Print this pageRole of a Ductile Detachment Horizon in the Development of Pull-Apart Basins in Physical Analog Models
FERILL, DAVID A., DARRELL SIMS, JOHN A. STAMATAKOS, BRET RAHE, and ALAN P. MORRIS
We conducted analog modeling experiments to evaluate the role of detachment deformation on structural style and fault topology of pull-apart basins. Basin development above a rigid detachment horizon (plastic sheet) initiates by formation of N (normal fault) shears which define basin edges, and D (displacement-parallel) and R (synthetic Riedel) strike-slip shears which define the primary fault segments beyond the ends of the basin. With continued displacement, R shears develop as cross-basin strike slip faults which terminate abruptly against basin-bounding normal faults. In the mature development stage, a through-going R shear develops that extends end-to-end across the basin connecting the active tips of the basin. Developmental sequence and structural style is dramatically different when the detachment horizon is placed beneath a ductile horizon (silicone putty). Models with a 5 cm thick sand pack above a 1 cm putty layer (above a plastic sheet) effectively detach the sand pack from the plastic sheet. In plan view, faulting initiates as R shears followed closely by antithetic R' shears to define fault- bounded horses in strike slip duplexes. Localized N shears accommodate basin development, and local oblique slip on R and R' shears forms trap-door structures. Early-formed R shears evolve from initial strike-slip to normal oblique- and normal dip-slip to form a system of connected pull-apart basins. The structural style of pull-apart basins is highly sensitive to depth of detachment horizon within a ductile substratum. These results suggest that deformation style within the detachment can be predicted from basin geometry. Domination of basin boundaries by normal faults suggests detachment within or at the base of a brittle layer. Alternately, domination of basin bounding faults by strike-slip and oblique-slip faults suggests detachment within a ductile layer.