The Mississippi embayment (ME) is the youngest of a series of intracontinental basins
in the central United States. It is a SSW-plunging sedimentary basin and a first-order
feature in North American Geology.
The ME is filled with unconsolidated Upper Cretaceous and younger sediments and is
underlain by the Reelfoot Rift, an ancient failed arm related to the breakup of the
super continent Rhodinia in Cambrian time. The broader Reelfoot Rift encompasses the
smaller Mississippi Valley graben (MVG), which contains the New Madrid Seismic Zone
The NMSZ generated at least three earthquakes in 1811-1812 exceeding moment magnitude
7.0 and paleoseismology indicates that at least five episodes of large earthquakes
capable of producing wide-spread liquefaction occurred in the NMSZ over the past 5000
The ME may be the most recent expression of tectonic activity associated with the
region; previous interpretations of potential field data and active source reconnaissance
seismic refraction/reflection experiments suggest significant crustal heterogeneity
beneath the embayment related to a long and complex geological history of rifting,
uplift, and subsidence covering at least two Wilson cycles.
Several models have been suggested for ME subsidence and for the generation of earthquakes
in the NMSZ but none has gained wide acceptance. We lack fundamental knowledge of
how upper crustal phenomena relate to deeper crustal and mantle processes that undoubtedly
exert significant control on regional subsidence and earthquake occurrence. The passage
of the EarthScope TA provides a unique opportunity to test our hypothesis that lithospheric
heterogeneity ultimately controls the dynamic processes associated with the ME and
More specifically, we hypothesize that the mantle lithosphere contains anomalous heterogeneity
related to buried Phanerozoic rift geology. Crustal rifting may have affected the
entire region presently buried beneath the northern ME but interpretation of potential
field anomalies suggests that the rifting was not accompanied by voluminous magmatic
intrusions. This contrasts sharply with older intracontinental rifts such as the Midcontinent
or Keweenawan rift system in which large volumes of mafic rocks were intruded along
much narrower grabens.
Comparing the lithospheric structure inherited from rifting below the ME to lithospheric
structure currently under investigation below the Midcontinent Geophysical Anomaly
(MCGA) related to the Midcontinent rift will expose the range of mantle dynamic processes
associated with continental rifting and may shed light on why some ancient rifts reactivate
and generate earthquakes while others do not. (See map above).
We will illuminate the dynamic processes associated with the ME and NMSZ by determining
crust and mantle velocity and anisotropy structure within and outside of the embayment.
The Northern Embayment Lithosphere Experiment (NELE) will consistent of TA densification
over a broad region to improve resolution of compressional and shear wave velocity
models. We will employ a relatively new joint inversion technique that uses gravity
data, transfer functions (similar to receiver functions), and source and ambient surface
wave dispersion measurements to constrain shear velocity.
Denser TA coverage along two embayment perpendicular and one embayment parallel line
will provide more finely detailed crust and upper mantle structure through both currently
aseismic and seismogenic crust. Traditional receiver function techniques will be replaced
with a newly developed transfer function method that will allow us to "see through"
the thick unconsolidated sediments in the region. Core phases, surface wave dispersion,
and gradiometry will provide anisotropy images over a range of depths.
Finally, we will image the lithosphere/athenosphere boundary using Ps and Sp conversions.
Joint interpretation of these images, generated using traditional and new techniques,
should help unlock the enigma of the NMSZ and reveal how ancient tectonic events continue
to influence the present via inherited lithospheric structure.