Download Past is Key: Tectonic Evolution of the Pacific

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).
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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.