Palaeomagnetic evidence for large-magnitude, low-angle normal faulting in a metamorphic core complex

Abstract

CONTROVERSY exists over whether large-magnitude extensional deformation can occur along low-angle, master detachment (normal) faults^1–16. Large amounts of Cenozoic extension occurred in the Basin and Range province of the western United States, and master detachment faults related to this extension are currently exposed as subhorizontal structures along crustal arches known as metamorphic core complexes (MCCs)^10,11,17. One set of models suggests that MCCs represent crustal-scale blocks bounded by originally high-angle normal faults which have tilted to sub-horizontal attitudes^1–6. More dynamic models propose isostatically induced flexural tilting of an initial subhorizontal fault, to an active high-angle fault, to final abandonment as a subhorizontal structure^7–9; these models require MCC detachment faults and their footwalls to have been tilted by 30–60°. Alternatively, MCC detachment faults may have originated as subhorizontal or low-angle structures^10–16. Here we test these models with palaeomag-netic data from undeformed portions of the South Mountains, a typical Cenozoic MCC in the southern Basin and Range province^18–24. Comparison with time-averaged expected directions of the geomagnetic field^25–27 yields no evidence for tilting of the footwall. We conclude that the South Mountains master detachment fault was active as a low-angle extensional structure, with a dip of ∼10°.