Allochthonous Salt Extrusion, Roof Dispersion, and Intrusive Import and Export of Salt in Squeezed Stocks

Tim P. Dooley, Martin P.A. Jackson, and Michael R. Hudec
The University of Texas at Austin, Austin, TX

It is an axiom that regional shortening readily squeezes precursor salt stocks and expels their salt as extrusive allochthonous sheets. We use analog models of laterally shortened stocks to explore how the sedimentary roof responds to this salt extrusion and how salt plumes are imported from and exported to the basal source layer during squeezing. Roof thickness controls the structural response to squeezing. During extrusion, thin roofs break up into rafts that disperse radially owing to traction from underlying salt flow. As roof rafts approach the front of the flow, they begin to ground or founder, which impedes glacial spreading and creates a scalloped extrusion front. Conversely, thick roofs are too strong to break up, preventing breakout and extrusion. Instead, shortening is accommodated by shallow, arcuate, roof thrusts soling out on the diapiric crest. Below thick roofs, some salt flows upward but remains buried as part of the hanging wall of these shallow, dome-related thrusts. The remaining salt from the squeezed stock is expelled downward to spread seaward as a major outward salt plume within the autochthonous source layer. Outward plumes are small or negligible in models having a thin roof because salt preferentially escapes extrusively at the surface. For both types of roof, major plumes of salt also flow inward from the updip source layer into the stock to renew its salt supply. This inward plume promotes allochthonous salt outpouring and roof breakup in thin-roofed diapirs. Injection of inward plumes pressurizes diapirs and resists diapiric pinch-off in thick-roofed models.