Mesozoic Rift System/Thesis

Aspects of the Structural and Stratigraphic Evolution of Early Mesozoic Rift Basins of Eastern North America

Abstract--Early Mesozoic synrift basins of eastern North America formed during the Triassic-Jurassic breakup of Pangaea. Situated along the Appalachian orogen, most of these basins were influenced by pre-existing structural control. Basins whose boundary faults experienced predominantly dip slip formed where northwest-southeast extension was oriented normal to pre-existing weaknesses; strike-slip margins formed where the extension direction was oriented at a low angle to pre-existing structures. My work has shown that dip-slip basins are generally characterized by: synsedimentary border faults (on which displacement is greatest at the center of the fault and decreases toward either end) with adjacent rider blocks and syndepositional transverse folds; largely post-depositional, synthetic intrabasinal faults; and onlap of younger strata onto the hanging wall margin of the basin, indicating that the basin had grown in size through time. Strike-slip basins are characterized by a mosaic of strike-slip and normal faults that bound a network of nested sub-basins; subsidence and stratigraphic thickness are generally much less than along dip-slip margins. Both strike-slip and dip-slip margins were active in eastern North America at the same time and represent local responses to regional extension rather than a historical progression of changing stress regimes.

Stratigraphy, basin modeling, and structural analysis indicate that: 1) basins started subsiding in the Middle Triassic with initial fluvial sedimentation; 2) the majority of the basins diachronously experienced a transition to lacustrine sedimentation as the volume of the basins exceeded the available volume of sediments; 3) toward the end of the Triassic, subsidence slowed in the northern basins and stopped in the southern basins, with resultant shallow-water lacustrine and fluvial sedimentation; 4) a rapid increase in extension and subsidence occurred coevally in all of the northern basins and resulted in deep-water lacustrine sedimentation in the earliest Jurassic and shortly preceded the widespread but shortlived (650,000-yr) flood basalt and related diabase intrusion event; 5) subsidence slowed following the igneous episode and terminated during the Early to Middle Jurassic; 6) a final pulse of extension culminating at ~175 Ma was associated with significant block faulting, stratal rotation, hydrothermal fluid migration, and the onset of seafloor spreading.

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