FROM FAULT DATES TO OROGENIC RATES

Radiogenic dating of deformation revolutionized our understanding of regional geology and orogeny. The distribution of timing of specific fault activity in fold-thrust belts is becoming sufficiently dense to make inferences about rates of deformation, which remains a grand challenge of crustal evolution. Newly-formed, illitic clays are a common feature of foreland fold-thrust belts, and the regional timing of fault-zone mineralization allows first-order constrains on the timing and progression of deformation. We will briefly discuss our technically-challenging, but otherwise robust method of clay dating using sample encapsulation and multiple Ar ages for each sample, followed by a comparison of results in three foreland fold-thrust belt settings: the US Appalachians, the Canadian Rockies and the Spanish Pyrenees. Fault ages in the southern Appalachians show that a large orogenic wedge was active over a relatively narrow window of time of ~4 m.y. (276-280Ma). Regional fault dating in the Canadian Rockies shows that multiple cycles (“pulses”) of orogenic wedge formation occurred, each lasting 4-8 m.y. and separated by longer periods of tectonic quiescence, instead of continuous forward progression of faulting; in a sense, this process can be described as “stick-slip orogeny”. Using regional fault ages, balanced sections and ~50% shortening, orogenic (longitudinal) strain rates in these major oceanic margin contractional settings are minimally 10E-14/sec. In contrast, fault ages in the eastern Pyrenees foreland wedge show modest (~40%) shortening over a relatively long time window of ~20 m.y. (30-50Ma), resulting in a strain rate that is at least an order of magnitude less for this (narrow basin) contractional setting. The difference in strain rates of these foreland wedges may reflect the nature of convergence in their respective tectonic settings. The rates also indicate that a fault is active for only a few m.y., accumulating finite displacements of several km to several tens of km in accordance with modern seismic (incremental) displacements and recurrence intervals.