Cretaceous tectonic evolution of coastal southern California: Insights from the Catalina Schist

The relatively high‐T, high‐P/T, subduction‐related basement in the Southern California Continental Borderland preserves important information regarding the late Mesozoic tectonic evolution of coastal southern California. Integrated petrologic, geochemical, and geochronological study of the subduction‐related Catalina Schist exposed on Santa Catalina Island documents the formation of lawsonite‐blueschist through amphibolite facies rocks during progressive underplating in a progressively cooling, newly initiated subduction zone. Petrologic and ⁴⁰Ar/³⁹Ar thermochronologic data demonstrate similar, perhaps shared, retrograde PT histories for all but the lawsonite‐albite facies rocks under pumpellyite‐actinolite facies conditions as early as ∼95–100 Ma. The ⁴⁰Ar/³⁹Ar data, together with the abundance of pumpellyite‐actinolite facies vein assemblages and lack of higher‐P/T (epidote‐ or lawsonite‐blueschist) over‐printing in high‐grade units, are consistent with the rapid emplacement of the Catalina Schist into the shallow‐ to mid‐crust by ∼ 95 Ma by combined underplating and tectonic/erosional removal of overlying rocks in the accretionary complex (cf. Platt, 1986). The Catalina Schist and the adjacent Peninsular Ranges Batholith potentially represent paired metamorphic belts formed along southwest North America at 120 to 90 Ma. Combining the evidence for the evolution of the Catalina Schist with the previously advanced hypotheses regarding tectonic evolution of the batholith, suturing of a fringing island arc to the continent may have triggered an oceanward trench stepout and the formation of a new subduction zone, at ∼ 120–115 Ma, in which the Catalina Schist was accreted and metamorphosed.