Post-Paleozoic activity - Lamont-Doherty Earth Observatory ...

366 . W. Manspeizer and Others
were available to them, and they relied heavily on the fault orien-
tations from Prowell (1983) to determine the direction of princi-
pal stress. New stress measurements reported by Zoback and
others (1984) suggest that the present maximum compressive
stress has a northeast-southwest orientation. This postulated stress
orientation is not substantiated by the orientation and slip direc-
tions of Pleistocene(?) faults, such as the one reported by Pavlides
and others (1983) in central Virginia.
Uplift and subsidence of the continental edge is further evi-
dence that regional tectonism has affected the Appalachians dur-
ing the Cretaceous and Cenozoic. Various geomorphic, geo-
physical, and chronostratigraphic studies have suggested that
relative changes in elevation are common occurrences in the
geologic past (see Walcott, 1970, 1972; Denny, 1974; Isachsen,
1975; Brown and Oliver, 1976; Zimmerman, 1977; Hack, 1979,
1982; Lyttle and others, 1979; Brown and others, 1980; and
Cronin, 1981). Hack (1979) emphasizes that the primary source
of sediment deposited on the coastal margin is from the uplift and
subsequent erosion of the exposed Appalachian crystalline rocks.
Combining the information from these various studies, Hack cal-
culates that uplift in the Appalachians over geologic time is about
40 m/m.y. Similar rates of deformation have been reported by
Denny (1974), Mathews (1975), and Judson and Ritter (1964).
Uplift of the landmass has been attributed to lithospheric bulging
adjacent to subsidence (Beaumont, 1978, 1979), glacial unload-
ing (Walcott, 1972), and mantle hot-spot migration (Crough,
1981). All of these proposed mechanisms are probably active in
the Earth's crust, but they fail to explain all the regional vertical
changes in the landmass, especially uplift and tilting, outlined by
Hack (1979). This conclusion, in conjunction with the regional
fault distribution discussed in this chapter, implies that deep
crustal tectonism, locally affected by other processes, is primarily
responsible for the present configuration of the Appalachians.
SUMMARY AND CONCLUSIONS
This section briefly summarizes the nature of faulting and
other tectonism in the eastern United States from the Cretaceous
to the present. Studies of reverse faults indicate that regional
compression has existed in the crust from the Early Cretaceous
through the Pleistocene. Intrusion of diabase dikes and rift basin
sedimentation from the Triassic to the Middle Jurassic suggest
that tensional forces associated with rifting were still present at
that time. The reversal of the maximum horizontal stress direction
from tension to compression probably was not a rapid process
and must have resulted in a period of little or no applied stress.
The long-term compression since the Early Cretaceous is appar-
ently responsible for rather uniform fault movement for the last
110 m.y. Slip rates may vary locally from 0.3 to 1.5 m/m.y., but
over geologic time, fault velocity closely approximates 0.5
m/m.y. Offsets in Holocene strata, however, have not been rec-
DUTCH GAP FAULT ZONE
A BROOKE FAULT ZONE Ã
0 BELAIR FAULT ZONE
0 ANDERSONVILLE FAULT ZONE
BRANDYWINE FAULT SYSTEM
A FALL HILL FAULT ZONE Ã
WARM SPRINGS FAULT ZONE
 STAFFORD FAULT SYSTEM
AGE OF OFFSET HORIZON (IN MILLIONS OF YEARS1
Figure 29. Movement histories of faults on the Atlantic coastal margin
showing age of offset horizon versus greatest vertical displacement of
horizon (Dutch Gap fault zone from Dischinger, 1989; Brooke and Fall
Hill fault zones from Mixon and Newell, 1978; Belair fault zone from
Prowell and O'Connor, 1978; Andersonville fault zone from H. E.
Cofer, Georgia Southwestern College, oral communication, 1983;
Brandywine fault system from Jacobeen, 1972; Warm Springs fault zone
from Reinhardt and others, 1984).
ognized, probably because of the lack of significant amounts of
fault movement.
Cretaceous and younger faults in the eastern United States
may form along preexisting Paleozoic or Mesozoic faults, but
they generally parallel local rock fabric. The location of the plane
of shearing in the regional geologic framework seems to depend
on both the local orientation of the maximum compressive stress
and the orientation and mechanical properties of planar geologic
elements (for example, faults, foliations, and cleavage). Shear
zones that are not oriented at right angles to the maximum com-
pressive stress will have oblique-slip reverse movement.
The presence of compressive stress and reverse fault move-
ment in the recent geologic record is inferred by seismic focal-
plane solutions and modern stress measurements. Other evidence
of possible crustal tectonism is uplift, tilting, and subsidence of the
landmass. Periodic uplift and erosion of source areas for coastal
sediment, such as the mohtainous terrane in the folded Appala-
chians, also suggest regional tectonism. This tectonism is of such
regional extent that smaller-scale processes such as lithospheric
loading or hot-spot-activity cannot account for its presence.
Therefore, faulting and related tectonism must be subtle evidence
of previously unrecognized plate tectonics in the eastern United
States.