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An Example from the Central Mojave Metamorphic Core Complex

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An Example from the Central Mojave Metamorphic Core Complex Large-magnitude continental extension: An example from the central Mojave metamorphic core complex John M. Fletcher* Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah 84112 John M. Bartley } Mark W. Martin* Isotope Geochemistry Laboratory and Department of Geology, University of Kansas, Lawrence, Kansas 66045 Allen F. Glazner Department of Geology, University of North Carolina, Chapel Hill, North Carolina 27599 J. Douglas Walker Isotope Geochemistry Laboratory and Department of Geology, University of Kansas, Lawrence, Kansas 66045 ABSTRACT of ductile deformation and either the proximity or relative volume of extension-related igneous rocks. This suggests that models that The central Mojave metamorphic core complex is defined by a invoke a single upper-crustal genetic relationship, such as magma- belt of Miocene brittle-ductile extension and coeval magmatism. tism triggering extension or vice versa, do not apply to the central The brittle-ductile fault zone defines a basin-and-dome geometry Mojave metamorphic core complex. that results from the interference of two orthogonal fold sets that Systematic variation in the relative timing of dike emplace- we infer to have formed by mechanically independent processes. ment and mylonitization suggests that, at the time of dike emplace- One fold set contains axes that lie parallel to the extension direction ment, rocks in the Mitchel Range were below the brittle-ductile of the shear zone and has a maximum characteristic wavelength of transition while those in the Hinkley Hills were above it. The Hink- about 10 km. The axial surfaces of these folds can be traced from ley Hills and Mitchel Range are separated by ;2 km in the dip the footwall mylonites, through the brittle detachment, and into direction of the fault zone, which suggests that the vertical thick- hanging-wall strata. However, folds of mylonitic layering have ness of the brittle-ductile transition probably was between 100 and smaller interlimb angles than those of the brittle detachment, sug- 950 m. gesting that the folds began to form during ductile shearing and continued to amplify in the brittle regime, possibly after movement INTRODUCTION across the fault zone ceased. Mesoscopic fabrics related to the transport-parallel fold set indicate that the folds record true crustal Brittle-ductile faulting as recorded in Cordilleran metamorphic shortening perpendicular to the extension direction. We interpret core complexes are now recognized throughout the world as an these folds to form in response to elevated horizontal compressive important mechanism of extension in previously overthickened con- stress perpendicular to the extension direction and suggest that this tinental crust. Examples include late- to post-Caledonian rifting on stress regime may be a natural consequence of large-magnitude the western margin of Norway (Norton, 1986; Andersen and extension. Jamtveit, 1990), extension in the Andean magmatic arc (Mpodozis The other fold set has axes perpendicular to the extension and Allmendinger, 1993), syncontractional spreading on the Ti- direction and a characteristic maximum wavelength of about 50 km. betan plateau (Burg et al., 1984; Burchfiel and Royden, 1985), col- Mesoscopic fabrics related to these folds include northwest-striking lapse of the Cycladic blueschist belt in the central Aegean Sea (Lee joints, kink bands, and en echelon tension-gash arrays. These fab- and Lister, 1992), and extension of continental crust at the tip of a rics formed after mylonitization and record both layer-parallel ex- propagating oceanic rift in the Solomon Sea (Davies and Warren, tension and northeast-side-up subvertical shear. The postmylonitic 1988; Hill et al., 1992; Baldwin et al., 1993). Additionally, spreading fabrics are kinematically compatible with rolling-hinge-style iso- at some mid-ocean ridges seems to involve such fault zones, based static rebound of the footwall following tectonic denudation. on the results from ocean drilling (e.g., Cannat, 1987; Cannat et al., The relative timing of extension-related magma intrusion and 1987; Dick et al., 1987a) and the study of ophiolites (e.g., Varga and ductile deformation varies through the central Mojave metamor- Moores, 1985; Harper, 1985). phic core complex. On the scale of the small mountain ranges that Much of the work on metamorphic core complexes in the past make up the central Mojave metamorphic core complex, no corre- decade has centered on three main areas: (1) understanding rela- lation was observed between either shear zone thickness or intensity tionships between magmatism and extensional deformation, (2) un- derstanding the characteristically nonplanar geometry of the low- angle fault zones, and (3) understanding mechanics of low-angle *Present address: Fletcher: Departamento de Geologia Centro de In- normal faulting. This paper addresses issues concerning each of vestigacio´n Cientı´fica y Educacio´n Superior de Ensenada, B.C. Ensenada, B.C., Mexico; Martin: Servicio Nacional de Geologia y Mineralia-Chile, these areas. Grupo de Geologia Regional, Avda. Santa Maria 0104, Casilla 1347, San- In many metamorphic core complexes, crustal extension oc- tiago, Chile. curred during igneous activity and genetic links therefore have been GSA Bulletin; December 1995; v. 107; no. 12; p. 1468–1483; 13 figures. 