Structure, emplacement and lateral expansion of the San Jose ´ tonalite pluton, Peninsular Ranges batholith, Baja California, Me ´xico S.E. Johnson a, * , J.M. Fletcher b , C.M. Fanning c , R.H. Vernon d,e , S.R. Paterson e , M.C. Tate d a Department of Geological Sciences, University of Maine, Orono, ME 04469-5790, USA b Departamento de Geologia, CICESE, Km 107 Carratera Ensenada-Tijuana, Baja California, Me ´xico c Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia d Department of Earth and Planetary Sciences, and Key Centre for the GEMOC, Macquarie University, Sydney, New South Wales 2109, Australia e Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089-0740, USA Received 8 May 2001; received in revised form 15 November 2002; accepted 17 January 2003 Abstract The 108 km 2 San Jose ´ pluton forms part of the Jurassic to Cretaceous Peninsular Ranges batholith of northern Baja California, Me ´xico. The pluton was formed by three nested, southward-migrating intrusive pulses, and the internal contacts between them indicate juxtaposition while the adjoining pulses were magmas. SHRIMP U – Pb zircon data indicate that the entire pluton was emplaced in less than 4.4 m.y.; ages of the individual pulses cannot be separated at the 95% confidence interval, owing primarily to low uranium content of zircon. Detailed structural data and geologic mapping are consistent with a component of asymmetrical, lateral expansion at the site of emplacement. The direction of maximum lateral expansion may have been controlled by thermal, compositional and resulting rheological gradients in the surrounding wall rocks. A carapace of predominantly solid-state deformation marks the northern two-thirds of the pluton, and the early stages of this fabric may have formed in the presence of a small amount of melt. Lack of evidence for syn- to post-emplacement regional ductile deformation around the pluton suggests that this carapace was deformed during the lateral expansion of the pluton. q 2003 Elsevier Ltd. All rights reserved. Keywords: Foliation development; Geochronology; Pluton Emplacement; Submagmatic flow; Subsolidus flow 1. Introduction This paper examines the structure and emplacement history of the 108 km 2 San Jose ´ tonalite pluton, located in the Sierra San Pedro Ma ´rtir region of northern Baja California, Me ´xico (Figs. 1 and 2). The combination of excellent exposure, lack of evidence for post-emplacement regional deformation, and a detailed structural data set make the San Jose ´ pluton an unusually good candidate for testing models of in-situ expansion. This paper emphasizes the importance of lineation data in both the pluton and surrounding country rocks for evaluating the kinematics of pluton ascent and emplacement. The Jurassic to Cretaceous Peninsular Ranges batholith of northern Baja California, Me ´xico, contains approxi- mately 400 plutons of mainly tonalitic and granodioritic composition (Gastil et al., 1975), although gabbro is common in the western part of the batholith. Unlike the Sierra Nevada batholith to the north, many of these plutons are completely separated by an interconnected framework of country rocks, similar to the isolated plutons of the White–Inyo Range of eastern California (e.g. Bateman, 1992). In the western part of the batholith, the weakly metamorphosed volcanic, volcaniclastic and volcanogenic sedimentary country rocks appear to have been deformed by only one regional folding event and commonly contain lithic tuffs that are useful for finite strain analysis (Johnson et al., 1999a). For these reasons, combined with the commonly excellent exposure and access, this is an ideal region for studying the structural evolution of mid- to upper- crustal magma plumbing systems (e.g. Johnson et al., 1999a, b; Tate et al., 1999; Tate and Johnson, 2000). The San Jose ´ pluton has many characteristics in common with plutons exposed at middle- to upper-crustal levels throughout the world, including: (1) an elliptical shape with low aspect ratio (, 2:1) in plan view; (2) multiple 0191-8141/03/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0191-8141(03)00015-4 Journal of Structural Geology 25 (2003) 1933–1957 www.elsevier.com/locate/jsg * Corresponding author. E-mail address: johnsons@maine.edu (S.E. Johnson).