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My research examines the tectonic evolution of continental
margins by combining detailed geologic mapping with a variety of laboratory
techniques, including geochronology, geochemistry, petrography and paleomagnetics.
This research is on a spectrum of topics, including volcanology, sedimentology,
structural geology, petroleum geology, economic geology, and geochemistry.
Most of my research has focused on Paleozoic to Mesozoic convergent margins
of the southwest Cordilleran US and Mexico. More recently, I have moved
to Cenozoic tectonics, mainly by examining the volcanology, structure
and landscape evolution of the Sierra Nevada in California. This new
project examines the controls of continental rifting as well as subduction
on magmatism, faulting, and landscape evolution.
I believe that my early papers established me as one of the pioneers of a field
that combines volcanology and sedimentology to solve tectonic problems. These
fields of research were considered quite separate before this time, yet volcano-sedimentary
terranes are extensive; this left huge gaps in our understanding of the evolution
of continental margins. My early papers represented some of the first studies
of submarine volcanoes that have been uplifted onto land, and were among the
first to recognize and describe large silicic centers in the stratigraphic record
(Busby-Spera, 1984a, 1986). Additionally, this work involved me at a relatively
early stage in a topic that became a major field of research, turbidite sedimentology
(Busby-Spera, 1985). I did this volcanological and sedimentological work by "seeing
through" upper greenschist grade metamorphism (Busby-Spera, 1984b), while
also mapping and dating deformational fabrics in both metamorphic and plutonic
rocks (Saleeby and Busby-Spera, 1986; Busby-Spera and Saleeby, 1987).
My research has helped us to recognize that extensional continental arcs are
common, particularly in the geologic record, and that continental arc extension
has played a major role in the growth of continents (Busby-Spera, 1988a; Riggs
and Busby-Spera, 1990; Schermer and Busby, 1994; Adams et al., 1997). I believe
my work also contributed to the understanding that extensional arcs are characterized
by abundant silicic calderas, in both continental and oceanic settings (Busby-Spera,
1988; Riggs and Busby-Spera, 1991; Adams et al., 1997; Busby et al., 2002; Busby,
2004: Busby et al., 2006). Our work in the Jurassic arc of California and Arizona
also addressed the question of the tectonic setting of what is perhaps the largest
erg field in the geologic record, by demonstrating that grabens within the extensional
continental arc accommodated great thicknesses of craton-derived eolianite sands
throughout Early and Middle Jurassic time (Busby et al., 1990; Riggs et al.,
1993; Adams et al, 1997; Busby et al., 2002; Busby et al., 2005). We were also
able to show that the volcanic and sedimentary styles in the Jurassic arc were
strongly controlled by climatic change, as North America moved very rapidly from
the horse latitudes to temperate latitudes (Busby et al. 2005). Through detailed
volcanological, sedimentological and structural analysis, we have defined distinguishing
characteristics for extensional intra-arc basins (Riggs and Busby-Spera, 1990;
Schermer and Busby, 1994; Busby et al., 2002). We proposed methods for distinguishing
between releasing bend basins and fault step-over basins in intra-arc strike
slip settings (Bassett and Busby, 2005; Busby and Bassett, in press). We were
also able to relate transpressional fabrics in Cretaceous arc plutonic rocks
of the Sierra Nevada to reconstructions for the Farrallon-Kula plates (Busby-Spera
and Saleeby, 1990).
The Cordillera records most of the global crustal growth during Phanerozoic
time, thereby serving as a modern/young analog for the genesis of the continents
in
Paleoproterozoic time. The Baja California Peninsula, where I have worked,
represents a volumetrically important component of this growth. In an earlier
paper (Busby
et al., 1998), I reconstructed the subduction component of crustal growth
in Baja, to propose an evolutionary trend for convergent margins facing
large
ocean basins. In a more recent paper (Busby, 2004), I added in the "along strike" component,
to propose that oceanic terranes in Baja were accreted to the margin by a "soft
docking" mechanism of sinistral transtension. In this way, my work has
contributed to the now widely-accepted idea that the Cordilleran margin is
the type example
of continental growth by protracted subduction combined with large-scale
along-strike translations. My models for the tectonic assembly of Baja have
drawn on detailed
studies, some of which are discussed briefly here.
Our sedimentological studies in Baja California mainly involve turbidite systems.
