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.
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