Squeezing a Large Canopy on a Slope: Suturing Patterns, Subsalt Thrusting and Suprasalt Extension and Salt Expulsion
We built a complex and lengthy model investigating canopy formation on slope from 14 feeders, shortened the resultant canopy by a mixture of gravity and a moving endwall, and subsequently loaded the canopy to remobilize the allochthonous salt. This is a data-rich model and this paper focuses on four aspects:
- 1.Suturing and salt flow during canopy coalescence: During the early stages of canopy formation the flow patterns of individual salt sheets was subradial with streamlines slightly skewed in the direction down regional slope. In the late stage of canopy coalescence the flowlines were dominated by inward flow from the canopy peripheries. The reason for this was the outer feeders flowed more vigorously as there was less competition for source-layer salt. Sutures between salt sheets are bowed in the direction of override and some became partially dismembered as they were deformed.
- 2.Deformation of salt and sutures during shortening: During shortening the individual salt sheets and associated sutures were remobilized and dispersed making it very difficult to reliably connect basal (lower) and apical (upper) suture lines due to the extreme strain and displacement. However, basal suture lines remain largely anchored in placed, apart from the effects of subsalt regional shortening or extension. Salt flow during shortening was grossly parallel to the regional shortening direction. However, canopy salt flows fastest where it is thickest and thus the chains of feeders channel the flow of salt streams within the canopy. This can create flow that is oblique to the regional slope and branch regularly.
- 3.Salt remobilization during sedimentary loading: During canopy loading a series of minibasins formed above the thicker salt in the feeders. The formerly lobate outline of the canopy evolved to a single lobe by further suturing and seaward expulsion of salt from the deflating first-generation canopy. Eventually a second generation canopy was formed that ramped seaward. Sutures formed during the coalescence and shortening stage were further fragmented and dispersed during canopy loading, with suture fragments now seen at the toe of the second generation canopy having traveled all the way from updip edge of the first-generation canopy, equivalent to about 80 km in nature.
- 4.Subcanopy structural geometry: When viewed from above during the shortening stage the model showed little signs of thrusts, as the canopy absorbed shortening by redistributing and thickening allochthonous salt. The view of the underside of the model revealed a complex subsalt network of thrusts and tear faults that link the feeders, similar to that seen in the Astrid thrust belt, Lower Congo basin. Three types of steep discontinuities form beneath a canopy during shortening: (1) Thrust Fault – Z-shaped or S-shaped base of salt with no remnant salt along the thrust as this never was a diapir. There is no true salt pedestal but asymmetric salt body is present in hangingwall and the footwall is commonly welded; (2) Thrust weld – Large asymmetric keel above weld and large pedestal below weld. Possible remnant salt along weld and likely to change geometry rapidly along strike as they pass into the thrusts linking feeders. Minor strain observed in the country rock as the feeders absorb the shortening by expelling salt into the canopy; (3) Nonthrust weld is like a thrust weld but steeper with little or no offset of strata across the weld surface. The weld widens upward into a keel and minor pedestal is present below the weld.
AAPG Datapages/Search and Discovery Article #90189 © 2014 AAPG Annual Convention and Exhibition, Houston, Texas, USA, April 6–9, 2014