Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Past is Key: Tectonic Evolution of the Pacific-North American Plate Boundary and Its Implications for Crust/Mantle Structure and Current Plate Boundary Strain Craig Nicholson Institute for Crustal Studies, University of California, Santa Barbara, CA 93106-1100 Many of the major elements of EarthScope are focused, at least initially, on evaluating the current deformation field (PBO, InSAR), velocity structure (USArray) and in-situ characteristics of an active fault (SAFOD) within the western United States. These physical characteristics, however, are all controlled by—or at least strongly influenced by—the prior tectonic history of the region. This is especially true for the most active components of the deformation field that are associated with the Pacific-North American plate boundary, as well as the current configuration of crust and uppermantle structure that reflects previous (and on-going) episodes of subduction, rifting, transform motion, and other effects of continental and oceanic plate interactions. At UC Santa Barbara, we have been developing improved kinematic models for the tectonic evolution of western North America and, in particular, the development of the Pacific-North America plate boundary [Nicholson et al., 1994; Atwater and Stock, 1998; Atwater, 2001]. These models have been able to provide workable explanations for such previously enigmatic features as: the widespread rifting and basin development in California during Miocene time; the large-scale tectonic rotation of the western Transverse Ranges; the transfer of Baja California (and other pieces of North American continental crust) to the Pacific plate; the change in direction of Basin-and-Range extension; the distribution of Miocene and younger volcanism in California; and the initiation and cumulative slip of major faults. These models thus provide a valuable, preliminary basis for evaluating and understanding plate dynamics, as well as testable hypotheses for the evolution and development of crust and upper-mantle structure. These kinematic models can be improved, however, by including more specific dynamic constraints, by extending the models to 3D, and by incorporating more recent observations, especially those that will be produced by EarthScope. A major element of these new tectonic models is the concept of microplate capture [Nicholson et al., 1994]. The basic premise of this concept is simple. As the east Pacific spreading ridge approached the western margin of North America, the intervening subducting Farallon plate began to fragment into various microplates, including the Monterey, Arguello, Guadalupe and Magdalena plates (Figure 1) [Lonsdale, 1991]. As the spreading ridge continued to approach the trench, increased coupling to the Pacific plate and/or to the over-riding North American plate caused the younger, upper part of the partially subducted microplate to detach from the older, colder, sinking slab. Once this occurred, the slab-pull force was effectively removed, and both subduction (beneath North America) and spreading (relative to the Pacific plate) ceased. In all cases, the remnant piece of partially subducted microplate was then captured by the Pacific plate. This meant that partially subducted oceanic lithosphere was now moving beneath western North America with Pacific plate motion. In the case of the Monterey microplate, this capture occurred just after anomaly 6 time (~19.5 Ma). The slip vector along the gently NE-dipping subduction interface changed from slightly oblique subduction to transtensional dextral transform motion. This change in slip vector, and shift of Pacific plate motion eastward along the already subducted Monterey-plate interface, implies that the San Andreas transform initiated as a system of low-angle faults that locally subjected the overriding continental margin to distributed basal shear and crustal extension. This basal shear, in combination with a unique plate geometry, was then responsible for the large-scale tectonic rotation of the western Transverse Ranges (WTR) (Figure 1). The model helps explain the timing of initial WTR rotation and basin formation, the sudden appearance of widely-distributed transform motion well inland of the margin in early-Miocene time, how and why the WTR uniquely rotated as a large coherent crustal block, and several other fundamental structural characteristics of central and southern California. The model also provides a direct mechanism for the transfer of North America continental crust to the Pacific plate. A prime example is Baja California, which began separating from North America (and taking on components of Pacific plate motion) once the underlying partially subducted Guadalupe and Magdalena microplates were captures at about 12 Ma (Figure 1) [Nicholson et al., 1994].. Although this model is at present qualitative, the model can provide relatively precise quantitative estimates of the relative position through time of offshore oceanic plates with respect to onshore California geology. This is because the relative position through time of various pieces of North American crust (that are now west of the San Andreas fault and moving with some component of Pacific plate motion) is largely a function of when the underlying microplate was captured by the Pacific plate, what the geometry of the underlying microplate was, and how much Pacific plate motion the microplate was able to impart to the over-riding piece of North American crust. Moreover, because the model is based on plate tectonic principles, it makes specific predictions about how and where plate boundary strain should be accommodated, and, in particular, the distribution of where remnant pieces of subducted oceanic lithosphere should still be present under the California margin. A critical element of the model is knowing how far under the continental margin the microplate extended at the time of capture. This geometry controls the degree to which motion of the partially subducted microplate can influence the over-riding North American crust and thus how plate boundary strain is (was) ultimately distributed. Better resolution and confirmation of where these remnant fragments of partially subducted oceanic lithosphere are, and what their geometry is, will thus provide an important critical framework for understanding the current pattern of plate boundary strain, and as such, should be an important research objective for EarthScope. The proper design and conduct of effect experiments to accomplish this objective, however, must necessarily have a fine appreciation for this previous tectonic history that has been active along the California margin for at least the last 20 million years. References Atwater, T., Sliding and spinning – How our plates have moved over the past 30 million years, Pacific Section AAPG/Cordilleran Section GSA meeting, Universal City (2001). Atwater, T. and J.M. Stock, Pacific–North America plate tectonics of the Neogene southwestern United States: An update, International Geology Review, v. 40, p. 373-402 (1998). Lonsdale, P., Structural patterns of the Pacific floor offshore of Peninsular California, in J.P. Dauphin and B.T. Simoneit, eds., Gulf and Peninsula Province of the Californias: American Association of Petroleum Geologists Memoir 47, p. 87–125 (1991). Nicholson, C., C.C. Sorlien, T. Atwater, J.C. Crowell and B.P. Luyendyk, Microplate capture, rotation of the western Transverse Ranges, and initiation of the San Andreas transform as a low-angle fault system, Geology, v. 22, p.491–495 (1994). PACIFIC PLATE ino NORTH AMERICA PLATE Z Z P W yF ~12 Ma JDFP yF RTJ IB OB RTJ GU A PLA D. TE E E ~5 Ma F 0 Ma MTJ MTJ FZ Salinia MTJ yF Z OB WTR WTR OB IB PACIFIC PLATE IB OB PACIFIC PLATE SB RTJ BAJA NORTH AMERICA PLATE T-AF BAJA GP MP NORTH AMERICA PLATE RTJ PACIFIC PLATE r Pen insu la Ran g es SLB SM B rra SG Mu N RTJ NORTH AMERICA PLATE oc rra Sierra Nevada nd rra NORTH AMERICA PLATE nia Sali s u lar R an g es Pen in P FARALLON PLATE Me Mu SG AR D Mu Z Z MTP Z MTJ PACIFIC PLATE SG yF FZ MTJ FZ Z MF MF rra ~16 Ma ino Z FF PACIFIC PLATE Mu oF oc SLB S TR MB MTJ er cin nd MAGD AL PLAT ENA Z ne JDFP Me C NORTH AMERICA PLATE rF FZ Pio do ~20 Ma WTR ee ino B Men FARALLON PLATE on oc evada nd Pi ~24 Ma JUAN DE FUCA Me PLATE Sierra N A 250 km Figure 1. Tectonic model for Pacific-North America plate interactions since 24 Ma [Nicholson et al., 1994]. Each time a partially subducted microplate is captured, a part of North America upper plate is transferred to the Pacific plate. These captured microplates strongly influence the subsequent pattern of plate boundary strain, as well as the development of crust and upper-mantle structure beneath western North America.