SHEAR ZONES OF THE WEST TEXAS GRENVILLE AND NEW ENGLAND APPALACHIANS COMPARED

Viewed at fine scales, shear zones of two major orogens, the Appalachian and Grenville, show maddening complexity in both geometry and chronology. The contrasts between them, however, are instructive. Each of the regions compared in detail here—part of the Bloody Bluff Fault Zone (BBFZ) near Framingham, Ma, and exposures of the Carrizo Mountain Group (CMG), the metamorphic core of the Grenville orogen near Van Horn, west Texas—has several strands of shear zones, with diachronous histories.

The area of the BBFZ studied has two km-scale shear zones that are internal to the Avalon Terrane; one cut by Neoproterozoic granites—thus pre-Appalachian—and one cutting through the granites (Nobscot shear zone), that may be a splay off the main, terrane-bounding, branch (Grimes and Skehan 1995). The Nobscot shear zone hosted dextral strike-slip motion at upper-greenschist facies. The Avalon-bounding branch in the study area is the Kendal Green shear zone, an ultramylonitic zone affecting several lithologies. This shear zone lacks lineations and unequivocal shear-sense indicators. Active at lower greenschist facies, it is assumed to post-date the Nobscot shear zone.

The CMG exposures, in contrast, lack pre-orogenic deformation. Also, the exposed and subsurface Texas Grenville lacks Avalon-style, 1000s-km-long accreted terranes. (The only known accreted island-arc terrane occurs along the SE edge of exposures in central Texas.) The CMG hosts garnet- to biotite-grade, NE- and E-trending reverse-/thrust-motion shear zones, related to overall NW tectonic transport (Grimes and Mosher 2003; Grimes and Copeland 2004). However, the kinematic linkages between the two trends of shear zones are unclear. The E-trending shear zones are along the frontal CMG exposures, parallel to the Streeruwitz Thrust, along which the CMG was thrust over platformal strata. Within the resulting foreland fold-thrust belt are folded synorogenic deposits. The frontal shear zones and Streeruwitz Thrust thus comprise a ductile-through-brittle thrust system that brought the CMG up from (Bt-grade) mid-crustal to near-surface levels. Curiously, though, CMG mylonites do not affect all rock types equally, like the Kendal Green, but are preferentially within metarhyolite. This may be due to the fine grain size of these pyroclastic rocks.