Mesozoic Tectonism and Pluton Emplacement in the Central Sierra San Pedro Martir, Peninsular Ranges, Baja California Norte, Mexico

G. Chavez1, J.M. Fletcher1 and S.E. Johnson1,2 1. Geology Dept., CICESE, San Diego, 2. GEMOC, Macquarie

The central Sierra San Pedro Martir contains several Cretaceous plutons of varying size, and the inferred crustal discontinuity that is thought to divide the Peninsular Ranges Batholith into a western belt of island arc affinity and an eastern belt of continental-margin arc affinity. The tonalitic Potrero and zoned tonalitic-granodioritic San Pedro Martir plutons contain a concentric internal foliation, whereas the mafic Santa Cruz and Encinosa plutons are unfoliated. Plutons show both discordant (e.g. Encinosa and western margin of Potrero) and concordant (e.g. western San Pedro Martir and eastern margin of Potrero) pluton-wall-rock relations. Discordant relations suggest emplacement by brittle processes such as diking and/or stoping, whereas concordant relations and ductile wall-rock deformation suggest processes such as diapirism, in-situ magma-chamber expansion, downward wall-rock flow and/or externally imposed stresses.

Finite strains around different plutons vary from nearly pure flattening to strongly constrictional, which may partly reflect complex interactions between regional and emplacement-related strains. Three main generations of folding and cleavages are observed. D1 and D2 folds and axial plane cleavages lie subparallel to the regional NW structural trend, and pre-date emplacement of the Potrero pluton. D3 folds and fabrics formed synchronously with emplacement of this pluton, and are most easily recognized on its NW and SE ends, which lie in the regional strain shadow. S3 varies from a weakly-developed spaced cleavage to a fully-transposed fabric at the pluton contact.

The inferred crustal discontinuity lies within a zone of strongly deformed rocks between the Potrero and Sierra San Pedro Martir plutons, where over a distance of 500 m, metamorphic grade varies from subgreenschist in the west to middle greenschist in the east, and is defined by a transition from nonporphyroblastic phyllite to andalusite-bearing muscovite-biotite schist. The discontinuity does not separate fundamentally different rocks, and if it was once a major fault zone, the evidence has been destroyed in this area by later, intense ductile deformation.

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