A PRELIMINARY STUDY OF DRAG ZONE GEOMETRY ADJACENT TO SALT DIAPIRS
Drag zones are produced immediately adjacent to diapirs where the surrounding overburden is rotated into a steeply dipping attitude subparallel to the diapiric walls. This preliminary study incorporates both seismic interpretation of specific diapiric case studies and analysis of analogue experiments. Experimental drag zones are characterized by the tectonic attenuation of pre and syn-kinematic horizons, coupled with a marked rotation of bedding toward the diapiric flanks. The width of the drag zone in case studies is approximately equivalent to the width of the diapir at the diapiric base, and decreases to 25% of the width of the diapir toward the crest. In general, vertical relief of bed upturn adjacent to the diapir also diminishes upward. The vertical relief of bed upturn in case studies is approximately 5% of the total diapir height near the diapiric crest and increases to 30% near the diapiric base. This is a consequence of the pre-movement layers and the lowermost syn-movement horizons having undergone the most protracted deformation history associated with diapiric growth. The width of the drag zone was found to increase proportionally with increasing diapir size, and with increasing diapir height. The total amount of upturn increased proportionally with the width of the drag zone. Experiments in which syn-kinematic sedimentation just kept pace with, but did not bury the diapir, display severely diminished drag zones which is interpreted to be due to an absence of active intrusion of diapiric material into overburden. Larger diapirs exhibit more significant drag zones due to a greater volume of polymer flow (and hence viscous drag forces) into the diapir.