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Description
The Cretaceous Peninsular Ranges batholith developed in a complex lithospheric setting involving oceanic crust on the west and continental crust on the east. Lying along the boundary between eastern and western plutonic zones is the regionally extensive Cuyamaca-Laguna Mountains shear zone (CLMSZ). The shear zone includes Jurassic plutons which intrude the Triassic and Early-Middle Jurassic Julian Schist and related rocks. The latter rocks are composed of continental and lesser amounts of volcanic material which were deposited in a basin along the margin of North America. Julian Schist and correlatives crop out on either side of a suture proposed by other investigators. Zircons from Jurassic plutons of the CLMSZ invariably contain an inherited Precambrian Pb component, and geochemical data as well as rare earth element modeling suggest derivation from subduction related I-type magmas variably contaminated by assimilation of sialic or pelitic material. These observations indicate that Jurassic plutons of the western zone were near a source of continentally derived materials during emplacement. At least two periods of ductile deformation produced the CLMSZ. Reverse-sense structures formed during D1 and are constrained, within the northern part of the shear zone, to have formed during the interval of -118 to -115 Ma and culminated by the emplacement of the 105 Ma Las Bancas tonalite. Subsequent d eformation generated a >12 km long normal-sense shear zone which transects D1 structures and formed during D2. D2 structures are the record of NE-SW extension and formed sometime between -105 and -94 Ma. Normal convergence between the Farallon and North American plates occurred during the interval of -125 to -115 Ma and is coincident with contractional deformation. This relationship suggests that the mechanically weak, thermally and melt softened CLMSZ may have yielded during the normal convergence event, resulting in the selective partitioning of strain into this belt during arc magmatism. D2 extensional structures in the CLMSZ are poorly understood but may have formed as the result of vertical loading or, alternatively, may represent a local response to a magmatically and structurally overthickened, gravitationally unstable crust. The results of this study imply that the Julian Schist may have been deposited across a cryptic join between oceanic lithosphere on the west and continental crust / lithosphere on the east. This model suggests that deformation within the CLMSZ is intra-arc in character and that the geochemical and petrologic variations noted across the Peninsular Ranges reflect the protracted development of a magmatic arc across a pre-Triassic lithospheric join.
