Mesozoic and Cenozoic extension recorded by metamorphic rocks in the Funeral Mountains, California

摘要

The Funeral Mountains metamorphic core complex, like so many of these complexes in the North American Cordillera, contains early ductile structures related to Mesozoic compressional tectonics that have been overprinted by structures related to later extension. The core-bounding detachment system formed in late Miocene time, and ductile extensional fabrics in its footwall have been viewed as evidence of a higher-temperature stage in the same event. Integrated structural analysis and geochronology, however, indicate that ductile fabrics with a transport direction similar to the northwest direction of movement on the detachment are part of an older extensional event of Late Cretaceous age. This event produced mylonitic fabrics, a dominant foliation, and well-developed mineral and stretching lineations. Mesoscopic folds and a widespread crenulation lineation that developed during a late stage in this event also are consistent with northwest-directed shear. Late Cretaceous mylonitic fabrics are related to extensional shear zones within the core complex that have small stratigraphic offsets but may represent significant structural and metamorphic discontinuities. Rocks in the footwall of the shear zones experienced mid-crustal depths that cannot be explained by the overburden available if the Funeral Mountains core rocks represent an intact block of Mesozoic crust. Instead, it appears that a significant section of mid-crustal material may have been excised in Late Cretaceous time by the combined Monarch Canyon and Chloride Cliff shear zones as the result of gravitational instability of an overthickened crust. In our interpretation, this early extension was made possible by a waning of compression within the Mesozoic thrust belt combined with a high regional heat flow due to the Sierran arc. This early extension took place 50 m.y. before development of the core-bounding detachment system, movement on which took place at low enough temperatures that deformation was predominantly brittle with some calc-mylonite fabric development. The youngest structures in the core are normal faults that cut the detachment and indicate a southwest transport direction orthogonal to those of both the earlier extensional events.