Download Enhancing Earthscope by Constraining Vertical Motions of the

Enhancing Earthscope by Constraining Vertical Motions
of the Continental Crust and Surface
J.A. Spotila, Virginia Tech; [email protected]
The Earthscope facility will have an enormous impact on
geoscience, largely because of its size, systematized approach, and
incorporation of new technologies to constrain key geologic problems.
With minor modification, the facility can be enhanced using other new
technologies and the same systematic strategy to address additional aspects
of tectonic and geodynamic processes and a broader spectrum of other
scientific questions. One data set that is not heavily emphasized by the
current vision of Earthscope is the vertical motion of the continental crust
and surface. By expanding the range of observations to systematically
constrain decadal surface uplift using space-based observation or absolute
gravimetry and the medial-term (103 -105 yr) and long-term (>106 yr)
denudational history of the continent using cosmochronology and lowtemperature thermochronology, the scientific value of Earthscope may be
enhanced. Three scientific motivations and some basic ideas on
implementation of these enhancements are described below.
1) Plate motion partitioning via vertical motion of continental crust
That mountain building is representative or even synonymous with
active continental tectonics illustrates the importance of vertical crustal
motion in some plate boundary systems. In many tectonic settings,
including examples along the Pacific-North America plate boundary, surface
uplift, exhumation, and mass redistribution via erosion and sedimentation
play important roles in accommodating long-term plate motion. The
kinematic data collected as the main objective the PBO facility will enhance
our understanding of how horizontal plate motions and strain are distributed
at the decadal timescale. In addition, surface uplift may locally be rapid
enough to be constrained using continuous GPS over the timescale of 5 to
10 years (i.e. >1 mm/yr). In these areas it will be very useful to compare
short-term vertical strain with the long-term record of vertical crustal
motions, which can be approximated by exhumation histories of rock or
depositional histories of adjacent basins. In addition, geologic studies of
exhumation may reveal important mechanisms of plate motion
accommodation where present-day uplift rates are too small to be measured.
Two examples illustrate why long-term vertical motions should be a
significant part of PBO's scope.
A major problem that all four branches of Earthscope will address is
the behavior and mechanics of the San Andreas fault. Key questions
include whether the fault is strong or weak, whether it deforms as a wrench
or partitioned system, how it accumulates and releases strain, and how it
has evolved over the long-term. In some locations, measurable components
of the San Andreas' slip budget are manifest as vertical tectonism. This is
true where local geometric complexities force convergence, such as at a
bend in the Santa Cruz Mountains, at the intersection with the Garlock fault,
or at the restraining bend of San Gorgonio Pass. Shortening and related
uplift and exhumation are also important along much of the fault's length,
because of its transpressive obliquity to fault motion. This includes the 5o
obliquity in central California that built the Coast Ranges and the greater
obliquity along the big bend in southern California that helped build the
Transverse Ranges.
In the former, the kinematics of convergent
deformation are best constrained by structural reconstruction of fold/thrust
belts. In the latter, constraining vertical tectonism as the wholesale uplift of
rocks is more important, as convergence in crystalline terranes is well
reconstructed using exhumation history. In one part of the Transverse
Ranges (Yucaipa Ridge), for example, narrow crustal slivers within the San
Andreas fault zone itself have exhumed at rates of up to 5 mm/yr and have
thus accommodated a significant component of plate motion via mass
redistribution. This partitioning of motion into vertical tectonism within the
fault zone has implications for models of the mechanics of transpression as
well as for the long-term evolution of the fault system. Constraining
exhumation and vertical crustal motions are thus important to understanding
the plate boundary processes associated with the San Andreas fault.
A second critical topic in tectonics is the importance of surface
processes to deformation partitioning in convergent plate settings. Studies
have shown that orographically-controlled precipitation gradients can lead to
concentration of exhumation along portions of mountain belts, thereby
influencing convergent tectonic systems. An example of where this may be
true is in the Chugach/St. Elias Mountains of southeast Alaska, where
underthrusting of the Yakutat plate has caused intense uplift and exhumation
in the North America plate over the past several million years. One reason
why this convergence may have been so well accommodated is the rapid
erosion of glaciers, which have dominated the mountain range for its entire
existence. Whether glacial erosion has played a deterministic role in plate
motion partitioning can be tested by comparing exhumation patterns to
patterns of precipitation and glaciation. Studying vertical tectonism is thus
critical to understanding this portion of the plate boundary, where a
transition is made from continental transform to subduction zone and plate
motion is manifest mainly as orogeny. This example also illustrates how
important the long-term record of vertical tectonism can be. Present surface
uplift of southeast Alaska is complicated by relaxation associated with large
earthquakes and isostatic rebound associated with deglaciation. To achieve
a record of how plate motion is accommodated by mass redistribution at the
surface thus requires studies of long-term mountain building.
These examples show how it is worth constraining vertical
tectonism in some locations to study tectonic processes. The basic tools
required for such constraints can include basic geologic data (e.g.
measuring vertical motions by correlating offset features), stratigraphy (e.g.
basin reconstruction), low-temperature thermochronometry (e.g. apatite [UTh]/He dating, Tc=60-80o C; or apatite fission track dating, Tc=105o C), and
drainage basin cosmochronology (e.g. 26 Al and 10 Be, exposure ages and
erosion rates over 10 7 -105 yr). Selective application of these techniques in
the PBO or SAFOD initiatives may provide useful constraints on vertical
tectonism, which may in turn enhance the understanding developed by
Earthscope of plate tectonic processes.
