EXPLORING SHAPE FABRIC DEVELOPMENT IN 3D STEADY AND NON-STEADY FLOW: IMPLICATIONS FOR CLAST-BASED VORTICITY GAUGES

Development of clast shape preferred orientation (SPO) is typically modeled with the assumption of rigid clasts embedded in a 2D steady flow. Clast-based kinematic vorticity gauges are by far the technique most commonly applied to naturally deformed rocks, and explicitly utilize such 2D models as a framework to interpret observed clast SPO on the XZ plane of finite strain of a deformed rock volume. Despite the popularity of clast-based vorticity gauges there is currently no agreement on the portion of the strain path recorded by these methods. In fact, clast SPO is frequently interpreted to record: 1) a mean vorticity for an entire deformation event; or 2) kinematics of only the latest flow increments. Thus, identical results obtained from clast-based techniques may be interpreted in strikingly different ways. This emphasizes the critical importance of a proper understanding of the strain memory of shape fabrics.

We address the question of strain memory using 3D modeling of a population of clasts and SPO development in both superposed deformations and continuously non-steady flow for a range of finite strain states. Results are compared to predictions of SPO development in steady flows that produce an identical finite strain. This approach allows comparisons of predicted clast SPO for multiple strain paths of identical mean vorticity number and provides insight into the portion of the deformation path recorded by populations of rigid clasts. Preliminary results suggest clast SPO is dependent on details of the strain path experienced by a deformed volume; however, clast SPO does not perfectly reflect either the mean vorticity or kinematics of the waning stages of flow. These results suggest that clast-based vorticity estimates alone are not sufficient to extract detailed kinematic information from a deformed rock volume.