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. 


go back to "MEETING SCHEDULE"