AT&G
- Spotila's active tectonics and geomorphology research
Origin of the Blue Ridge Escarpment, southern
Appalachians
Scope: Great escarpments occur along numerous
passive margins and are somewhat of a mystery. In many ways, they appear
as young landforms expected from tectonically active regions, yet generally
occur where active mountain building has long since ceased. Some of these
escarpments are thought to have been built by normal passive margin processes;
such as flexure associated with sediment loading on the coastal plain and
continental shelf, isostasy associated with local erosion and parallel
escarpment retreat, or rift-flank uplift. Others have suggested active
faulting as a cause. Where the Blue Ridge Escarpment of the Appalachians
fits in is not certain, however, because its basic "kinematics" (what
happened where, when, etc.) of erosional evolution have not been
evaluated. We studied the erosional history of this landform with a
3-fold approach. First, we explored the erosional kinematics using
topography as a proxy for geomorphic evolution. The idea was to search
for signals in digital topography that would correlate and therefore test for
the presence of offset peneplains and erosion surfaces, or quantify the
differences in maturity or present erosion rate (qualitatively of course!)
between domains. This turned out to be more complicated than first
thought, when we started to realize just what other factors (climate, soil
type, type of erosional agent, transport vs. supply limited conditions, etc.)
influence parameters such as drainage density, hypsometry, and so on. The
second approach was more successful. This was to look for evidence in the
fluvial geomorphology (topographically and geologically) for migration of the
asymmetric divide atop the escarpment. Things like beheaded/underfed
stream channels and fluvial deposits on the lip of the escarpment (very well
rounded, large cobbles in terraces right at the divide) show that there has
likely been migration of the escarpment on the order of 10's of km.
Finally, we mapped out the pattern of exhumation in the area with (U-Th)/He
dating. This revealed a large exhumational (not topographic) bulge in the
inner Piedmont. It suggests that the most exhumation over the past 100
million years or so has been focused to the east of the escarpment, not
immediately below it (and certainly not from atop the Blue Ridge upland to the
west of the divide). This exhumation pattern is consistent with flexure
and isostatic rebound associated with erosional retreat of an escarpment. See Spotila et al. (2004) for more
details.
Personnel: Greg Bank (M.S., 2001),
with assistance from Bill Henika (VPI), Chuck and Nancy Naeser, Pete Reiners,
and Lee Daniels.
Funding: Geological Society of America student
grant, the Byron Cooper scholarship (Virginia Tech) to Greg.
Links
Other folks working on the long-term landscape evolution of the
Appalachians including Dave Harbor (Washington
and Lee), Frank Pazzaglia
(Lehigh), Greg
Hancock (William and Mary), and Paul Bierman (U. Vermont).
Below is a shaded DEM (90 m resolution) of the region, with
the Blue Ridge escarpment clearly visible.
Below is a slope map from the same DEM. The slopes along the Blue Ridge
escarpment average 24 degrees. This
is quite rugged for a landform along a passive margin.
Below is a geologic map of the escarpment in southern
Virginia (shown as blue line).
Note how the escarpment does not correspond to lithology. This is unique from other major
landforms in the southern Appalachians, which are largely lithologically
controlled.
Perspective 3-d image of the Blue Ridge Escarpment in
Virginia and North Carolina based on digital elevation models.
Elevation profile across the Blue Ridge escarpment,
illustrating the distance in km along river networks to sea level in either
direction. The top of the
escarpment is an asymmetric divide between the Gulf of Mexico drainage on the
west and the Atlantic drainage on the east.
The map of apatite helium ages below shows that cooling ages
are youngest in the Inner Piedmont (in red), just east of the escarpment. The interpretation is that this area
has experienced greater denudation over the Cenozoic, associated with erosional
retreat of the escarpment. See
Spotila et al. (2004) for details on inferred exhumation rates and evolution of
this landform.
The helium ages plotted on the elevation profile below again
show the younging towards the southeast, or towards the coast. This pattern has been observed on other
great escarpments worldwide (e.g. Brazil, southern Africa). In fact, the ages we observe are nearly
identical in magnitude and pattern to apatite helium ages along the SE
Australia great escarpment (Persano et al., 2002). The pattern of ages there is thought to represent erosional
retreat of the escarpment following rifting ~80 Ma. If the same erosional history has produce the ages along the
Blue Ridge escarpment shown below, it implies erosional retreat of the
escarpment significantly later than the ~200 Ma rifting event along eastern
North America.
Comments to: spotila@vt.edu