2) Geodynamics of the crust and lithosphere
Unprecedented data on the thermal, compositional, and rheological
structure of the crust and upper mantle to be developed by USArray will
greatly enhance our understanding of the lithosphere's dynamic behavior. It
will be interesting to see, for example, how topography relates to crustal
thickness or mantle density structure. Although surface elevations may be
useful in understanding the structure and processes in the lithosphere
below, a greater understanding may be achieved if the history and modern
rates of change in surface elevation can be measured.
There are numerous examples of where structure and processes
within the upper mantle and crust may be reflected in topography and
surface uplift. The 2-km-high Colorado Plateau of the western U.S. is one,
as its origin has been linked to crustal thickening via magmatic injection or
intra-crustal flow, buoyancy associated with mantle density gradients and
thermal structure, delamination of the mantle lithosphere, or simply
thickening via convergent tectonics. USArray may shed light on what is
actually beneath the Plateau today, and it would be useful to know whether
the surface elevations are increasing or decreasing accordingly. A similar
(non-U.S.) example of the importance of lithospheric structure on orogenic
plateaus is Tibet, where topography may directly represent the viscosity of
the flowing lower crust. Other examples of where topography and uplift
may be important in understanding the behavior of the crust and upper
mantle include California's Transverse Ranges, where topography appears
associated with a high density anomaly in the upper mantle that may
represent a sinking slab, or the southern Sierra Nevada, where a high
density anomaly does not correlate with topography.
For much of the western U.S. and Alaska, surface elevation change
at the decadal timescale may be measured by continuous GPS of the PBO
and InSAR, depending on the magnitude of motion and the timeframe
studied. To fully understand the dynamic behavior of the lithosphere that
will be imaged by Earthscope, however, it may be important to constrain the
decadal change in surface elevation at locations spread across the entire
continent. In almost all cases, we would expect such change to be
unmeasurable, given the lack of geologic evidence for sustained vertical
motion. There are locations, however, where rapid elevation change has
been suggested by leveling studies over the previous century, such as the
Adirondack Mountains in New York, the Blue Ridge Mountains in North
Carolina, and the coast of Maine. Some of these may represent change
associated with glacial rebound, an important process worth constraining.
Some of these examples have also been interpreted by geodesists as
representing epeirogeny, in which broad scale warping of the continental
surface occurs rapidly over short, but unsustained periods. An example of
this view can be seen in a widely-used undergraduate geology textbook
(Understanding Earth; Press and Siever, 3rd Ed., 2001), as a map of
present-day uplift and subsidence rates across the entire U.S. is shown,
including rates of over 1 cm/yr uplift across the midwest and southern
Appalachians (Figure 21.19, p. 500). Although this map is not well
constrained, it illustrates that epeirogeny as a dynamic lithospheric process
is still so poorly known that it remains a poorly defined element in
introductory geologic instruction.
Expansion of decadal surface uplift measurements beyond the region
covered by the PBO could lead to an ultimate goal of creating a map of
surface uplift and subsidence rates for the entire U.S. For example,
continuous GPS stations (vertical accuracy gives resolution of about 1
mm/yr in 5-10 yr) or sites of repeated measurements with absolute, fallingmass gravimetry (accuracy of 0.01 µGal can give vertical resolution of up to
1 mm/yr) could be spread across a moderate-resolution grid (~500 km)
throughout areas not covered by the PBO, either as an expansion of PBO or
a broadening of the tools of USArray. Observations of this type are already
being made along the East Coast, where absolute surface motions are being
measured to constrain the stability of tide gauges. The resulting pattern
could subsequently be compared to numerous structural aspects of the crust
and upper mantle.
It would also be interesting to compare the observations of recent
surface uplift across a continent-wide grid with constraints on long-lived
vertical crustal motion. Geomorphic studies could thus be useful, as
sustained topographic change is generally associated with perturbation of
erosional and depositional systems. For example, at each location where
decadal surface uplift is measured, it would be worthwhile to constrain
long-term (106 -107 yr) exhumation rates using low-temperature
thermochronometry or medial-term (103 -105 yr) erosion rates using basinscale cosmochronology and sedimentology. A comparison of present-day
surface uplift and long-term could explore the long-term evolution of
geodynamic processes and would also relate importantly to other scientific
problems.
3) Landscape evolution of the continent
Inclusion of measurements of decadal surface uplift or long-term
exhumation will improve our knowledge base and understanding of tectonic
and geodynamic processes. It will also open doors to study other Earth
processes in a systematic way. The long-term evolution of landscapes is of
considerable interest but is still only partly understood. Major steps in our
understanding of the processes involved are coming from many sources,
including complex numerical simulations of landscape evolution, theoretical
and empirical studies of individual erosional processes (e.g. fluvial bedrock
incision), and quantification of how particular landscapes have developed
through time (e.g. at the basin scale or mountain scale). Earthscope now
presents the opportunity to systematically control how the surface of the
continent is actually changing and has changed over the past thousands and
millions of years. For example, if basin-scale erosion rates (103-105 yr
timescale) are measured across even a minimum-resolution grid spanning
the continent, the role of climate and other parameters in erosional processes
can be systematically tested. If longer-term exhumation is constrained
across the continent, maps of exhumation rate may be compared to known
tectonic histories to investigate the timescale of topographic decay in
mountain belts. The keys to these examples are that the data will be
collected across regions where it may not yet exist and will be collected in a
systematic way that can be easily accessed and synthesized. Other types of
data important for surface processes, such as hydrologic or meteorologic
data, could also be collected systematically at each USArray and PBO site.
These data would be particularly useful given that high-resolution
topographic and remote sensing data may become available for many of the
PBO and USArray localities. This approach would thus take advantage of
the research strategy of Earthscope, to increase our understanding of
geomorphic and atmospheric processes.