T044-02
From Fault Dates to Orogenic Rates: Fault Gouge Dating in the Spanish Pyrenees and Application to Global Fold-Thrust Belt Strain Rates

Monday, 14 December 2020: 16:11
Virtual
Samuel Haines, Earth Observatory of Singapore, Singapore, Singapore, Ben van der Pluijm, University of Michigan, Ann Arbor, MI, United States, James S Hnat, Royal Dutch Shell plc, 'S-Gravenhage, Netherlands, Dinu Pana, Alberta Geological Survey, Edmonton, Canada and Judith Hubbard, Nanyang Technological University, Earth Observatory of Singapore, Singapore, Singapore
Recorded Presentation
Abstract:
The rates of orogenic processes have been explored only since the advent of radiometric dating provided absolute time constraints on the ages of events. Bulk shortening and bulk strain rates from fold-thrust belts are rarely reported, despite the data to do so being available, in the form of shortening estimates from balanced cross sections and time constraints over which shortening occurred. To acquire these constraints from a small fold-thrust belt, we present ages of authigenic illite in fault gouge from 5 thrust faults in the south-eastern Pyrenees fold-and-thrust belt dated by 40Ar/39Ar methods, in conjunction with previously balanced sections. The ages suggest in-sequence thrusting for the prowedge of the eastern Pyrenees during the Eocene and early Oligocene, ranging from 49.5 + 4.4 Ma for the inboard Freser Antiformal Stack to 31.9 +/- 3.9 Ma for the frontal L’Escala thrust. Unlike many foreland fold-and-thrust belts, foreland progression of ages is preserved, although overlapping of ages of the frontal thrusts suggests that the interpretation of a pulse of orogenic activity around 35 Ma is also possible. Shortening rates for the Ter-Freser section range from 1.6 – 3.0 km/Ma, with strain rates ranging from 7.0 x 10-16 to 1.6 x 10-15 s-1. We place these rates in context by compiling bulk shortening and strain rates from 56 balanced cross sections from 17 orogenies globally, sampling fold-thrust belts with a range of structural styles, ages and scales. Fold-thrust belt bulk shortening rates more closely match geodetic strain rates of ~1.0 x 10-16 – 1.0 x 10-14 s-1, than the ‘classical’ geological strain rate of ~10-14+/-1 s-1 of Pfiffner & Ramsey (1982). Although constraints on the onset of thrusting are the single largest source of error, structural style appears to be the most significant control on shortening and strain rates, more so than either prowedge/retrowedge position or plate convergence rate.
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