206 Strain and Displacement: Patterns, Gradients, Partitioning, and Reconstructions

This session aims to bring together researchers from structure-related disciplines to facilitate discussions of approaches that inform our understanding of strain and displacement. We welcome contributions from observations, experiments, theory, and modeling that characterize deformation from micro to tectonic scales.

Tuesday, 21 October 2025
1:30 PM - 5:30 PM
HBGCC, 217C

Comparing strain rates in an exposed plate-boundary shear zone to plate tectonic displacement rates during high-magnitude extensional deformation

Paper number: 206-8
Program: Technical Sessions
Time: 3:40 PM – 3:55 PM

Authors

Abstract

We studied the Turon de Técouère massif of the French Pyrenees, which exposes a 40-200 m thick shear zone that was active during high-magnitude extension in the lithospheric mantle.  The highest differential stress (~200 MPa), as determined by microstructural observations from the exposed rocks, is recorded at conditions of ~850 °C, ~0.6 GPa; below this temperature, deformation is facilitated by reaction-assisted deformation because of the polyphase nature of the rock.  Because the rock is dominantly olivine, we calculated strain rates using olivine flow laws.  Plate motion rates were previously estimated from tectonic analyses for this Cretaceous, magma-poor hyperextended plate margin.

In simple shear deformation, there is a linear relationship between strain rate, displacement rate, and shear zone width.  Combining strain rate estimates determined from microstructural analyses and displacement rates determined from tectonic analyses, the estimated thickness of the shear zone should be 8-20 m.  Thus, calculated thicknesses, based on the estimated strain rates, suggest shear zone thicknesses that are ~1/4-1/10 of those observed at Turon de Técouère.  There are significant implications.  First, the entire extensional plate boundary is preserved in the Turon de Técouère massif, as the thickness of the shear zones easily accommodates the plate motion.  Second, the highest differential stress occurs at conditions of ~850 °C, ~0.6 GPa, suggesting that this deformation occurs at the strongest part of the lithosphere.  At these conditions, the mantle must deform continuously:  At the peak differential stress, the shear zone cannot vary in time, and must vary in space.  Because the calculated thickness of the exposed shear zone is significantly less than the observed thickness, we propose a “jump around” shear zone.  That is, the zone of active shearing moves around within a thicker zone, fundamentally challenging our conceptions of strain localization with increasing strain.  Last, because stresses vary in space and time in the shear zone, while strain rates remain approximately constant within the high-strain domain, we suggest that displacement rate is the driving boundary condition at the divergent plate margin. 

Geological Society of America Abstracts with Programs. Vol. 57, No. 6

doi: 10.1130/abs/2025AM-412639

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