Extension of the Anaconda metamorphic core complex: 40Ar/39Ar thermochronology and implications for Eocene tectonics of the northern Rocky Mountains and the Boulder batholith

Abstract

Thermochronologic data defi ne the extension and exhumation history of the Anaconda metamorphic core complex and have implications for the Eocene tectonic setting of the northern Rocky Mountains. The 40 Ar/ 39 Ar data indicate that relatively rapid extension on the Anaconda detachment started at ca. 53 Ma and continued through ca. 39 Ma. Apatite fi ssion-track data reveal that faulting and exhumation of the footwall continued until ca. 27 Ma. The average displacement rate on the Anaconda detachment was on the order of 1 mm/yr between ca. 50 and 39 Ma based on the lateral gradient in mica 40 Ar/ 39 Ar ages in the direction of fault slip. The total displacement along the Anaconda detachment in Eocene and Oligocene times is estimated to be ≥25-28 km based on reconstruction of the Cretaceous Storm Lake Stock with its detached roof, which is now exposed in the Deer Lodge Valley. Extension exhumed crust from ~12 km depth and exposed middle-greenschist-facies mylonites in the easternmost part of the Anaconda complex footwall. On a regional scale, the Anaconda detachment dips east beneath the Cretaceous Boulder batholith, indicating that the batholith and the Butte mineralization were transported east in the hanging wall. The Anaconda metamorphic core complex formed at the transition between the Cordilleran hinterland and the foreland at the same time as extension occurred in the Bitterroot and Priest River metamorphic core complexes but exhumed a shallower part of the Eocene crustal section than the contemporaneous complexes to the west.

Thermal evolution of the Hengshan extensional dome in central South China and its tectonic implications: New insights into low-angle detachment formation

“…The most preferable cooling paths in the ductile shear zone and unmylonitic granitoids both indicate a rapid cooling during~94/96-80 Ma. 7 mm/yr for the Chemehuevi detachment fault, John and Foster, 1993; 7 mm/yr for the Raft River detachment fault, Wells et al, 2000; 3-4 mm/yr for the Sierra Mazatan detachment fault, Wong and Gans, 2003), but similar to those estimated in the central Basin and Range Province (e.g.,~0.5-1 mm/yr for the Ruby detachment fault, Gifford et al, 2007;~0.9 mm/yr for the Anaconda detachment fault, Foster et al, 2010).…”

“…2). Generally, the spatial variation of cooling ages within a certain extensional terrane depends predominately on their elevations or distances from the normal fault (e.g., Ehlers et al, 2003;Reinecker et al, 2008;Foster et al, 2010), whether and how footwall cooling ages from this study vary in response to such factors are discussed below. Fig.…”

“…The rate of slip on the Hengshan detachment fault is estimated by plotting cooling ages versus distance along the fault trace (Fig. 10), i.e., the inverse slope on the age-distance plot (distance/time, Foster et al, 2010). Linear regression of muscovite 40 Ar/ 39 Ar data on the plot indicates slip rates of 0.14 mm/yr for 108-94 Ma and~1 mm/a for 94-91 Ma, while biotite data give slip rates of~1 mm/yr for 94-90 Ma and~0.4 mm/yr for 90-86 Ma (Fig.…”

“…The application of thermochronology to extensional domes or metamorphic core complexes is particularly useful in approaching tectonic exhumation of the mid-crustal rocks, and the slip rate, magnitude, initial geometry of the low-angle detachment fault (e.g., Wong and Gans, 2003;Foster et al, 2010;Lin et al, 2013). In this contribution, we present new moderate to low-T thermochronological data along an east-west geological transect across the lower plate of the Hengshan extensional dome (Fig.…”

“…These crustal-scale structures are commonly created in the late stage of orogeny in response to gravitational instability of previously thickened crust (e.g., Crittenden et al, 1980;Wernicke, 1981Wernicke, , 1995Buck, 1991), and their occurrences thus offer opportunities to study crustal upwarping, deformation and denudation in extensional tectonic regimes (e.g., Davis and Lister, 1988;Foster et al, 2010;Jolivet et al, 2010). One of the most conspicuous features during their generations refers to the large-magnitude exhumation of footwall units, as high-grade metamorphic rocks are exhumed from N12 km mid-lower crustal depth through the ductile-brittle transition to the near surface conditions (e.g., Lister and Davis, 1989).…”