1468 LARGE-MAGNITUDE CONTINENTAL EXTENSION inferred between the two processes. Elevated mantle heat flow formation of the undulations. Rock fabrics related to these two fold and/or the emplacement of mafic magma at the base of the crust has sets have important implications for the origin of the dome-and- been proposed to cause thermal softening, triggering extension (e.g., basin fault geometry, as well as for the kinematics and dynamics of Rehrig and Reynolds, 1980; Sonder et al., 1987; Wernicke et al., large-magnitude extension. 1987; Gans et al., 1989). Lister and Baldwin (1993) recently pro- This paper defines the areal extent and characterizes the rock posed a more intimate genetic relationship, suggesting that mid- types and fabrics of a large-displacement extensional shear zone crustal magmatism triggers extension and controls the location of exposed in the central Mojave Desert. Field relations and structural the brittle-ductile transition in the crust. They suggest that mid- data in this paper complement geochronologic data from the central crustal magmatism is responsible for many characteristic features of Mojave metamorphic core complex published separately by Walker core complexes, including ductile deformation of footwall rocks, et al. (in press). Extension-related intrusive rocks are used to limit spatial patterns of cooling ages, and characteristic geometric fea- the timing of deformation and to recognize and correlate Miocene tures of the fault zones. Abundant extension-related intrusions and rock fabrics throughout the multiply deformed terrane. Field rela- excellent exposures in the central Mojave metamorphic core com- tions in the central Mojave metamorphic core complex also help to plex permit tests of such proposed genetic relationships. characterize the brittle-ductile transition during crustal extension. Brittle-ductile detachment faults in core complexes character- istically define dome-and-basin structural topography that results Geographic and Tectonic Setting from interference of two orthogonal sets of upright folds (Spencer, 1982, 1984; Davis and Lister, 1988; Yin, 1991; Yin and Dunn, 1992). Miocene crustal extension and basin development are recog- Several models have been proposed to explain the origin of these nized in the central Mojave Desert from the Rodman Mountains to folds, and each makes specific predictions for (1) macroscopic ge- the Buttes (;90 km along strike; Fig. 1). A single master detach- ometry of layered rocks in the upper and lower plates, (2) meso- ment fault may underlie the entire belt. However, differences in scopic finite strain recorded in the upper and lower plates, and (3) structural style and extension magnitude permit the belt to be di- relative timing of mylonitization, deposition of rift sediments, and vided into two main segments. The southeastern segment in the Figure 1. Geologic map of the Miocene extensional belt in the central Mojave Desert, California (The Buttes to Rodman Mountains) showing the mylonitic and nonmylonitic segments of the detachment fault system. Modified in part after Dokka (1989) and Walker et al (in press); see text. Geological Society of America Bulletin, December 1995 1469 FLETCHER ET AL. Rodman Mountains and Newberry Mountains exposes no footwall Mitchel Range and provides the thickest exposures of the brittle- mylonites nor extension-related plutonic rocks. In contrast, the ductile fault zone, as well as the most intense mylonitic fabrics. The northwestern segment of the fault system, exposed from the Mitchel Hinkley domain, which includes the Hinkley Hills and Lynx Cat Range to the Buttes, is characterized by well-developed footwall Mountain, contains exposures of the fault zone that are structurally mylonites and widespread extension-related igneous intrusions. up-dip from the Mitchel domain. The Buttes domain, which in- Based on the offset of regional pre-Tertiary lithologic markers in the cludes the Buttes and surrounding hills, makes up the westernmost central Mojave Desert, ;40–50 km of northeast-directed displace- exposures of the brittle-ductile detachment in the central Mojave ment occurred across the northwestern brittle-ductile fault segment metamorphic
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