My students and I were among the first to do outcrop studies of a submarine canyon
(Morris and Busby-Spera, 1988) and a deepwater valley-levee complex (Morris and
Busby-Spera, 1990). These systems in Baja have since been subjected to intense
(unpublished) field study by numerous petroleum industry workers in search of
outcrop analogs for their subsurface reservoirs. Our study of a fault-controlled
submarine canyon on Cedros Island (Smith and Busby-Spera, 1993a) has reservoir
analogs in the North Sea; it was also one of the first places where the Platt
model for extensional unroofing of blueschist terranes was applied (Smith et
al., 1991; Busby, 2004). Within another turbidite succession, we discovered the
only known example of Pacific-margin mega-landslides generated by the Chicxulub
Cretaceous-Tertiary bolide impact (Busby et al., 2002).
Our volcanological studies in Baja California have focused on large, structurally-intact
pieces of oceanic arcs and backarc basins. The backarc basin represented by the
Gran Canon Formation and its substrate likely remains perhaps the best-preserved
backarc basin exposed on land (Busby-Spera, 1987, 1988b; Critelli et al., 2002).
The Alisitos arc provides a time-integrated view of extensional oceanic arc evolution,
from surficial to mesozonal levels, at a level of detail that cannot be gained
in modern oceanic arcs (Busby et al., 2006). Additionally, our process-oriented
papers have demonstrated the importance and nature of magma-wet sediment interactions
in eruption, deposition and resedimentation in the oceanic arc environment (Busby-Spera
and White, 1987; White and Busby-Spera, 1987).
My interests in volcanology took me to the Appalachians, where I joined a US
Geological Survey project on an Ordovician volcanic-hosted massive sulfide deposit.
This project gave me the chance to devise predictive models for deepwater silicic
volcanism (Busby et al., 2003; Kessell and Busby, 2003; Busby, 2005). This work
followed on an earlier paper describing and interpreting welding in submarine
calderas (Kokelaar and Busby, 1992). My interests in turbidite sedimentology
have taken me to the Maritime Alps and the Pyrenees, where I have taught field
courses for the petroleum industry, and to San Clemente, California, where we
published a new interpretation of a turbidite system that is visited by hundreds
of petroleum geologists and university professors and students every year.
My major project for the past few years, funded by the Petrology program at NSF,
combined field mapping with geochronology, petrology and paleomagnetic studies
to characterize the Cenozoic tectonic evolution of the Sierra Nevada (Busby et
al, 2007a, 2007b, in press; Putirka and Busby, 2007, in press; Garrison et al.,
2007, in press). We have just been awarded two NSF grants (from the Tectonics
program and the Petrology program) to continue our new work in the Sierra Nevada.
This work is addressing the following questions: What can Cenozoic volcanic-volcaniclastic
strata tell us about the evolution of the Sierran landscape, and how does this
compare with models for landscape evolution of the western U.S.? What can these
rocks tell us about the paleogeographic and tectonic evolution of the Ancestral
Cascades arc, and what is the origin and significance of high-K2O volcanism?
What is the age and nature of range-front faulting in the Sierra Nevada, and
what is its relationship to the birth of the future plate boundary? I have two
PhD students involved in this work.
A third PhD student has come to UCSB to work with me on a new project, studying
the largest silicic igneous province on earth, in the Sierra Madre Occidentale
of Chihuahua, Mexico. Our goal is to determine the paleogeographic, volcanological,
sedimentological, petrologic and structural evolution of this world-class igneous
province, by targeting a sector that we believe escaped disruption by younger
tectonics events (based on my preliminary field work with Elena Centeno Garcia
of UNAM). This work is important for determining the tectonomagmatic phenomena
responsible for the formation of large silicic igneous provinces, a common but
poorly understood feature of active continental margins globally through time.
Also with Elena Centeno Garcia, I am preparing two manuscripts on the tectonic
assembly of western Mexico, one that uses detrital zircon to define Paleozoic
to Mesozoic terranes and their sites of origin, and another that uses geochemical
data to define tectonic assemblages and processes. Additionally, I have begun
working on a multi-PI detrital zircon proposal to submit to the Continental Dynamics
program. This study represents a return to Cordillera-wide analysis (Saleeby
and Busby-Spera, 1990), by using it as a natural laboratory to determine how
continents are assembled. In addition, I am spending the 2007-2008 academic year
on sabbatical leave at the University of Granada (Spain), working with Antonio
Azor on Neogene tectonics and sedimentation in the Betic Cordillera.
Throughout most of my career, I have balanced the challenges of being a research
science professor (and field geologist) with the demands of being the mother
of three daughters. I have thus served as a role model for female undergraduate
and graduate students who hope to combine motherhood with a challenging career. |
