Structure, Petrology and Geochronology of the Kings-Kaweah Mafic-Ultramafic Belt, Southwestern Sierra Nevada Foothills, California

Abstract

Mafic and ultramafic igneous and metaigneous rocks with associated metasedimentary rocks are exposed as a 125 km-long northwest trending belt between the Kings and Tule rivers in the southwestern Sierra Nevada Foothills, California. These rocks form part of the western wall of the Sierra Nevada batholith; they pose some key problems concerning the structural, petrologic, and geochronological evolution of the pre-batholith tectonic framework and of the batholith itself. Detailed and reconnaissance field data is integrated with petrographic, zircon geochronological, and geophysical data to develop a sequence of structural and petrogenetic events for the Kings-Kaweah belt and the related rocks of the southwestern Sierra Nevada foothill region. The rocks of the region can be broken into five main units. 1) Tectonically emplaced Carboniferous ophiolite remnants which form the 125 km-long Kings-Kaweah ophiolite belt consisting of two sub-units: a) the Kings River ophiolite complex which consists of four large tectonic slabs separated by movement zones composed of tectonically broken and mixed ophiolitic material and by cross-cutting plutonic rocks of the batholith (a complete ophiolite stratigraphic succession is preserved within the tectonic slabs which range up to 17.5 km in length); and b) the Kaweah serpentinite matrix melange which is the more highly disrupted and internally mixed equivalent of the Kings River ophiolite complex. 2) Metasedimentary rocks of uncertain age that bound the ophiolite belt along its eastern margin or occur as small infolds within the Kaweah serpentinite melange. These rocks consist of two sub-units: a) a sequence of interbedded quartzose flysch, chert, and limestone now metamorphosed to quartz-mica schist, quartzite, and marble; and b) volcanogenic mudstone deformed into slate and now partly metamorphosed to quartz-mica schist and hornfels. The quartz-flysch sequence was either tectonically juxtaposed against the ophiolite belt or deposited unconformably above it, but subsequent deformation and metamorphism have obscured the original contact relations. The volcanogenic slate appears to have been deposited unconformably on both disrupted ophiolite and the quartzose flysch sequence. 3) Jurassic plutons that cut the disrupted Kings River ophiolite complex, and have protoclastic and hot-subsolidus deformational features which indicate magmatic emplacement and cooling in a tectonically active environment. 4) Early Cretaceous plutonic rocks that cut the ophiolite belt and the adjacent metasedimentary rocks which range in composition from olivine-hornblende melagabbro to hornblende­ biotite granodiorite. These plutonic rocks appear to be one petrogenetically related suite of rocks, and they appear to have been preferentially emplaced into the structurally weakened zone provided by the disrupted ophiolite belt. 5) Middle to late Cretaceous plutonic rocks composed mainly of quartz diorite and granodiorite that were emplaced along the eastern margin and further east of the ophiolite belt and adjacent metasedimentary rocks. The Kings-Kaweah ophiolite belt originated at an oceanic spreading zone in the Carboniferous and was subsequently transported into the proximity of the North American continental margin by sea-floor spreading processes. It was then tectonically disrupted and emplaced along a lithospheric plate juncture prior to the mid-Jurassic. Sometime during disruption and emplacement the ophiolite came in contact with the quartz-flysch sequence. During the middle and late Jurassic the structural discontinuity defined by the disrupted Kings-Kaweah ophiolite belt operated as an active tectonic suture while the surface expression of the plate juncture migrated westward. During the active life of the Kings-Kaweah.

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