Post-Paleozoic activity - Lamont-Doherty Earth Observatory ...
Post-Paleozoic activity - Lamont-Doherty Earth Observatory ...
Post-Paleozoic activity - Lamont-Doherty Earth Observatory ...
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0 100 200 300 Miles ^<br />
Figure 27. Map of the eastern United States showing Cretaceous and<br />
Cenozoic fault provinces defined by type of fault motion (data from<br />
Prowell, 1983).<br />
ally dependent on the observed displacement of geologic units or<br />
contacts of an appropriate age range. Hence, most of the observed<br />
faults shown on Plate 5B are within the Coastal Plain geologic<br />
province, where datable, subhorizontal sedimentary horizons are<br />
most abundant. Most of these faults, however, are known to<br />
displace the underlying Appalachian crystalline rocks as well.<br />
The occurrence of datable Cretaceous and Cenozoic materials in<br />
the exposed Appalachians is far more limited, which is undoubt-<br />
edly the primary reason for the small number of reported faults in<br />
this region. In addition, seismologic evidence and seismic reflec-<br />
tion profiling, in conjunction with limited geologic data, have<br />
been used to assign an age and location to a few faults shown on<br />
Plate 5B. These criteria are far less definitive than field observa-<br />
tions of faulting, but they offer evidence of tectonism in otherwise<br />
unevaluated areas.<br />
The abundance of fault data along the inner margin of the<br />
Coastal Plain could be taken as an indication of a concentration<br />
of tectonic <strong>activity</strong>, but it may only be a reflection of opportune<br />
geologic conditions. Reports of faults along the inner edge of the<br />
Coastal Plain typically describe crystalline rocks faulted against<br />
Cretaceous or younger sediments. Contacts of this sort are far<br />
<strong>Post</strong>-<strong>Paleozoic</strong> <strong>activity</strong> 363<br />
more obvious than those involving the juxtaposition of similar<br />
rock types, and therefore, local geologic conditions may be a<br />
major factor in the grouping of faults shown on Plate 5B. Accord-<br />
ingly, areas showing no faults should be taken to represent not a<br />
lack of tectonic <strong>activity</strong> but rather a lack of sufficient information.<br />
Some of the fault data shown on Plate 5B and in Prowell<br />
(1983) were used by Prowell (1976) and Howard and others<br />
(1978) to characterize regions of the eastern United States on the<br />
basis of fault style. An updated version of their interpretations is<br />
shown in Figure 27. The eastern United States is herein subdi-<br />
vided into four tectonic provinces characterized by the type of<br />
faults within it. These provinces are: (1) the Atlantic Coast prov-<br />
ince, (2) the Gulf Coast province, (3) the West Georgia<br />
transition province, and (4) the Interior province. The Atlantic<br />
Coast province and the West Georgia transition province cover<br />
the majority of the rocks described in this volume and therefore<br />
will be discussed in detail. A description of the fault styles in the<br />
other provinces, found in Prowell (1988), helps provide an un-<br />
derstanding of the tectonic processes operating in eastern North<br />
America during the late Mesozoic and Cenozoic.<br />
Atlantic Coast Province<br />
The Atlantic Coast province is characterized by Cretaceous<br />
and younger northeast-trending reverse fault zones and fault sys-<br />
tems up to 100 km long. Vertical displacements as great as 76 m<br />
have occurred since the Early Cretaceous, and progressively<br />
smaller offsets have been recognized in rocks spanning the Ceno-<br />
zoic. Although a component of lateral slip has been reported for<br />
many reverse faults, dip-slip reverse motion is dominant. The<br />
strikes of the fault zones tend to be more northerly in the northern<br />
part of the province and more easterly in the southern part of the<br />
province, but the strikes are typically within 45 degrees of north.<br />
The dips of the fault zones range from 40 to 85 degrees, and the<br />
dip of an individual zone may vary depending on the physical<br />
properties of the rocks in the adjacent fault blocks. Deformation<br />
associated with the faulting is extremely brittle in hard rocks, and<br />
slip surfaces consist of coarse breccias and soft gouge. Coastal<br />
Plain strata are typically less sheared, and drag folding is well<br />
developed. Secondary thermal mineralization is not observed,<br />
which indicates that heating and recrystallization (dynamic met-<br />
amorphism) are not part of this process.<br />
West Georgia Transition Province<br />
In western Georgia and central Florida(?), a zone of transi-<br />
tion between the reverse fault and normal fault provinces of the<br />
Atlantic and Gulf regions can be recognized. The faults in this<br />
West Georgia transition generally are east-west-trending vertical<br />
faults or fault zones as much as 30 km long. Vertical displace-<br />
ments in early Tertiary strata are as great as 60 m, and lateral<br />
displacement is apparently minimal. These near-vertical faults are<br />
commonly flanked by smaller secondary reverse faults (Rein-<br />
hardt and others, 1984), suggesting that compression is a factor in