COLLISIONAL MODEL FOR A GRENVILLE-AGED OROGENIC BELT: LLANO UPLIFT, CENTRAL TEXAS

The Llano uplift of central Texas exposes the core of a collisional orogenic belt that formed along the southern margin of Laurentia during Grenville time. A collisional model is proposed that reconciles differences in structural stacking, apparent tectonic transport, and deformation conditions between the eastern and western portions of the uplift. The eastern uplift primarily records arc-continent collision whereas the western uplift records continent-continent collision, reflecting different parts of an orogenic belt and most likely different crustal levels.

Our model proposes that subduction with a southward polarity resulted in collision of an exotic arc with Laurentia and telescoping of the intervening basinal sediments, followed by overriding of the arc and margin of Laurentia by the southern continent. Transport toward Laurentia and the formation of discrete shear zones coupled with polyphase deformation at upper to uppermost amphibolite facies conditions, as observed in the eastern uplift, are consistent with this model. The model further proposes that convergence led to subduction of the Laurentian margin, resulting in high P metamorphism, but buoyancy forces due to partial subduction of continental crust under the southern continent (with or without slab break off) resulted in uplift, similar to that predicted for the Alpine orogeny by finite element models. Such uplift most likely resulted in retrotransport away from Laurentia, producing the apparent opposite structural stacking observed in the western uplift. Polyphase deformation intimately associated with partial melting, lack of discrete shear zones, and uppermost amphibolite facies conditions postdating the eclogitic transitional to granulite facies conditions are compatible with this orogenic history.

Our model suggests that the Llano uplift exposes different segments of the orogenic belt, and most likely different crustal levels, and records different parts of the overall Grenville orogenic history along the southern margin of Laurentia.