STRIKE SLIP ATTACHMENT HYPOTHESIS FOR CRETACEOUS EXHUMATION OF THE FOSDICK MOUNTAINS MIGMATITE DOME, ANTARCTICA

Exhumation of a migmatite dome in the Fosdick Mountains (FDM), West Antarctica, occurred during the rapid transition from convergent to divergent tectonics along the active margin of Gondwana between 110-95 Ma. Rapid cooling documented by ⁴⁰Ar/³⁹Ar thermochronology is consistent with exhumation by extensional processes; however the obliquity of structures internal to the dome may be better explained by transcurrent mechanisms, perhaps involving strike-slip reactivation of a pre-existing structure during the change from regional contraction to extension. Here we develop a transcurrent attachment model for formation and exhumation of the Fosdick gneiss dome. An “attachment zone” couples deformation upon discrete brittle faults in the rigid upper crust to relatively continuous deformation in the middle crust.

The FDM form an elongate (80 x 15 km) E-W-trending asymmetrical dome with a tiered internal structure formed by shallowly dipping high strain zones that alternate with domains of km-scale recumbent folds. The calculated pressure trajectory records transit of dome rocks from ~26 km (790 MPa) to depths of 16-17 km (500 MPa), or possibly as low as 8.5 km (260 MPa), bracketed between 105-99 Ma by U-Pb monazite ages on concordant and discordant leucogranites. Mineral lineations are poorly developed within the dome, and folds show inconsistent vergence. The shear zones and folds resulted from dominant flattening deformation during tectonic exhumation; however, AMS lineations trend NE, and early upright and late recumbent folds have a consistent ~N60E trend, suggesting a component of stretching oblique to the long axis of the dome. The fold trend is parallel to the stretching direction determined from brittle kinematic analysis of faults and minor structures in surrounding ranges.

The internal fabrics and elongate geometry of the Fosdick Mtns dome resemble outputs from strain models of trantensional attachment zones. The strain models predict high strain flattening foliation directly beneath the strike slip fault, with foliation dip and strain intensity decreasing away from the fault. Foliations define an antiform centered beneath the fault, with linear fabrics oblique to its axis. Our work suggests that transtensional attachment zones may be optimal sites for exhumation of mid-crustal rocks and gneiss domes.