EARLY CRETACEOUS INTRAARC DUCTILE STRAIN IN TRIASSIC-JURASSIC AND CRETACEOUS CONTINENTAL-MARGIN ARC ROCKS, PENINSULAR RANGES, CALIFORNIA

The Cuyamaca-Laguna Mountain shear zone (CLMSZ) lies along the axis of the Peninsular Ranges batholith, separating it into an eastern and we stern plutonic zones. The shear zone involves Triassic-Jurassic and Ea rly Cretaceous plutonic units which intruded the Triassic Julian Schis t and transects the eastern edge of a cryptic lithospheric boundary, s eparating oceanic crust on the west from continental crust on the east . The Julian Schist crops out on either side of the cryptic lithospher ic boundary and is interpreted to represent an overlap sequence. This structural/stratigaphic relationship indicates that the contrasting li thospheric types must have been juxtaposed prior to approximately the Triassic time, and as a result, the CLMSZ probably developed in an int ra-arc setting. At least two periods of deformation produced the polyg enetic CLMSZ. Structures that formed during D-1 include S-1 and L(1). In Triassic-Jurassic and Early Cretaceous orthogneisses, S-1, a pervas ive NW striking and NE dipping mylonitic gneissosity, obliterates near ly all traces of an older magmatic fabric. L(1) plunges steeply to the NE, lies within the plane of S-1, and is locally a well-developed str etching lineation. D-1 structures can be traced from the similar to 11 5 Ma Oriflamme Canyon protomylonite and in the similar to 118 Ma Pine Valley granodiorite developed while these plutons were incompletely so lidified. Hence D-1 probably occurred between similar to 118 and simil ar to 115 Ma and had culminated in the 105 My emplacement of the Las B ancas tonalite. Normal convergence, similar to 125 to 115 Ma, between the North American and Farallon plates is coincident with D-1 and the syntectonic emplacement of the Pine Valley granodiorite and the Orifla mme Canyon protomylonite. This relationship suggests that the mechanic ally weak, thermally and melt-softened cryptic lithospheric interface between oceanic and continental lithosphere may have yielded during th e normal convergence event, resulting in the concentration of strain i nto the CLMSZ during arc magmatism. Such a conclusion underscores the possibility of using intra-arc structures to deduce convergence patter ns, as elegantly argued in several recent papers. A >12-km long normal sense shear zone transects D-1 structures and formed during D-2. meso scopic structure associated with the normal sense shear zone includes S-2, L(2), and C-2. D-2 structures are the record of NE-SW extension b etween similar to 105- and similar to 94 Ma. They may be related to th e vertical loading of the CLMSZ by the hanging wall block of the westw ard verging Santa Rosa and Borrego Springs mylonite belts or they may represent an early, local response to magmatically and structurally ov erthickened, gravitationally unstable crust. In the latter interpretat ion, D2 structures are the harbingers of Tertiary-aged, gravity-driven collapse of the SW Cordilleran margin.