TRANSTENSIONAL SHEAR ZONES IN SOUTHERN LAURENTIA, IMPLICATIONS FOR 1.4 GA TECTONICS

Controversy has surrounded the origin of 1.4 Ga plutons. These plutons have anorogenic geochemical signatures consistent with formation during extension. In contrast, structural studies of 1.4 plutons have demonstrated shortening at a regional scale. Reconciling these two results is critical to deciphering the tectonics of 1.4 Ga plutonism. The Cimarron Mountains of New Mexico and the Wet Mountains of Colorado reveal new aspects of tectonism at 1.4 Ga. In the Cimarron Mts., the Fowler Pass shear zone, a steep, NW-striking shear zone juxtaposes amphibolite grade gneisses with greenschist grade rocks. Stretching lineations plunge NW, and kinematic indicators consistently show NE-down normal oblique motion. The monoclinic symmetry of kinematic indicators and consistent direction of mineral lineations demonstrate simple shear. Geochronologic data indicate that movement on the shear zone occurred between 1430 and 1380 Ma concurrent with metamorphism. In the Arkansas River canyon in the Wet Mountains, the north-striking, subvertical Five Points Gulch shear zone truncates an ENE-plunging antiform in middle amphibolite grade gneisses, placing it against a thick package of aluminous gneisses. Lineations are weakly developed in the folded package on the west, but consistent N-NE-plunging mineral lineations penetrate the gneisses to the east. Foliations and lineations are nearly identical in orientation to those in the Fowler Pass shear zone. However, kinematic indicators record both dextral-reverse and sinistral-normal displacement. Timing of the motion is poorly constrained, but age dates indicate that rocks cooled through hornblende blocking temperatures at 1.42 Ga. Comparing structural data from the Wet and Cimarron Mountains suggests that 1.4 Ga deformation involved both normal-NE down and sinistral transcurrent shearing. Considered together with other work in the Southwest, the emerging pattern suggests that transcurrent shearing dominated the strain field at 1.4 Ga. If found to be regionally extensive, an intercontinental transcurrent system could reconcile conflicting interpretations of the 1.4 Ga event by providing a mechanism for contemporaneous extension, with emplacement of A-type granites, and horizontal shortening.