Geochronological evidence for continuous exhumation through the ductile‐brittle transition along a crustal‐scale low‐angle normal fault: Simplon Fault Zone, central Alps

Abstract

[1] Major low-angle normal faults juxtapose different structural levels of the crust that record both brittle and ductile deformation. Field relationships alone cannot establish whether these different responses to deformation represent (1) parts of a single process of exhumation along the detachment or (2) two separate events, with the later, more discrete brittle detachment exhuming a fossil ductile shear zone from depth. These two general models are critically assessed for the lowangle normal Simplon Fault Zone (SFZ) in the central Alps. The SFZ shows a spatial transition from a broad ductile mylonitic shear zone to a discrete brittle detachment with identical kinematics. The age of the ductile shear zone and ductile-brittle transition is controversial. We present a detailed geochronological study based on fission track, 40 Ar/ 39 Ar, and Rb/Sr microsampling dating, coupled with structural, petrological, and chemical analyses that provides tight constraints on SFZ timing. Discontinuous mineral cooling ages over a broad range of temperatures across the fault zone argue for fault activity between 20 and 3 Ma. On the basis of synkinematic white mica in low-temperature shear zones and necks of foliation boudinage, the brittle-ductile transition in the footwall could be dated at ∼14.5-10 Ma. Overall, the data presented here are consistent with a continuous transition from ductile shearing to a more localized zone of brittle deformation within the same geological framework, over a period of ∼15 Ma. The SFZ is therefore an example of a telescoped crustal section within a single major lowangle fault, involving a continuous period of exhumation rather than a two-stage structure.

Metamorphic core complexes: windows into the mechanics and rheology of the crust

“…The rocks in the Tauern window were exhumed in part by slip on a major normal fault (the Brenner Line) that outlines the western end of the window, and that has many of the characteristics of a detachment fault (Selverstone 1988). A similar situation exists in the central Alps: medium-to high-grade metamorphic rocks, including relict eclogites, in the Lepontine Dome reached peak temperatures at around 30 Ma (Vance & O'Nions 1992) and were exhumed in part along the west-directed Simplon fault to the west (Campani et al 2010) and the east-directed Turba fault to the east, both of which exhibit many of the features of detachment faults (Nievergelt et al 1996). Few workers consider these large volumes of metamorphic rocks as core complexes, however, presumably because they were exhumed during continuing continental collision.…”

Two‐ and three‐dimensional thermal modeling of a low‐angle detachment: Exhumation history of the Simplon Fault Zone, central Alps

“…In the central region of the SFZ (Figure 2a), the amphibolite facies mylonites from the footwall reached minimum temperatures of 500°C with peak metamorphic temperatures in the footwall at the start of exhumation of 580–620°C [ Vance and O'Nions , 1992; Todd and Engi , 1997]. For these rocks, Campani et al [2010] calculated a closure temperature of ∼440°C for 40 Ar/ 39 Ar on muscovite. This is significantly lower than the minimum temperatures reached in this region, implying that 40 Ar/ 39 Ar ages for muscovite stable in the amphibolite facies assemblage from these footwall rocks are cooling ages [ Campani et al , 2010].…”

“…The SFZ provides an excellent basis for such a study for several reasons. (1) Many cooling ages are available from a wide range of different thermochronometers, from high temperature (white mica Rb‐Sr) to low temperature (apatite fission track) [e.g., Jäger et al , 1967; Hunziker , 1969; Hunziker and Bearth , 1969; Purdy and Jäger , 1976; Wagner et al , 1977; Soom , 1990; Baxter et al , 2002; Keller et al , 2005; Campani et al , 2010]. (2) The general controversy is directly relevant because two different kinematic models for footwall exhumation have been proposed previously (Figure 1): one considers exhumation to have occurred along an initially shallowly dipping detachment (model A, Figure 1a) [ Mancktelow , 1985; Grasemann and Mancktelow , 1993] whereas the other considers exhumation through a rolling hinge system (model B, Figure 1b) [ Wawrzyniec et al , 1999; Axen et al , 2001; Wawrzyniec et al , 2001].…”

Deciphering orogenic evolution

“…1). Activation of right-lateral faulting occurred mainly along the Penninic Front, connecting the Insubric Line via the Simplon extensional fault (Campani et al, 2010). Fig.…”

“…: Post-collisional crustal and upper mantle strain field of Western Alps with polarities and ages of ductile crustal deformation (references below), and direction of upper mantle deformation inferred seismic anisotropies (Barruol et al, 2011). Age and polarities of displacements are from Rolland et al (2009a) in the Aar Massif, from Campani et al (2010) in the Simplon Fault zone, from Rolland et al (2008) in the Mont Blanc, from Sanchez et al (2011a) in the Mercantour. The legend displays: 1, the Dauphinois (or Helvetic) zone, rimmed by the Jura and Alpine frontal thrusts.…”