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New U-Pb zircon geochronology from the Riggins region of west-central Idaho refines the timing of contractional deformation across the Salmon River suture zone (SRSZ), a broad north-to northeast-striking belt (>25 km wide) of high strain recording Jura-Cretaceous island-arc-continent collision. Laser ablation -inductively coupled plasma -mass spectrometry (LA-ICP-MS) yields mid-Cretaceous crystallization ages on formerly undated plutonic rocks sampled from the Salmon River canyon. In the Crevice pluton (∼105 Ma), the development of steep to moderate northerly striking gneissic foliation (S1) was followed by tops-to-the-west slip on shallow mylonitic shear zones (S2) and brittle overprinting via systematic joints (Jn) of regional extent. Together, these structures form the pluton's internal architecture. Subvertical gneissic foliation in the adjacent Looking Glass pluton (∼92 Ma) indicates ductile deformation was ongoing in the Late Cretaceous. Prior to this investigation, penetrative fabrics in local arc volcanogenic, plutonic, and continental rocks have been unequivocally linked to post-collisional dextral transpression on the narrow (<10 km wide) western Idaho shear zone (WISZ). As an alternative to this model which requires spatially overlapping but temporally distinct orogenic belts (WISZ-SRSZ), we consider a protracted history whereby regional synmetamorphic structures accumulated over a pre-118 Ma to post-92 Ma interval without an overprinting orogen-scale ductile shear zone. In our view, a progressive deformation history more accurately accounts for the time-transgressive nature and structural continuity of fabrics observed across the arc-continent transition. This tectonic history proposed for western Idaho may be analogous to other long-lived accretionary margins in the North American Cordillera (e.g., Omineca Belt of southeastern British Columbia).Résumé : Une nouvelle géochronologie par U-Pb sur zircon pour la région Riggins du centre-ouest de l'Idaho précise le moment de la déformation par contraction à travers la zone de suture de Salmon River (SRSZ), une large ceinture (>25 km) de direction nord à nord-est de grandes contraintes enregistrant des collisions au Jurassique-Crétacé entre des îles d'arcs et le continent. La spectrométrie de masse à source plasma à couplage inductif avec ablation laser (LA-ICP-MS) a donné des âges de cristallisation au Crétacé moyen à partir de roches plutoniques prélevées dans le canyon de la rivière Salmon non datées antérieurement. Dans le pluton Crevice (∼105 Ma), le développement de foliation gneissique (S1) à direction nord et à pendage fort à modéré a été suivi par un décrochement « sommet vers l'ouest » sur des zones de cisaillement mylonitiques à faible profondeur (S2) et une surimpression cassante par des joints (Jn) systématiques d'étendue régionale. Ensemble, ces structures forment l'architecture interne du pluton. Une foliation subverticale des gneiss dans le pluton adjacent de Looking Glass (∼92 Ma) indique qu'une déformation ductile était en cours au Crétac...

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Tectonic and orogenic processes, reflecting the dynamic nature of the planet, provide myriad examples of the failure of Earth materials under load. Despite this wealth of data, the shear localization process remains a difficult physical modelling problem, lying at the frontiers of complex and non-linear systems research. We present a non-conventional continuum-physics approach to address this problem, based on the mathematical properties of differential grade-2 (DG-2) materials. We choose this material because it is both frame-indifferent, and general enough to include other, simpler materials as special cases. DG-2 materials in pure shear exhibit a dynamic rescaling mechanism, associated with localized shearing, which links the spatial and temporal scales of this process in a self-consistent manner, independent of the observer. On typical thermal timescales, the thermomechanical competence of DG-2 materials depends on the ratio of thermal to mechanical diffusivities, κ/χ. On this basis, we hypothesize the effective rigidity of Earth materials, pertaining when the thermomechanical competence is greater than unity. This theory, applied to the whole Earth, suggests the existence of isopycnal ‘detachment’ zones at systematic, globally correlated depths beneath orogens, consistent with a variety of geological data.

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Abstract-Calcite crystals within the Kaibab limestone in Meteor Crater, Arizona, are examined to understand how calcite is deformed during a meteorite impact. The Kaibab limestone is a silty finegrained and fossiliferous dolomudstone and the calcite crystals occur as replaced evaporite nodules with impact-induced twinning. Twinning in the calcite is variable with deformational regimes based on abundances of crystals with twins and twin densities within crystals. The twins are similar to those that are seen in low tectonically deformed regimes. Low levels of shock are inferred from minor peak broadening of the X-ray diffraction patterns (XRD) of the calcite crystals. In addition, electron spin resonance (ESR) spectroscopy data also indicates low shock levels (<3.0 GPa). Quantitative shock pressures and correlation between the XRD and ESR results are poor due to the inferred low shock levels in conjunction with the analytical error associated in calculation of the shock pressures.