Development of three genetically related basins associated with  detachment-style faulting: Predicted characteristics and an  example from the central Mojave Desert, California  J- Department of Geology, University of Kansas, Lawrence, Kansas 66045-2124  R. P. Fillmore*  J. D. Walker  J. M. Bartley Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah 84112-1183  A. F. Glazner Department of Geology, University of North Carolina, Chapel Hill, North Carolina 27599-3315  ABSTRACT Three types of sedimentary basins may develop in large-magnitude detachment-type extensional systems. Possible early Miocene examples of these basin types have been iden- tified in association with northeast-directed extension on the central Mojave metamorphic core complex, southern California. From west to east the basins are (1) the Tropico basin, a flexural basin formed on the unextended footwall behind, and bounded by, the detach- ment breakaway zone; (2) the Pickhandle basin, a supradetachment half graben bounded to the southwest by the detachment breakaway and to the northeast by the hanging wall; and (3) the Clews basin, an intra-hanging-wall basin that formed on the upper plate of the detachment system. Different basin types may be recognized by the nature and geometry of their strata. The footwall flexural basin should be shallow and areally extensive, with basin fill dominated by fine-grained, low-energy deposits. The supradetachment basin typically will be elongate normal to the extension direction and characterized by a thick sequence of volcanic and coarse-grained deposits that reflect its fault-controlled margins. Intra- hanging-wall basins may be of half- or full-graben geometry and will vary in dimension depending on the spacing of the transfer elements. Recognition of these basin types is potentially useful in delineating features of the extensional system that are important to its reconstruction, including the detachment breakaway zone and the boundaries of the ex- tensional system parallel and normal to the extension direction. INTRODUCTION Structural and geophysical considerations suggest that three basin types may form dur- ing detachment-style extension (e.g., Kusz- nir et al., 1987; Weissel and Karner, 1989; Yielding, 1990; Fig. 1): (1) a supradetach- ment basin at the breakaway, (2) a footwall basin formed in the relatively unextended area behind the breakaway fault, and (3) in- tra-hanging-wall basins formed by faulting of the upper plate. The pattern and charac- ter of basin fill should reflect depositional conditions, which are controlled, to a first order, by the structures that form the basin. Therefore, the position within the exten- sional system should be evident in the char- acteristics of the extension-related deposits. To tie the characteristics to a field exam- ple, we describe a single extensional system in the central Mojave Desert, California, that is interpreted to record the different structural settings outlined above. Deposi- tion of Miocene volcanic and sedimentary rocks was coeval with northeast-directed ex- tension on the central Mojave metamorphic core complex. From southwest to northeast (Figs. 1, 2), these strata are (1) the Tropico Group, interpreted to fill a footwall basin; (2) the Pickhandle Formation, deposited in a basin that formed adjacent to the break- away escarpment (Fillmore and Walker, un- published); and (3) the Clews Formation at Alvord Mountain, interpreted to have accu- mulated well within the hanging wall of the northeast-rooting detachment system (Fill- more, 1993). GEOLOGIC SETTING Mylonitic footwall rocks, exposed in the Waterman Hills, the Mitchel Range, and the Buttes (Fig. 2), constitute the central Mo- sw jave metamorphic core complex. Northeast movement of the hanging wall is recorded by kinematic indicators in footwall mylonites (Dokka and Woodburne, 1986; Glazner et al., 1988; Dokka, 1989; Bartley et al., 1990a). Synkinematic dikes in the Waterman Hills are dated at 23 ± 0.9 Ma (U-Pb zircon anal- ysis, Walker et al., 1990). The variably my- lonitized Waterman Hills granodiorite is dated at 22 to 23 Ma by zircon discordia, and 40 Ar/ 39 Ar cooling ages from biotite of 20.1 and 20.9 Ma suggest extensional unroofing over a period of 1 to 3 m.y. (Glazner et al., 1992). Fission-track ages from apatite show a pronounced cooling at —19 Ma (Dokka and Baksi, 1988). Basins formed during this extensional de- formation are filled by the Tropico Group, Pickhandle Formation, and Clews Forma- tion (Dokka and Woodburne, 1986; Walker et al., 1990; Fillmore, 1993). The timing of basin formation is determined by radiomet- ric ages from these sequences. The lower Tropico Group is conformably overlain by the Red Buttes andesite (21.2 ± 0.5 Ma; K-Ar; Dokka and Baksi, 1988; Linn, 1992) and unconformably by Saddleback basalt (19.9 ± 0.7 Ma; K-Ar, Armstrong and Hig- gins, 1973, recalculated for new decay con- stants). These rocks are overlain by evapor- itic shales of the upper Tropico (Dibblee, 1958). The age of the lower, volcanic rock- rich part of the Pickhandle Formation has been bracketed between 23.8 and 21.4 Ma ( 40 Ar/ 39 Ar; Fillmore, 1994; Walker et al., NE Footwall Uplift  Basin  Tropico basin Supradetachment  Breakaway Basin  Pickhandle basin Intra-Hanging-Wall  Basin  Clews basin approximate scale  ^Present address: Department of Geology, Northern Arizona University, Flagstaff, Ari- zona 86011. Figure 1. Schematic cross section of basins formed early in development of metamorphic core complex. This is referenced to central Mojave extensional system and shows relative locations of basins and related tectonic elements. Continued deformation may significantly disrupt these basins. GEOLOGY, v. 22, p. 1087-1090, December 1994 1087