TECTONIC EVOLUTION OF THE EASTERN LLANO UPLIFT, CENTRAL TEXAS: A RECORD OF GRENVILLE OROGENESIS ALONG THE SOUTHERN LAURENTIAN MARGIN

Mesoproterozoic metamorphic rocks exposed in the eastern Llano Uplift, central Texas, were involved in a ~1150-1116 Ma Grenville-age orogenic event along the southern margin of Laurentia. Collision of an exotic arc and a southerly crustal block with Laurentia tectonically telescoped and stacked three distinct lithotectonic domains. A major ductile shear zone forms the contact between the Coal Creek ensimatic arc terrane and rocks with Laurentian affinities. Directly north of the arc boundary, supracrustal rocks of the Packsaddle domain represent basinal sedimentary and volcanic rocks deposited along the Laurentian margin. Farther north and juxtaposed beneath the Packsaddle domain along a shear zone, granitic gneisses of the Valley Spring domain, which consist of both plutonic and supracrustal rocks, represent a Laurentian continental-margin arc.

Recent mapping in the northeastern uplift has documented that the Valley Spring domain records a polyphase deformational history equivalent to that observed in the Packsaddle domain of the southeastern uplift. Early deformation occurred under granulite facies metamorphic conditions and was accompanied by partial melting and formation of foliation-parallel leucosomes, consistent with deformation deeper in the orogenic pile. The shear zone separating these two domains contains mylonitic rocks that show northeastward tectonic transport similar to that shown by the shear zone separating the ensimatic arc from the Packsaddle domain. The structurally highest Valley Spring domain igneous complex is highly attenuated into thin sheets within this zone.

Thus the Grenville orogenic event recorded in the eastern Llano Uplift is characterized by polyphase ductile deformation synchronous with transitional amphibolite-, granulite- to eclogite-facies dynamothermal metamorphism. Deformation progressed from northeast-directed ductile thrusting and folding (D1, D2) that accommodated collision-related crustal thickening and contraction to polyphase, regional-scale folding (D3, D4, D5) accommodating continued, collision-related north- to northeast-directed contraction. The kinematics of the deformation in the eastern uplift is directly related to the northeastward collision of an exotic arc terrane with the generally east-trending Laurentian margin.