Over the past few years my principal field area has been the Peninsular Ranges batholith (PRB) in Baja California, where I have mapped approximately 600 square kilometres at 1:10,000 and 1:20,000, and another 600 in reconnaissance or by air photo. This project is collaborative with scientists from the University of Southern California, CICESE (México), the Australian National University, and Macquarie University; its underlying aim is to better understand the formation and evolution of continental crust. My colleagues and I have taken a multidisciplinary approach, involving structural and lithological mapping, structural and metamorphic analyses, geochemistry, geochronology, and tectonic syntheses. My role has mainly involved mapping, structural analysis and tectonic synthesis. The activities I describe below were previously funded by the Australian Research Council and the Méxican Conséjo Nacional de Ciéncia y Tecnologia, and is currently funded by the National Science Foundation. At the bottom of this page you will find some photos of the field area. Erwin Melis has just returned from his first field season in Mexico, where he is working towards his PhD. If you watch his page you'll see what he is up to there (migmatites, high-T, low-P metamorphisms, magmatism and structure).
The Mesozoic PRB in Baja California, México, contains one of the world's best preserved, most completely exposed and readily accessed transitions between adjacent ocean-island and continental-margin arcs. A boundary of variable width between the eastern and western portions of the batholith is known from geological, petrological, geochemical and isotopic asymmetries along its northern 900 km. This boundary is particularly well exposed in the Sierra San Pedro Martir area, where it is expressed as a strongly deformed zone up to 2 km wide juxtaposing rocks of different age, origin, metamorphic grade and deformational intensity. Recent mapping and SHRIMP U-Pb zircon ages in this region provide new constraints on the age and origin of this boundary north and west of the Sierra San Pedro Martir pluton. On the basis of our work in this area, we suggest that the exposed boundary may represent a suture zone that formed by collision of the western ocean island arc with the North American margin, which began ca. 115 Ma ago. This story was published in Geology in 1999.
While mapping in Baja, I discovered several previously unrecognized magmatic ring complexes. A ring complex is any intrusive complex that contains cone sheets and/or ring dikes. The Cretaceous Zarza Intrusive Complex is perhaps the best-preserved multiple-center, cone-sheet-bearing ring complex documented in North America. Cutaway block diagrams illustrate the 3-D structure of the complex. The 7 square kilometre elliptical complex hosts three nested, southward-migrating intrusive centers, and the northern and central centers show the same evolutionary sequence of: (1) intrusion of concentric, gabbroic cone sheets; (2) intrusion of massive gabbro cores; and (3) development of subvertical, ductile ring faults. Ring-fault kinematics indicate that both centers moved down relative to the surrounding country rocks, suggesting collapse into an underlying magma chamber. The southern center is composed of approximately equal proportions of gabbro and tonalite, and lacks cone sheets. Aluminum-in-hornblende barometry on the tonalite indicates an emplacement pressure of 2.3±0.6 kbar. The Zarza complex is surrounded by a ductile deformation aureole, and bedding in the country rocks is inward-dipping and inward-younging around the entire complex (see block diagrams). Excellent preservation of the intrusive history allowed us to evaluate the origin of the aureole, and the three most applicable models are: (1) collapse of the complex into its underlying magma chamber, (2) sinking of the complex and its chamber after solidification, and (3) formation of the aureole prior to emplacement of the complex. The preserved structural and intrusive relationships provide information on the dynamic evolution of subvolcanic magma chambers, and suggest that the complex may have been overlain by a caldera. Ring complexes probably represent magma transfer zones between high-level magma chambers and surface calderas, and so studies of these complexes can provide important clues to how magma ascends through Earth's upper crust. Detailes of our structural and geochemical work on these ring complexes can be found in Johnson et al., 1999a; Johnson and Tate, 1999; Tate et al., 1999.
In addition to mapping ring complexes, I have also conducted very detailed mapping in and around the San Jose pluton. The 108 km2 San José pluton forms part of the Jurassic to Cretaceous Peninsular Ranges batholith of northern Baja California, México. 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 3.2 ± 2.9 m.y. Ages of the first two pulses can be separated at the 95% confidence interval, and indicate minimum and maximum times between pulses of 0.3 and 6.1 m.y., respectively. Detailed structural data and geological mapping are inconsistent with end-member ascent and emplacement processes such as ideal hot-Stokes diapirism or 100% dike-fed expansion. We favor a model that involves some component of visco-elastic diapirism, but the pluton appears to have undergone asymmetrical expansion and so dike-fed growth may also have occurred. Our results challenge the application of end-member magma ascent and pluton emplacement models. The direction of maximum lateral expansion may have been controlled by thermal, compositional and resulting rheological gradients in the surrounding wall rocks. The northern two-thirds of the pluton is marked by a carapace of solid-state deformation. Lack of evidence for syn- to post-emplacement regional deformation around the pluton suggests that this carapace may have resulted from in-situ chamber expansion. See this map for over 2000 bedding and foliation measurements. See this block diagram, which shows a N-S section through the pluton. Trend lines in the block diagram follow bedding and foliation. See this map for locations of samples we collected for petrology and chemistry. A paper detailing the emplacement history of the pluton is currently in review with the Journal of Structural Geology.
This photograph shows what I consider to be the boundary between the western and eastern Peninsular Ranges batholith in my field area. The low material in the foreground is part of the Alisitos Formation. The sharp color change marks the Main Martir Thrust, and the start of the green unit on the map. In the background is a vast area of orthogneisses, migmatites, and sheeted plutons.
Aerial view of the contact of the San Jose pluton - a 100 square km tonalite.
Metasedimentary xenoliths in a granitoid at Bahia de Los Angeles, on the east coast of the Baja peninsula. At lower left is a swarm of xenoliths. In the main part of the photo is a long, narrow metasedimentary xenolith at the core of concentric dark and light bands in the granitoid. How did these bands form?
Folded metasediments in the Alisitos Formation just east of the San Jose pluton (see field map for location of the pluton).
An odd looking microgranitoid enclave in one of the tonalite plutons.
Sillimanite-bearing migmatite in the eastern part of the batholith.
An orthogneiss, or deformed granitoid, from just east of the Main Martir Thrust. In thin section the rocks shows mylonitic microstructures.