2011 - Cooperative Institute for Research in Environmental Sciences ...

Anne Sheehan
Deep Structure Beneath Rocky Mountain
Foreland Arches and Sedimentary Basins
FUNDING: NATIONAL SCIENCE FOUNDATION
The Bighorns Arch
Experiment is an integrated
seismological and
structural geology investigation
to understand
how basement-involved
foreland arches form; how
they are connected to plate
tectonics; and what they
reveal about the rheology
of the continental
lithosphere. Basementinvolvedarches—characteristic
of the Laramide
orogeny that formed
the Rocky Mountains 60
million years ago—are
uplifts of deep crystalline
basement rocks, which
are rocks beneath what is
often several kilometers or more of sedimentary cover. They
are typically flanked by deep sedimentary basins. Hypotheses
for the formation of basement-involved arches include
subcrustal shear during shallow subduction, crustal detachment,
lithospheric buckling and domino-style lithospheric
faulting. All of these hypotheses predict different lower
crustal and crust-mantle boundary geometries beneath
foreland arches. These hypotheses and others are being
tested by combining the near-surface geology of the Bighorn
CIRES undergraduate
assistant Jeremiah
Silver installs a
short period
seismometer
in Bighorn
Mountains, Wyo.
ANNE SHEEHAN
Arch of northern Wyoming with a combined active/passive
source EarthScope Flexible Array seismic experiment. The
Bighorn Arch was chosen because it is minimally affected by
pre- and post-Laramide tectonic events, preserving the sedimentary
sequence. It has a relatively planar, west-dipping
backlimb inclined into the Bighorn Basin and a more abrupt,
east-dipping forelimb facing the Powder River Basin.
The passive seismic experiment uses naturally occurring
seismic sources, such as earthquakes and ambient
noise. The active-source experiment includes the use of
controlled sources, 20 single-fired shots ranging from 500 to
2,000 lbs. in size. The seismic experiment was a three-phase
deployment with a 15-month deployment of 41 broadband
seismometers (deployed summer/fall 2009); a six-month deployment
of 170 three-component short-period seismometers;
a four-month deployment of three five-element seismic
arrays (supported by an Air Force Research Laboratory seismic
discrimination contract); an active-source experiment
with 20 single-fired shots recorded on 1,800 geophones; and
a passive-source geophone deployment with 850 geophones
deployed in passive-source mode for 12 days. The combination
of these approaches is being used to develop structural
crustal images at high resolution at all levels of the crust.
Ongoing analysis with the data collected includes the
use of seismic mode-converted waves to map out subsurface
interfaces such as the crust-mantle boundary. Vertical
component multiply-reflected waves from distant earthquakes
(teleseisms) have been used to map out sedimentary
basin structure. This is the first time to our knowledge
that recordings of distant earthquakes on a dense array of
industry-style geophones have been used to map out sedimentary
basin structure at this level of detail. This technique
shows great promise, and as passive-source seismology is
increasingly applied in oil and gas and geothermal reservoir
modeling, the reverberation image method could be used to
extract structural information from the passive data. Ambient
noise surface wave tomography analysis has been performed
and provides constraints on crustal shear velocity.
CIRES Annual Report 2011 49