My work on mid-Tertiary detachment basins has focused on Miocene sedimentary sequences exposed in the NE Whipple Mts (SE California), Aubrey Hills (W Arizona) and Buckskin Mts of western Arizona (see map, below). These sedimentary rocks are found in the upper plates of low-angle extensional detachment faults, which accommodated large-magnitude regional extension during Miocene time. Working with several graduate students at Northern Arizona University (NAU), we used detailed sedimentology and physical stratigraphy to reconstruct the geometry and sediment-dispersal systems of syn-extensional sedimentary rocks. This allowed us to evaluate the dynamic structural controls on upper-plate basin development and syn-extensional stratal tilting.
We conclude that a large rollover growth structure formed in the hanging wall of the Whipple detachment fault shortly prior to emplacement of the voluminous War Eagle landslide (megabreccia). Syn-depositional growth of this rollover structure occurred in response to initiation and slip on a secondary breakaway fault at about 14-15 Ma, on the northeast flank of the rising core of the Whipple detachment lower plate. Work by other people in the Whipple Mountains (Greg Davis, Jack Dunn, An Yin, Liz Forshee, Kathi Beratan, Jane Nielson, and others) has been important for helping us to achieve our current understanding of the complex processes that controlled development of Whipple detachment upper-plate basins. In addition, recent work by Julia Miller and Barbara John in the Chemehuevi Mountains to the north has shown that a secondary breakaway was initiated at the same time (~14-15 Ma) on the northeast margin of the Chemehuevi lower plate. The similar timing, style, and structural position of secondary breakaways in the Whipple and Chemehuevi Mountains suggests that these structures are linked and may be regionally significant. The full implications of this have yet to be explored.
Below are several figures and brief explanations that outline our thinking on the above conclusions, excerpted from Dorsey and Becker (1995).
Figure 1. Geologic map of the Whipple and Chemehuevi Mountains region, redrafted from Howard and John (1987).
Figure 2. Geologic map of the Whipple Mountains and surrounding areas, showing upper- and lower-plate rocks in the highly extended detachment terrane. The box shows the area in which we concentrated our work.
Figure 3. Schematic model of secondary breakaway development in Whipple Mountains, inspired by models of Wernicke (1984), Spencer (1984), and Spencer and Reynolds (1989). (1) Early detachment fault system, with initial breakaway located in Piute and Old Woman Mountains, or possibly in western Mopah Range. (2) Progressive uplift and cooling of lower plate. (3) Fault slip ceases on southwest flank of dome, secondary breakaway is initiated on northeast flank, upper-plate rollover structure is initiated, and lower-plate derived megabreccias are transported for the first time into the flanking basin. Stratigraphic and structural evidence for this scenario is presented by Dorsey and Becker (1995, Basin research v. 7, p. 151-163).
Figure 4. Interpretive tectonic map showing inferred link between secondary breakaway in Whipple and Chemehuevi Mountains (from Dorsey and Becker, 1995).
Here is a list of Papers, Theses, and Abstracts resulting from this research.