Anders, Mark H.1, Christopher D. Walker1, Nicholas Christie-Blick1
(1) Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY
ABSTRACT: Fault Deformation Studies of Rootless Detachments: Implications for Assessing Regional Extension
A comparison of deformation at the base of large carbonate slide blocks (1 to 104 km3) and in rooted normal fault zones reveals marked differences that are useful in the differentiation of such structures. Brittle faults are associated with abundant evidence for repeated movement: cross-cutting veins, cataclastic flow structures and reworked breccia. Healed microfractures in quartz grains exhibit an exponential increase in density as the fault is approached. Cataclastic rocks are characterized by a clear reduction in grain size in the same direction. In contrast, the basal surface of the slide blocks that we have studied exhibits no evidence for repeated movement, suggesting catastrophic emplacement. In many places, such blocks are underlain by a distinctive layer of conglomerate composed of rounded particles of mixed lithology in a fine-grained carbonate matrix. This layer was clearly involved in the movement of the overlying block, in every case being injected upwards as dikes. The basal layer is in some cases underlain by thin gouge with directional grooves. The conglomerate is locally faulted, but in most cases it is remarkably undeformed. We interpret the conglomerate to represent fluidization during slide block emplacement. Application of these criteria to several prominent detachment faults in the Basin and Range Province (Beaver Dam and Mormon Mountains) suggests that these features are rootless and, in marked contrast to what has long been accepted, associated with no crustal extension. Clearly, misidentification of block slides as rooted detachments will result in overestimates of extension; therefore, careful examination of these low-angle structures for distinguishing characteristics is paramount.
AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004