THE EVOLUTION AND TIMING OF MULTI-PLANE DETACHMENT SYSTEMS: FROM DUCTILE TO BRITTLE

The Attic-Cycladic Complex (Greece) is characterized by multiple bivergent low-angle detachment systems; crustal-scale structures that accommodate differential extension of bedrock. These detachment systems have been generalized into two cases: 1) A singular detachment plane accommodating ductile-then-brittle deformation. 2) A multi-plane detachment system, wherein plutonism results in deactivation of an initial detachment surface and relocalization of strain along younger detachment horizons at higher structural levels and lower temperatures as indicated by thermochronometry and cross-cutting relationships. New results from field mapping, structural analysis, and thermochronometry of Mt. Hymittos may suggest another type of Cycladic-style detachment system. Bedrock mapping identified a paired ductile-then-brittle detachment system. The detachments are ~500 m apart and divide the local tectonostratigraphy into three packages: low-grade phyllites and marbles in the uppermost hanging wall package, and high-pressure greenschist-facies schists and marbles in the lower two packages. Ductile mylonites in the footwalls of both detachments grade into brittle-ductile mylonites and finally into cataclastic fault cores. Flanking structures, stepped porphyroclasts, sigmoids, SCC’ fabrics, rotation of boudinaged layers and alignment of clasts in cataclasite fault cores indicate consistently top-S senses of shear from ductile to brittle conditions. The consistency of the kinematics and styles of deformation of both detachments indicate the structures accommodated the same extension. Notably there is no evidence of intrusive rocks within the massif. The presence of a paired detachment system without spatially associated intrusions suggests that plutonism is not the only mechanism that leads to a multi-plane detachment geometry. Moreover, thermochronometry on samples from the footwalls of both detachments produced late Oligocene to early Miocene white mica ⁴⁰Ar/³⁹Ar ages and mid-to-late Miocene zircon (U-Th)/He ages, suggesting that the activity of the detachments overlapped in time though at different pressure and temperature conditions. Thus, multi-plane detachments may occur without the influence of plutonism, and deformation along these detachments may be contemporaneous.