Rift basin inversion around the margins of the North
Atlantic Ocean: Chronology, causes, and consequences
Abstract--Many extensional basins that formed during early Mesozoic
continental rifting along the margins of the North Atlantic Ocean have
been inverted. In the Fundy basin, the geometry of inversion-related structures
(reverse faults with up to 4 km of displacement, strike-slip fault zones
associated with positive flower structures, and folds), suggests NW-SE
contraction occurred after NW-SE extension associated with rifting. The
deformation post-dates the youngest synrift deposits in the Fundy basin
and is hypothesized to be Middle Jurassic in age, based on similar inversion-related
structures in offshore basins. Contractional deformation of late Early
to Middle Jurassic age may also have affected the conjugate margin of NW
Africa. In the southern basins of eastern North America, postrift shortening
may have begun somewhat earlier based on the following: (1) NW- and N-striking,
earliest Jurassic(?) age diabase dikes cut across these basins and indicate
a NW-SE-oriented s1, which is incompatible with the NW-SE extension responsible
for basin formation, normal faulting, and sedimentation; (2) none of the
southern basins contain Jurassic sedimentary rocks or lava flows, indicating
that the basins may have stopped subsiding prior to Jurassic time; and
(3) in the Richmond basin, NNW-striking earliest Jurassic(?)-age diabase
dikes cut across folds and reverse faults that resulted from NNW-SSE shortening.
Thus, NW-SE extension had not only ceased prior to earliest Jurassic time
but also been replaced by NW (to NNW)-SE (to SSE) shortening in the southern
basins. Therefore, inversion in the southern basins occurred while extension
was continuing in the northern basins, and initiation of inversion was
diachronous along the Atlantic margin. The most likely cause of basin inversion
was compression related to ridge push during the initial stages of seafloor
spreading (when the crust was weakest as a result of extensional thinning
and magmatism). If this hypothesis is correct, then initiation of seafloor
spreading may have been begun earlier in the south and propagated northward,
and basin inversion may be an integral part of the transition from rifting
to drifting. Compressional deformation was not just restricted to the early
Mesozoic, however, for NE-striking reverse faults cut coastal plain deposits.