ANATOMY OF A 100 MA SUTURE, NORTHERN PENINSULAR RANGES BATHOLITH

Within the northern Peninsular Ranges Batholith, major structural dislocation occurred during suturing of relatively thin Mesozoic oceanic crust to older, thicker continental crust. During suturing, the style of ongoing pluton emplacement radically changed from passive to dynamic, and prebatholithic rocks underwent pronounced structural and thermal changes. In the Diamond Valley Reservoir area, a pronounced thermal gradient resulted from the suturing. West of the suture are greenschist facies rocks of medium-P/T type. The suture-affected rocks are subdivided into four structural domains that include five mineral isograds. Penetrative structural features synchronous with suturing extend over a 5 km wide zone. Mineralogic changes are evident in a 3.5 km wide zone. Structures related to the suturing include three progressive structural transpositions and folding. Regional S₁ schistocity was transposed to S₂, which in turn was transposed to S₃ that was folded and transposed to S₄. Thermal effects resulting from structural dislocations is a low-P/T type metamorphic gradient. Progressive changes in mineral assemblages of andalusite, cordierite, sillimanite, garnet, and K-feldspar reflect changes from greenschist facies to lower granulite facies. East of the suture rocks are amphibolite facies medium-P/T type. West of the suture, plutonic rocks predating suturing intruding greenschist facies rocks indicate pressures of 2-3 kb. Pressures of plutonic rocks contemporaneous with suturing indicate pressures of 5-6.5 kb while those to the east of the suture average 4.5 kb.