See more from this Division: Topical Sessions
See more from this Session: Mid- to Lower Crustal Deformation Processes: Strain, Kinematics and Relationships to Upper Crustal Structures
Abstract:
The >5 km thick, cross-cutting Capricorn ridge shear zone (CRSZ) records S-side-up shear. Here, S1 and S2 foliations are transposed into a steeply SW-dipping S3 foliation with lineations plunging steeply SSE. Parallelism of grain shape fabrics and lithologic boundaries, and locally sheath folds, all indicate high strain.
Fabric mapping within the CRSZ delineates strain gradients recording part of the strain path. Mesoscale structures such as boudinage and conjugate extensional shear bands record a component of stretching perpendicular to the lineation. Strain markers such as mineral grains, feldspar porphyroclasts and polycrystalline feldspar blebs define plane to oblate fabric ellipses (SPO and ELLIPSOID programs; Launeau and Robin, 2005). Together, these observations reflect the conversion of an L-dominated, but structurally heterogeneous, older fabric into an S-dominated fabric during a deformation combining flattening with shear.
By removing rotation due to uplift, the CRSZ restores to a moderately dipping (30-50°N), N-side-down normal shear zone, whose strike was subparallel to the inferred Proterozoic plate boundary. The restored lineation is clockwise from the normal to the plate boundary, suggesting oblique divergence. If the restoration is correct, the CRSZ would have been oblique to shallow-crustal faults expected to form in oblique divergence. This geometry, and the evidence for a flattening deformation, illustrate the challenge in relating smaller-scale and crustal-scale kinematics, and in formulating models for whole-crust deformation using deep crustal exposures whose shallower correlative structures have been removed.
See more from this Division: Topical Sessions
See more from this Session: Mid- to Lower Crustal Deformation Processes: Strain, Kinematics and Relationships to Upper Crustal Structures