Anatomy of a rift system: Triassic-Jurassic basins
of eastern North America
Abstract--Basins containing the early Mesozoic Newark Supergroup
formed during the incipient rifting of Pangaea. The basins are characterized
by the following: (1) The border fault systems (BFS) represent reactivated
older faults. (2) A regionally persistent NW-SE to WNE-ESE extension direction
reactivated NE- to N-striking structures as predominantly normal dip-slip
faults, forming half-graben in their hanging walls, and E- to ESE-striking
faults as strike-slip faults, forming transtensional basins. (3) The half-graben
are lozenge-shaped basins in which subsidence/fault slip was greatest at
or near the center of the BFS and decreased to zero toward either end.
(4) Transverse folds in the hanging walls immediately adjacent to the BFS
formed as a result of higher-frequency variations in subsidence. (5) Subsidence
also decreased in a direction perpendicular to the BFS, commonly resulting
in "reverse drag," which need not necessarily indicate the presence of
listric faults. (6) Intrabasinal faults are overwhelmingly synthetic and
predominantly post-depositional; rider blocks adjacent to the BFS indicate
progressive footwall incisement as a result of footwall uplift. (7) Younger
strata progressively onlap pre-rift rocks of the hanging wall block; this
indicates that the basins grew both in width and length as they filled.
(8) In all basins initial sedimentation was fluvial, reflecting an oversupply
of sediment with respect to basin capacity; this gave way diachronously
to lacustrine sedimentation as basin capacity exceeded the volume of sediment
supplied. (9) Sediments were derived largely from the hanging wall block,
which sloped toward the basin, and from streams that entered the basin
axially; a direct footwall source was minor, owing to footwall uplift.
(10) In strike-slip-dominated basins, subsidence was considerably less
than in dip-slip basins, and mosaics of strike- and dip-slip faults are
common.