Independence Dike Swarm
Independence Dike Swarm
Cretaceous and Jurassic, mostly earliest Cretaceous to latest Jurassic, ~146-148 Ma
Occurs in the White-Inyo Mountains, east south-central and southern Sierra Nevada, and the Spangler Hills on the east end of the El Paso Mountains (just east of the southeast corner of the map area). Swarm extends over 250 km along a north-northwest trend in the map area from outcrops adjacent to the Garlock fault. Unit occurs to the southeast of the map area in the Granite Mountains across the Garlock fault, offset about 65 km to the east by left-lateral offset on the fault (Smith, 1962), and from there extends over 250 km to the south (James, 1989). Chiefly very fine-grained mafic diorite, quartz diorite, and granodiorite, but also includes microcrystalline granite and granodiorite porphyry and elsewhere basalt and basalt porphyry. Dikes in Sierra Nevada are pervasively metamorphosed. Individual dikes mostly strike NNW, sub-parallel to trend of swarm. Swarm consists of hundreds of dikes, filled with mainly mafic rocks and are individually about 3 m in width. The swarm may be the root of linear-fissure-array volcanoes. Swarm interpreted to be dextrally offset post-emplacement about 65 to 130 km across Owens Valley based on offset of the axis of maximum dilation (Bartley and others, 2007). Dikes are thought to be predominantly ~148 Ma (Chen and Moore, 1982), but also include dikes as young as Late Cretaceous (Coleman and others, 2000), though most studies (e.g. Bartley and others, 2007; Glazner and others, 2008) exclude the younger dikes in discussion of the dike swarm. Latest Jurassic to earliest Cretaceous dike emplacement overlapped in time with the formation of wall rock mylonite zones which accommodated shear varying in sense from sinistral to west-side-down reverse displacement. Many of the dikes have a foliation related to the orientation of mafic minerals that suggests sinistral movement of dike walls during magma cooling (e.g. Moore, 1963). Swarm intruded along dilatant fractures within a regional sinistral shear system (Glazner and others, 1999) and(or) extension related to western migration of the subduction margin following the Nevadan orogeny (Chen and Moore, 1979). Associated wall rock deformation denotes partitioning into the magmatic arc of the sinistral component of strongly oblique Late Jurassic subduction. Unit constitutes a major temporal break between suites of older, pre-Late Jurassic and younger Early Cretaceous granitic plutons in the eastern Sierra Nevada, though roughly coeval with the Guadalupe igneous complex and other plutons emplaced in the western metamorphic belt during or immediately following the Nevadan Orogeny. The presence of younger, Cretaceous dikes indicates that the region maintained a preferred orientation for dike emplacement over an extended period of time. The younger dikes lack foliation suggesting sinistral offset during cooling (Coleman and others, 2000), so do not support continuation of the sinistral component of deformation into the Cretaceous. Instead, Cretaceous dike intrusion may follow the orientation of previously developed regional deformation. The pattern of dikes in this unit shown on the map is schematic and depicts longer dikes and general area of shorter dikes.