Direct dating of ductile shear zones and calculation of uplift/exhumation rates can be done using various radiometric dating techniques.
But radiometric dating of shallow crustal faulting, which occurs in the crust's brittle regime, has remained difficult because the low temperatures typical of shallow crusted faults prevent the complete syntectonic mineral recrystallization that occurs in deeper faults.
Our other gouge age group (52 Ma, early Eocene) apparently has no surviving syntectonic deposits in the foreland basin due to post-tectonic uplift and erosion (Sears, 2001). Isotope ages (59 Ma) from dikes that cut the Lewis thrust show that movement had ceased by this time, and that our 51.5 ± 2.2 Ma gouge age cannot be correct.
Our data show either thrusting in the foreland that was contemporaneous with extension in the hinterland (De Celles, 2004), or that our “young” date for Lewis thrusting is yet another measurement demonstrating the short duration of the transition from compression to extension.Attribution: You must attribute the work in the manner specified by the author or licensor ( but no in any way that suggests that they endorse you or your use of the work).Price makes a case based on his experiences, but we stand by our date. But, Catuneanu and Sweet (1999) describe reciprocal sequence stratigraphy with two tectonically driven cycle pairs (pulse and quiescence deposits) between early Maastrichtian (72 Ma) and middle Paleocene (61 Ma).The timing of their first pulse agrees with our older gouge age group; their second pulse at ~68 Ma also agrees with one of our dates.Ma indicating a Middle Eocene age of faulting and a Middle Triassic age of the wall rocks. AU - Li, Chuanyou PY - 2011/4/15Y1 - 2011/4/15N2 - Paired together, fault gouge dating and low-temperature thermochronometry overcome the limitations and assumptions inherent in each independent technique.
Comparing this dataset with published thermochronology from hanging wall rocks supports the interpretation that the West Qinling fault initiated at ~. Ma and continued until at least Middle Miocene time and that authigenic clay growth occurred at ambient temperatures of ~. Lack of overprinting of younger clay ages at this site may indicate that rocks were out of the thermal window for authigenic clay formation during later faulting episodes. TY - JOURT1 - Direct dating of Eocene reverse faulting in northeastern Tibet using Ar-dating of fault clays and low-temperature thermochronometry AU - Duvall, Alison R. Here we establish timing of brittle faulting along the West Qinling fault of northeastern Tibet by dating several size fractions of fault gouge clay that represent variable populations of illite polytypes.
This result requires the western North America stress regime to have changed from contraction to extension in only a few million years during the Eocene.
We also estimate the uplift/exhumation age and sedimentary source of these rocks to be 172 Myr.
A37) describes a single biotite K/Ar age of 58.3 Ma (no uncertainty given) from a “quartz monzonite porphyry sill” that “intruded into the [Eldorado] thrust zone.” Without seeing it, a sill is not a clear cross-cutting relationship, and if the K/Ar uncertainty were several Ma, then the sill age may overlap with our gouge Ar-age.
It is hard to accept this one age as an unassailable constraint on termination of thrusting. An abrupt transition to an extensional regime following thrusting has been dated in several ways, with results seen by Price as inconsistent with our ~52 Ma thrust gouge age. (2007) summarize the “regime change” data, which are not necessarily at odds with our thrust age.
For all localities, undeformed mudstone country rock was also dated, using our methods; some localities were multi-sampled progressively away from the gouge zone.