COMPARISON OF THE PIONEER METAMORPHIC CORE COMPLEX (IDAHO, U.S), DEATH VALLEY TURTLEBACKS (CALIFORNIA, U.S.) AND NAXOS CORE COMPLEX (GREECE) WITH IMPLICATIONS FOR THE CONTROLS ON THE 3-D GEOMETRIES OF CORE COMPLEXES

Our recent work in the Pioneer Metamorphic core complex (PCC; Idaho, USA) has revealed striking similarities with the well-known core complexes associated with the Death Valley turtlebacks (DVTB; California, USA). These core complexes share several common features with Naxos core complex (NCC, Greece), particularly elongate footwall domes parallel to the extension directions and curviplanar detachment systems. Comparison of these core complexes provide insight into processes leading to their formation.

Both the PCC and DVTB footwalls display tight elongate ~8-12 km long, doubly plunging folds of synextensional magmatic sheets and high-grade non-coaxial fabrics formed in a mid-crustal extensional shear zone that formed at similar depths (P~3.5-4.5 kb PMMCC, P~3 kb DVTB). In both complexes the extensional shear zone was subhorizontal and active prior to emplacement of the magmatic sheet, but shearing continued following solidification. Folding led to initiation of lower-relief lower-T detachment zones ranging from complete (PCC) to partial (DVTB) abandonment of older higher-T shear zones. Folding of progressively older shear zones in the DVTB is interpreted to result from horizontal shortening a stepover in a strike-slip fault system; in the PMCC it is unclear as to the cause of the horizontal extension-perpendicular shortening.

The PCC exposes migmatitic rocks and a syn-extensional high-strain high-T fabric with stretching lineations oblique to the motion on the detachments and direction of ductile shearing in the overlying rocks; thus, the deepest exposed levels record evidence for decoupling flow of thermally/magmatically weaked middle crust. In sharp contrast, the deepest exposed levels in the DVTB preserve ductile fabrics associated pre-extensional thrusting.

The NCC also displays an elongate footwall dome of ductile extensional fabrics that is up to twice as long and may have formed at twice the depth as the PCC & DVTB. The strain field that formed the folds/domes in the migmatitic core, as well as the progressively less folded higher level shear zones remains debated.

Through comparison of these three core complexes we discuss the controls such as the role of magmatism, rheological layering, pre-existing geometries, 3-D regional strain fields on formation of these types of core complexes.