George Davis | Core Complexes and Megaboudinage, Aegean and Basin & Range

Outcrop-scale strain localization and boudinage in Basin and Range and Cycladic core complexes convey insight into crustal-scale development of detachment tectonics.

George H. Davis, Department of Geosciences, The University of Arizona, Tucson, USA AZ 85721 C. Mehl, L. Jolivet, and O. Lacombe, Laboratoire de Tectonique, UMR 7072, Université Pierre et Marie Curie, T 46-00 E2, case 129, 4 place Jussieu, 75252 Paris Cedex 05 France

The Miocene detachments of the southern Arizona metamorphic core complexes within the Basin and Range and the Cycladic Oligo-Miocene detachments contain abundant outcrop-scale structures and fabrics that reveal a progressive ductile through brittle continuum of deformation.   Furthermore, for any subregional system, there is kinematic coordination of direction and sense of strain and displacement throughout the ductile through brittle continuum.  Though attention on fabrics and structures generally focuses on overprinting of mylonities by cataclasites, it is the outcrop-scale boudinage, both in abundance and character, which particularly reveals strain localization in general, and the control and emergence of shear bands, shear zones, and semi-brittle to brittle faults. 

Outcrop-scale structures, in any number of tectonic environments, have historically provided insight regarding km-scale macrostructures, and their origins.  This may be the case with boudinage in core complex and detachment settings.  Specifically, it is possible to imagine that boudinage tectonics at a crustal scale, produced by collapse and spreading in the one case (core complexes in the Basin and Range province) and back-arc spreading in the other (the Aegean Cyclades) may have similarly influenced the development of km-scale shear zones and shear bands that, through time, evolved as ductile-brittle detachment faults.  The Aegean shows the complete range from outcrop-scale to crustal-scale suggesting that major detachments originated as shear zones between crustal-scale boudins.  Even the asymmetry of structures seen at outcrop-scale reflects at the scale of the crust with top-to-the-north shear sense over a large part of the Agean Sea. 

One appeal of seeking failure criteria at the plastic/brittle interface is the inherent difficulty of explaining detachment faults by Anderson mechanics.  Undoubtedly there are places where it can be shown that domino tectonics participated in the formation of metamorphic core complexes and detachment faults.  Undoubtedly there are places where (continuous) stress rotation driven by weak faults created unconventional initial orientations of some normal faults.  Yet, overall, it is useful to consider non-Andersonian mechanics as having spawned the detachments, which, upon formation, can participate in whatever follows, from rolling hinges to cross-cutting by later structures. 

[Davis, G.H., Mehl, C., Jolivet, L., and Lacombe, O., 2005, Outcrop-scale strain
localization and boudinage in Basin and Range and Cycladic core complexes
convey insights into crustal-scale development of detachment tectonics: EOS,
Transactions of the American Geophysical Union, 2005 Fall Meeting, American
Geophysical Union, invited paper, T24C-01.]