I report here new stratigraphic definitions for some of the only exposed supracrustal rocks in southern Laurentia that coincide with the 1.4-1.3 Ga southern granite-rhyolite province (SGRP). Although SGRP emplacement added substantial volumes of crust to southern Laurentia, the exposed stratigraphic record for this time is scant. However, coeval rocks comprise the 1370-1332 Ma (at least) Carrizo Mountain Group (CMG), which, together with intruding 1286 ± 3 Ma mafic sills, makes up the metamorphic core of the Grenville orogen near Van Horn, west Texas. The volcanic rocks have within-plate geochemistry; their Sm-Nd characteristics suggest correlation with the then-juvenile SE margin of the GRP. The oldest known part of the CMG is the Carrizo Peak Subgroup, predominantly ca. 1370 Ma pyroclastic rhyolites, with lesser amounts of thinly-bedded quartzite and phyllite. To SE across a fault contact, the Hackett Peak Subgroup, > 2 km thick, is the most lithologically varied of CMG subgroups, comprising feldspathic to micaceous quartzite, 1332 +7/-3 Ma flow-banded metarhyolite, and lesser volumes of schist, phyllite, and metacarbonate. The age of the Wylie Mountains Subgroup, exposed in mountain ranges to SE, relative to that of the other subgroups is unknown. However, as it comprises feldspathic quartzite, meta-arkose, and feldspathic schist, layered at 100-m-scales, its contrasting lithology suggests it is not coeval.

The Carrizo Peak and Hackett Peak Subgroups coincided with, respectively, the ca. 1.37 Ga peak of SGRP activity, and 1.35-1.32 Ga intrusion in and around the SGRP margins. The CMG rhyolite and arenite protoliths suggest rifting. However, there is no evidence for the opening of a Mesoproterozoic ocean--specifically, the CMG is not part of a continuous, margin-parallel, clastic slope-rise wedge. It, together with possibly coeval subsurface basins to NE, represents a “feathering” of the Laurentian margin. Given that the SE Laurentian margin was convergent during the earliest Mesoproterozoic, the CMG and the vast felsic magmas emplaced in the SGRP proper suggest evolution to a transform margin, analogous to the Cenozoic events leading to extension and ignimbrite flare-up in western N. America. Resumed subduction led to the earliest stages of the Grenville orogenesis.