KINEMATIC CONSTRAINTS ON RODINIA ASSEMBLY: LLANO UPLIFT, TEXAS

Mesoproterozoic rocks in the southeastern Llano Uplift, central Texas, record ductile deformation and high-P, high-T regional metamorphism associated with a Grenville-age (~1.15-1.12 Ga) collisional orogenic event along the southern margin of Laurentia. Deformation progressed from NE-directed ductile thrusting and folding, which accommodated collision-related crustal thickening and shortening, to polyphase, regional-scale folding accommodating continued, post-collisional N- to NE-directed shortening. Minor late- to post-contractional, NNE-oriented extension followed, most likely related to orogenic collapse.

We interpret deep-seated orogenesis to be the result of collision between Laurentia and a southern continent, plus an older exotic arc, with tectonic telescoping of intervening assemblages during a Grenville-age orogeny. None of the structures mapped are consistent with the southern margin of Laurentia being a dextral transform boundary as predicted by models for continental blocks colliding from the east. Instead, structures and metamorphic conditions are consistent with crustal thickening related to direct north to northeastward collision of a southern continent.

Our results clearly rule out tectonic models for assembly of Rodinia that attempt to explain orogenesis along the southern Laurentian margin by collision of a continent along the eastern margin. Those models require NW-directed transport or transcurrent motion along the entire southern margin. Strong similarities between the Canadian and Texas Grenville orogens exist because both culminated in continental collision, but the differences in kinematics require collision by separate continents from different directions. No kinematic link needs to exist between the orogenic belts along Laurentia’s eastern and southern margins in reconstructions of Rodinia involving a southern continent. Collision of a southern continental block would be independent of the culminating, northwestward directed continental collision along the eastern Laurentian margin. Future reconstructions of Rodinia need to consider kinematic as well as paleomagnetic and temporal constraints and should include a southern continent along the southern margin of Laurentia at ~1.15-1.12 Ga with an overall northward transport direction.