Structure, metamorphism, and geodynamic significance of the Catalina Schist terrane

Abstract

This guide begins with an overview of the internal structure and petrology of the Catalina Schist terrane as exposed on Santa Catalina Island, California, followed by a discussion of the tectonic setting and exhumational history of the terrane, and the Cenozoic tectonic and geological evolution of the Inner Borderland, within which it lies. The guide then presents an itinerary for a three-day field trip from 9–11 May 2020. Next, we present a tectonic model for the formation of the Catalina Schist, followed by a discussion of its relationship to the Pelona, Orocopia, Rand, and related schists in southern California.

Is the Inverted Field Gradient in the Catalina Schist Terrane Primary or Constructional?

“…(2022) suggested on the basis of lawsonite pseudomorphs enclosed in garnet from a high‐grade block that it passed through the lawsonite‐eclogite field at around 22 kbar, and was subsequently heated to ∼800°C at 10 kbar during decompression. An early high‐pressure history for the blocks is consistent with the local preservation of eclogite facies assemblages (Platt et al., 2020). For the purposes of this discussion we take the range 650–730°C and 13.4–14.4 kbar for the peak metamorphism of the Amphibolite Unit as a whole (Figure 4).…”

“…The present sequence is not more than ∼700 m thick at maximum, though Platt et al. (2020) estimate that the Amphibolite Unit was originally at least 2 km thick. The lack of any blueschist facies overprint in the Amphibolite Unit suggests that it was juxtaposed with the lowest grade rocks at pressures of <7 kbar (equivalent to ∼ 26 km depth).…”

“…The entire body has a strong deformational fabric, lacks primary textures, and shows evidence of partial melting during metamorphism (Sorensen & Barton, 1987). The Mg‐rich bulk composition, and the presence of a consistent compositional layering, suggests that it may represent a body of cumulate gabbro (Platt, 1976). The mafic amphibolite is overlain by metasedimentary rocks, comprising coherent (but strongly deformed) migmatitic paragneiss and quartzite.…”

“…The intermediate and high‐grade metamorphic sequence on Catalina is not more than 700 m thick at present (Figure 2), though its components may originally have been much thicker. Measured normal to the metamorphic foliation, the coherent mafic amphibolite is ∼2,000 m thick, for example, (Platt et al., 2020). The present structure is a result of essentially post‐metamorphic faulting (Platt, 1976).…”

“…Bailey (1941) originally recognized the presence of the high‐grade rocks on Catalina, and described them as being in thrust contact with lower grade schists that he referred to as Franciscan. Platt (1975, 1976) showed that rocks of intermediate metamorphic grade, which he referred to as the Greenschist Unit, are in tectonic contact above blueschist facies rocks (the Blueschist Unit), and both are tectonically overlain by the high‐grade rocks (Amphibolite Unit). Grove and Bebout (1995) subsequently showed that the intermediate grade rocks include slices of epidote blueschist and albite‐epidote amphibolite grade, and Grove et al.…”