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 low‐angle 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 low‐angle fault, involving a continuous period of exhumation rather than a two‐stage structure.
In southeast Karakorum (northwest Himalaya, Pakistan), kilometric size migmatitic domes were exhumed in a context of north-south shortening during Neogene times. The domes are characterized by a conical shape, and ductile deformation criteria indicate both radial expansion and extrusion of the migmatitic core relative to the surrounding gneisses. Most of the domes are aligned along the dextral, strike-slip Shigar fault that is parallel to the N130°E Karakorum fault. Along the Shigar fault, exhumation of the domes is mainly vertical with a slight dextral component.
Abstract The South Karakorum margin, east of the Himalayan syntaxis, consist of an E–W elongated zone of young (10–3 Ma) high‐grade metamorphic rocks (M2) and related migmatitic domes. This late tectono‐metamorphic event post‐dates the Palaeogene (55–37 Ma) phase of thickening of the belt featured by NW–SE structures and associated M1 amphibolite facies metamorphism (0.7 GPa, 700 °C). This M2 metamorphism is characterised by low‐pressure, high‐temperature conditions coeval with migmatite formation in response to a thermal increase of c . 150 °C compared to M1, culminating at a temperature of c . 770 °C and a pressure of 0.5–0.6 GPa. Rapid exhumation of migmatitic domes, at a rate of 5 mm yr −1 , was accommodated by vertical extrusion, in the core of E–W crustal‐scale folds. These crustal‐scale folds formed in response to N–S syn‐collisional shortening and were enhanced by thermal weakening of the migmatised continental crust. M2 metamorphism is spatially and temporarily associated with granitoids showing a mantle affinity, firmly suggesting that this could be the advective heat source for the granite and syenite generation and the subsequent migmatisation of the mid‐crustal level. Such relationships between a mantle‐related magmatism and a high‐temperature metamorphism in a convergent shortening context are suggestive of the breakoff of the subducted Indian slab since 20 Ma.
