson and Shirey, 1990) or as a far-field extensional response to compressional stresses during the Grenville orogeny (e.g. Whitmeyer and Karlstrom, 2007), or a combination of the two (Stein et al, 2015). Northward propagation of the MCR was inhibited by the strong Superior lithosphere, and the shape and position of the rift was influenced by the stable craton margin (Ola et al, 2016).…”

The end of Midcontinent Rift magmatism and the paleogeography of Laurentia

“…The magmatic products of the Midcontinent Rift are exceptional in that they are both voluminous and largely confined to an intracratonic rift (Green, 1983;Stein et al, 2015). The total volcanic output is dominated by basaltic lavas and has been estimated at ~1.5 × 10 6 km 3 (Hutchinson et al, 1990;Cannon, 1992), characteristic of large igneous province magmatism fed by a mantle plume.…”