HOW SENSITIVE ARE RIGID GRAINS TO CHANGING FLOW VORTICITY?

Clast-based vorticity estimation techniques have become a common tool for investigating flow conditions of ductile high strain zones due to their ease of use and apparent wide applicability. Several vorticity gauges exist that apparently record a similar portion of the deformation path. For example, rigid grain analysis and quartz c-axis / strain ratio methods likely record much of the ductile flow history, while oblique dynamically recrystallized grain shape foliation is sensitive to ISA and likely records the latest flow increments. Because several analytical techniques are currently available, single appropriate specimens may be analyzed using different gauges. If the techniques used record a similar portion of the flow path, agreement between independent estimates may be considered evidence for a steady flow history, at least for the recorded portion of the path. A brief literature survey indicates ~50% of the rocks for which this has been done produced distinct, non-overlapping vorticity estimates. This highlights the need for a more thorough examination of the most commonly applied vorticity gauges.

If the specimens described above truly indicate a flow history with temporally variable vorticity it is important to know approximately how vorticity markers will respond to these changes. It is currently unknown how rapidly rigid grains equilibrate to a change in vorticity. Some investigators suggest that vorticity values extracted using rigid grain techniques approximate an average flow vorticity while others have suggested that rigid grains equilibrate rapidly to changes in vorticity and therefore record only the last increments of ductile deformation.

To obtain first-order results of the consequences of variable flow vorticity on rigid grains we have used Ghosh and Ramberg's (1976, Tectonophysics 34, 1-70) analytic solutions for the orientation of suspended rigid inclusions in a flow that is instantaneously steady but with continually changing vorticity. This simplified approach allows us to explore the sensitivity of rigid grains using the same governing equations upon which the rigid grain techniques are built. Our model indicates rigid grains are somewhat sensitive to changing flow vorticity, but the temporal duration of distinct flows appears to have a large impact on final clast orientation.