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

tomac Formation. They also report that successively smaller dis-
placements were found in strata of Paleocene, Eocene, Miocene,
and Plio-Pleistocene age in the same general area. Prowell and
O'Connor (1978), Reinhardt and others (1984), and Dischinger
( 1979; 1989) report similar sequential displacements on other
Appalachian fault zones.
The oldest reported displacements along reverse faults in the
Appalachians are found in early Cretaceous strata in Virginia and
Maryland. Nothing is presently known about initiation of com-
pression and reverse faulting in the Appalachians, but some infer-
ences can be made from other types of Mesozoic regional
tectonism. Triassic rifting, Triassic and Jurassic rift basin sedi-
mentation, and lower and middle Jurassic diabase intrusions are
commonly associated with extensional stresses created during
continental separation. An extensional stress field would gener-
ally prohibit the formation of compressional reverse faults and
would therefore place a lower age limit on the propagation of
reverse faults. This suggests that the reverse faults could have
formed as early as the Late Jurassic, if no significant amount of
time was required for reversal of the stress field from extension to
compression.
The geologic evidence of late Cenozoic fault movement is
poor, largely because of the limited distribution of well-defined
late Cenozoic materials and the small amounts of fault move-
ment. Late Cenozoic fault movements have been reported by
Mixon and Newel1 (1977, 1978), Pavlides and others (1983),
Prowell (1983), and Reinhardt and others (1984) in Virginia,
Maryland, and Georgia. These reports show clear evidence of
relatively young tectonism in the eastern United States. The fault
described by Pavlides and others (1983) is the youngest known
reverse fault involving crystalline basement (Fig. 28). The fault is
located proximal to the Paleozoic Mountain Run fault zone and
offsets the base of Pleistocene(?) colluvium about 1.5 m. The
location of the faulting within the mylonitic rocks of the older
Mountain Run fault zone has tentatively been attributed to reac-
tivation of this old zone of weakness by late Cenozoic
compression.
Fault Slip Rates
Comparison of amounts of offset in different chronostrati-
graphic horizons provides a basis for calculating fault slip rates
over geologic time. Wentworth and Keefer (1983) compiled data
published by Mixon and Newell (1978), Prowell and O'Connor
(1978). and Behrendt and others (1981) for three fault zones in
the eastern United States and concluded that the average rate of
vertical displacement is 0.9 m/ m.y. New and more detailed data
have been used to construct the slip-rate curves shown in Figure
29. The new slip-rate curves imply that fault movement in the
eastern United States has ranged from about 0.3 to 1.5 m/m.y.,
with an average of about 0.5 m/m.y., since the Early Cretaceous.
This observation is an important element in the evaluation of
recent faulting in the eastern United States and the assessment of
the seismic potential of these faults. The consistency of fault
. Post-Paleozoic activity 365
Figure 28. Paleozoic phyllonite (left) faulted over Pleistocene (?) col-
luvium (right) along a small reverse fault near Everona, Virginia. Vertical
displacement is 1.5 m (photo looking northeast). Photo by D. C.
Prowell.
movement through geologic time indicates that the compressive
stress responsible for the deformation was relatively uniform and
unidirectional.
RELATIONSHIP TO OTHER TECTONIC FEATURES
Other types of Cretaceous and Cenozoic tectonism have
been recognized in the eastern United States and compare favor-
ably with the faults found in the Atlantic Coast fault province.
The relatively small number of seismic events recorded in eastern
North America in the last 250 years (see York and Oliver, 1976)
may be explained by the small slip rates of regional reverse faults.
Where localized seismic networks have provided focal-plane so-
lutions, the resulting fault-plane solutions have typically been
attributed to reverse fault movement. Seismicity consistent with
reverse faulting has been recognized along the Ramapo fault zone
in New York (Ratcliffe, 1971; Aggerwal and Sykes, 1978; Yang
and Aggerwal, 1981), near Charleston, South Carolina (Tarr and
Rhea, 1983), and at the North Anna Reservoir in central Virginia
(Dames and Moore, Inc., 1976). Seismological data in the eastern
United States, however, are far from conclusive proof of recent
reverse-fault activity because many seismologists disagree over
the interpretation of seismic evidence. In addition, no observed
fault displacements can presently be attributed to historical seis-
mic events.
The crustal stress in the pre-Cretaceous rocks beneath the
Appalachians is probably responsible for the origin and orienta-
tion of the reverse faults. Zoback and Zoback (1980) summarized
the state of stress in the conterminous United States and described
the eastern United States as in a northwest-southeast compres-
sional regime. However, very few actual stress measurements