PROGRESS IN UNDERSTANDING THE STRUCTURE AND 
TECTONIC DEVELOPMENT OF THE EASTERN NORTH 
AMERICAN RIFT SYSTEM

SCHLISCHE, Roy W., Department of Geological Sciences, 
	Rutgers University, Busch Campus, Piscataway, NJ 08855-
	1179

The Triassic-Jurassic rift system in eastern North America provides an 
unparalleled natural laboratory for studying the processes and 
products of continental extension and breakup. Here I focus on five 
key developments that have contributed significantly to our 
understanding of the structural geology and tectonic evolution of these 
rift basins.
	1. Acquisition of new data.-Over the last two decades, 
regional and local geologic mapping, drilling and coring, and seismic 
reflection profiling have vastly increased our structural and tectonic 
database. It is now clear that these basins are predominantly half-
graben, with generally synthetic intrabasinal faults and fault-
perpendicular folds that in many cases are related to fault 
segmentation.
	2. Role of preexisting structures.-The rift system is located 
within the Appalachian orogen, and thus the border fault systems of 
the rift basins consist of reactivated structures. The attitude of the 
reactivated fault with respect to the rift-related extension direction 
controlled the nature of the reactivation (dip-slip dominated vs. strike-
slip dominated), which affected the amount of basin subsidence and 
types of associated structures. The uniform dip-direction of 
preexisting faults over large areas accounts for the lack of half-graben 
polarity reversals within rift zones (e.g., Newark-Gettysburg-Culpeper 
rift zone).
	3. Application of fault-population studies.-In the last 10 
years, considerable progress has been made in our understanding of 
the geometry and scaling relationships of populations of normal fault 
systems. This information is directly applicable to rift-basin structural 
geology in that half graben are large normal-fault-bounded basins. 
The most relevant features of normal fault systems to basin geometry 
are: (a) Displacement is greatest at or near the center of a normal fault 
and decreases systematically to the fault tips; displacement also 
decreases with distance perpendicular to the fault. (b) Normal fault 
systems are segmented, and many fault segment boundaries are areas 
of (at least temporary) displacement deficits. (c) As displacement 
builds up on a normal fault, the fault increases in length. 
Consequently, rift basins consist of scoop-shaped depressions that 
grow longer, wider, and deeper through time. In the case of segmented 
border fault systems, the scoop-shaped depressions are separated by 
intrabasinal highs.
	4. Integrating stratigraphy and structural geology.-The 
sedimentary deposits of half-graben are, of course, influenced by 
basin geometry; consequently, we can use stratigraphy to infer aspects 
of basin evolution and structural geology. On a local scale, thickness 
variations of fixed-period Milankovitch cycles are particularly useful 
for assessing variations in basin subsidence and for determining 
whether of not structures formed syndepositionally. On a regional 
scale, (a) the lack of Jurassic strata in the southern basins likely 
indicates that they stopped subsiding before the northern basins did; 
(b) high accumulation rates in Early Jurassic strata in the northern rift 
basins indicate accelerated basin subsidence during eastern North 
American magmatism; and (c) the presence of a tripartite stratigraphy 
(basal fluvial unit, middle deep-water lacustrine unit, upper shallow-
lacustrine and fluvial unit) in most basins indicates that they share a 
similar evolutionary trend, most likely related to the infilling of basins 
growing larger through time.
	5. Recognition of inversion structures.-Although post-rift 
contractional structures have long been recognized, recent works 
shows that the magnitude of post-rift shortening was greater than 
previously thought and the initiation of shortening and basin inversion 
was diachronous. In particular, shortening in the southern basins 
began after synrift deposition and prior to the eastern North America 
magmatic event (~201 Ma), while rifting and subsidence continued in 
the northern basins. Inversion in the northern basins occurred between 
early Middle Jurassic and Early Cretaceous time. Post-rift shortening 
is attributed to ridge-push forces and continental resistance to plate 
motion during the initiation of seafloor spreading, which itself was 
diachronous along the North American margin.