Lithospheric weakspots, not hotspots: New England-Quebec and Shenandoah anorogenic magmatism in the context of global plate tectonics, intraplate stress and LIPs

Abstract

We explore the origins of anorogenic post-breakup magmatism in two areas of the mid-Atlantic Appalachians: the New England-Quebec Province (ca. 130–120 Ma) and the Shenandoah Province (ca. 49–47 Ma). Radiometric rock ages and other data do not support claims that this magmatism occurred when these sites were located above postulated Great Meteor and Bermuda mantle hotspots/plumes. We propose instead that the sites are persistent lithospheric ‘weakspots’ favorable for magma ascent during relatively short intervals of a few Myr when global-scale plate motion reorganizes every 20–30 Myr. Magma ascends into the crust when compressive intra-plate stress is relaxed. Weakspots in the plate, not fixed mantle hotspots, can explain why anorogenic magmatism occurred at the same two sites also much earlier (by ca. 50 Myr in the New England-Quebec province and ca. 100 Myr in the Shenandoah Province), and why the Bermuda volcanoes formed not later, but coevally with the Shenandoah Province, 1400 km along the postulated hotspot trace. The plume hypothesis also fails to explain why the New England-Quebec magmas were emplaced at the same time as anomalously productive magmatism along the northern mid-Atlantic Ridge and coincident with the breakup of Iberia from the Grand Banks, sites almost 2000 km distant from the New England-Quebec Province. Moreover, New England-Quebec radiometric age distributions suggest that distant magmatic events and continental breakup affecting other plates were global plate reorganization events that may be ‘recorded’ by volcanism at weakspots. Shenandoah-Bermuda magmatism happened during the Pacific plate motion change recorded by the Hawaii-Emperor Bend. The ca. 720 Ma Robertson River Igneous Suite of anorogenic plutons in Virginia, USA, may be an old analog of the Shenandoah Province exploiting the same lithospheric weakspot. The New England-Quebec magmatic period 130–120 Ma is also the time over which the geomagnetic reversal frequency slowed, reaching zero at the onset of the Cretaceous normal superchron (C34n) at ca. 120 Ma. This event was recorded at the mid-Atlantic Ridge axis as the J-Anomaly Ridge and a large increase in spreading half-rate from 1 to 2.5 cm/a. Thus, geomagnetic reversal frequency may also be related to plate tectonics.