Abstract:

A lineament study from various sources was conducted to develop a conceptual kinematic model for the Richmond Triassic basin area in Virginia. The procedure involved mapping at different scales: color infrared U-2 photos (1:130,000), Landsat MSS images (1:1,000,000), and rectified drainage maps (1:250,000). The lineaments were digitized and subjected to a correlation package that compared the statistically significant peaks (>95%) and troughs (< 5%) within 5 km2 grid cells. Spatially filtered significant 10° trends were contoured according to length-weighted frequencies to obtain lineament zones. These lineament zones were compared to mapped structures, outcrop patterns, Triassic diabase dikes, and aeromagnetic alignments and gradients.

Correlations of lineament zones with mapped structures and geophysical gradients strongly support the interpretation that they represent the surface expressions of boundaries of subsurface structural domains. The basin is bounded on the west by the northeast-trending Hylas fault zone, and the eastern margin is cut by 60°-70° northeast-trending lineament zones that segment the basin into discrete blocks. Numerous ±15° north-trending lineaments correlate remarkably well with the north-trending diabase dikes, yet none of the mapped dikes apparently cut across the basin margins as other Triassic basins do in Virginia and elsewhere.

Detailed analysis of lineament patterns appears to indicate left-handed strike-slip movement along the Hylas fault. At this time, the dikes intruded the basin, but were refracted into the fault zones despite pervasive north-trending fractures. Present-day expression of lineaments is probably due to Cenozoic reactivation of preexisting fractures.

The kinematic model postulated here is entirely compatible with existing models for Mesozoic tectonism in the Virginia coastal plain.

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