PALEO-MAXIMUM THERMAL STRUCTURE OF THE
TRIASSIC TAYLORSVILLE (VIRGINIA) BASIN: EVIDENCE
FOR BORDER FAULT CONVECTION AND IMPLICATIONS
FOR DURATION OF SYN-RIFT SEDIMENTATION AND LONG-
TERM ELEVATED HEAT FLOW
MALINCONICO, MaryAnn L., Department of Earth and
Environmental Sciences, Columbia University, P.O.
Box 1000, Palisades, NY 10964-8000
Vitrinite reflectance profiles from several core holes in the
Taylorsville basin, Virginia, of the Early Mesozoic Newark rift
system provide a snapshot of maximum paleo-temperature structure.
In the central part of the basin, paleo-maximum geothermal gradients
of 40šC/km are linear with depth. At the border fault, however, a high
gradient overlies an isothermal section interpreted as a convection
cell. This structure is predicted by hydrologic models of conductive/
convective heat transfer in continental rift basins with permeable
border fault alluvial fan deposits and lower permeability basinal
lacustrine sediments (Person and Garven, 1994). The expected low
geothermal gradient due to meteoric recharge at shallowest depths at
the border fault, however, is not present, possibly due to post-rift
erosion.
Burial history reconstruction using estimates of eroded section
from vitrinite reflectance profiles, paleomagnetic stratigraphy, and
stratigraphic correlations with the Newark basin (P. LeTourneau, pers.
comm.) suggest that deposition in the Taylorsville basin did not
continue into the Jurassic like the Newark and other basins to the
north and west. This is the first firm evidence that the rifting-related
subsidence of the Taylorsville basin ceased by the Triassic-Jurassic
boundary and supports the hypothesis of Schlische and Ackermann
(1995), based on stress patterns from dike orientations, that seafloor
spreading and rift basin inversion may have begun as early as the
Early Jurassic for the southern Newark Supergroup Basins.
Kinetic thermal modeling of reflectance data using the burial
history construction plus published fluid inclusion data (Tseng et al.,
1995) demonstrate that the thermal maturation patterns can be
produced by burial alone under a long term syn- to post-rift
geothermal gradient of 40šC/km. The fluid inclusion studies indicate
the elevated 40šC/km geothermal gradient lasted at least until 160
million years ago. Inversion and unroofing beginning in the earliest
Jurassic resulted in differential erosion between core hole sites before
Early Cretaceous Coastal Plain deposition, 120 million years ago.
The long-lived elevated (40šC/km) geothermal gradient in this
study and that of Tseng et al. (1995) is not expected in continental rift
basins. This gradient may not be so much due to rifting but inherited
from the unroofing of the doubly thickened crust of the Alleghenian
orogeny metamorphic axis, upon which the Taylorsville basin sits.
The cooling history of the basin has been shown to be consistent with
the thermal decay curve for Alleghenian metamorphism in eastern
Virginia (Roden and Miller, 1995), and Wintsch et al. (1992) have
shown that an apparent 40šC/km gradient existed from the late
Permian in the extension of this metamorphic axis in eastern
Connecticut. A long-term regionally elevated gradient may explain
young fission track ages in Paleozoic and older basement along the
rift system for which rift-related hydrothermal/ magmatic activity has
been discounted as a cause (Kohn et al., 1993), and explain the lack of
a post-rift thermal subsidence phase for individual Newark
Supergroup basins.
In summary, the Taylorsville basin is a positive test of
published numerical models of conductive/ convective heat flow in
rift basins. The thermal structure of the Newark/ Hartford basins,
which may have extensive hydrothermal effects, can then be tested
against the Taylorsville basin. The Taylorsville basin also has
provided more evidence of diachronous cessation of rifting along the
Newark rift system. The long-term elevated geothermal gradient
experienced by the basin suggests the Newark rift basins may perhaps
best be interpreted as local breaks along the regionally heated Atlantic
rift margin that subsided as a whole, marked by the onlap of the
Coastal Plain, after the initiation of sea-floor spreading.
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