Early Cretaceous intra‐arc ductile strain in Triassic‐Jurassic and Cretaceous continental margin arc rocks, Peninsular Ranges, California
Authors: Thomson Celeste N. , Girty Gary H. 
Published: February 2004

The Cuyamaca‐Laguna Mountain shear zone (CLMSZ) lies along the axis of the Peninsular Ranges batholith, separating it into an eastern and western plutonic zones. The shear zone involves Triassic‐Jurassic and Early Cretaceous plutonic units which intruded the Triassic Julian Schist and transects the eastern edge of a cryptic lithospheric boundary, separating oceanic crust on the west from continental crust on the east. The Julian Schist crops out on either side of the cryptic lithospheric boundary and is interpreted to represent an overlap sequence. This structural/stratigraphic relationship indicates that the contrasting lithospheric types must have been juxtaposed prior to approximately the Triassic time, and as a result, the CLMSZ probably developed in an intra‐arc setting. At least two periods of deformation produced the polygenetic CLMSZ. Structures that formed during D1 include S1 and L1. In Triassic‐Jurassic and Early Cretaceous orthogneisses, S1, a pervasive NW striking and NE dipping mylonitic gneissosity, obliterates nearly all traces of an older magmatic fabric. L1 plunges steeply to the NE, lies within the plane of S1, and is locally a well‐developed stretching lineation. D1 structures can be traced from the ∼115 Ma Oriflamme Canyon protomylonite into the adjacent Julian Schist and are represented by a well developed S‐C mylonitic structures indicative of NE–SW contraction. D1 structures in the Oriflamme Canyon protomylonite and in the ∼118 Ma Pine Valley granodiorite developed while these plutons were incompletely solidified. Hence D1 probably occurred between ∼118 and ∼115 Ma and had culminated in the 105 My emplacement of the Las Bancas tonalite. Normal convergence, ∼125 to 115 Ma, between the North American and Farallon plates is coincident with D1 and the syntectonic emplacement of the Pine Valley granodiorite and the Oriflamme Canyon protomylonite. This relationship suggests that the mechanically weak, thermally and melt‐softened cryptic lithospheric interface between oceanic and continental lithosphere may have yielded during the normal convergence event, resulting in the concentration of strain into the CLMSZ during arc magmatism. Such a conclusion underscores the possibility of using intra‐arc structures to deduce convergence patterns, as elegantly argued in several recent papers. A >12‐km long normal sense shear zone transects D1 structures and formed during D2. Mesoscopic structure associated with the normal sense shear zone includes S2, L2, and C2. D2 structures are the record of NE–SW extension between ∼105‐ and ∼94 Ma. They may be related to the vertical loading of the CLMSZ by the hanging wall block of the westward verging Santa Rosa and Borrego Springs mylonite belts or they may represent an early, local response to magmatically and structurally overthickened, gravitationally unstable crust. In the latter interpretation, D2 structures are the harbingers of Tertiary‐aged, gravity‐driven collapse of the SW Cordilleran margin.