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

Mesoproterozoic metamorphic rocks exposed in the Llano Uplift, central Texas, were involved in a ~1150-1116 Ma Ga Grenville-age orogenic event along the southern margin of Laurentia. Structural mapping in the western uplift documents a different kinematic history than that of the well-studied eastern uplift.

The western uplift records multiple fold generations (F1-F5) and penetrative axial planar foliations (S1-S5). Later ductile shear zones are common. Four generations of intrusive granitic sills and dikes provide age constraints on deformation. Early deformation occurred under granulite facies conditions between 1253 -1126 Ma, whereas later events occurred at amphibolite facies conditions between 1126 - 1076 Ma. Metamorphic fabrics record a change in conditions between F2/S2 and F3/S3, although both deformations were at granulite facies. This change is attributed to an influx of fluids associated with intrusion of granitic magma.

The eastern Llano Uplift records a similar, essentially contemporaneous, polyphase deformational and magmatic history, but the orientation and style of all but the latest structures differ. Only the youngest fold generation (F5) can be directly correlated in orientation and timing from the western to the eastern uplift. Structures in the western uplift are more easterly or northeasterly trending than the dominantly southeast-trending structures in the eastern uplift. Also, no mylonite zones separate major lithotectonic domains in the western uplift, unlike the eastern uplift, and the general direction of tectonic transport is southwest, opposite that of the eastern uplift.

The difference in kinematic history is best explained by the exotic arc terrane located only in the southeastern uplift. We propose that the kinematics of the deformation in the eastern uplift was directly related to the northeastward collision of the arc terrane with the generally east-trending Laurentian margin, whereas deformation in the western uplift was controlled by collision of a southern continental block.