Structure of the Carrizo Mountain Group, southeastern Carrizo Mountains, west Texas: A transpressional zone of the Grenville orogen

Abstract

The west Texas segment of the Grenville orogen includes an oblique, dextral, transpressive zone. The orogen exposed near Van Horn, Texas, displays a transition from the mid‐amphibolite‐facies metamorphic core to a foreland fold and thrust belt. In this paper, detailed structural mapping and analyses, presented for key exposures of the metamorphic rocks, the Mesoproterozoic Carrizo Mountain Group, show a southeastward increase in number of deformational phases and intensity of deformation. The central portion is dominated by oblique northwest convergence, which produced two phases of northwest verging folds and synchronous, oblique reverse and dextral shearing. The thick parts of pre‐tectonic mafic plutons, which had been emplaced as locally discordant sills, control the spatial distribution of structures. Two dominant phases of folds form Type 0 interference patterns (essentially coaxial and coplanar; i.e., fold tightening was dominant). This central belt also contains en echelon reverse to oblique shear zones and comprises a transfer zone within the orogen between structurally higher and lower major shear zones. Overall, the mapped part of the central Carrizo Mountains represents localized dextral transpression within a larger region dominated by reverse motion, possibly indicating a change through time from orthogonal to oblique contraction. Further south in the Bass Canyon region, the earlier fold generations are refolded into Type 2 (“mushroom”) interference patterns. These structures show a westward vergence and are kinematically linked to those further north across a zone of disharmonic folding and heterogeneous shear. The structure and kinematics of the polyphase deformation and ductile shearing in the metamorphic portion of the orogen is compatible with that seen in the foreland fold‐thrust belt, suggesting that in both foreland and hinterland, transpression was a characteristic feature of the west Texas Grenville orogeny. The timing and kinematics of deformation in the Van Horn exposures contrasts with that seen in the Llano uplift, but compares favorably with those of the Natal belt, South Africa, a relationship with implications for recently proposed reconstructions of Rodinia.