HR: 1330h
AN: T12A-0439    [PDF]
TI: How Synthetic Fault Arrays and Missing Lower Crust Support Symmetric Rifting Models
AU: * Nagel, T J
EM: nagel@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, 61 Route 9W, Palisades, NY 10964 United States
AU: Buck, W R
EM: buck@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, 61 Route 9W, Palisades, NY 10964 United States
AB: Continental rifts are often associated with synthetic normal faults generally dipping towards the rift center where rifting has proceeded to seafloor spreading. We use an advanced numerical technique to study the formation of these fault arrays. The model has a particular vertical and horizontal strength stratification. It consists of upper crust, lower crust and mantle. The upper crust comprises a brittle upper layer and a weak plastic lower layer. The lower crust and the upper mantle deform through temperature dependent viscous creep and are strong enough not to flow into the rift center. In the center, the model has a pronounced thermal perturbation, which acts as a weak spot in the lower crust and mantle. The formation of synthetic faults requires a consistent shear stress at the base of the brittle layer. In our model, such a shear stress arises, because the brittle portion of the upper crust responds differently to extension than the strong deeper layers of the lithosphere. The predefined perturbation in the thermal structure triggers strain localization in the lower crust and mantle, as the higher extension rate at this weak site sucks up hot mantle and further increases the thermal anomaly. In the brittle upper crust deformation is accommodated through normal faulting. Slips along individual fault planes originate topographic and elastic stresses in the surrounding rocks. If this effect outweighs strain weakening on existing faults, new faults appear and deformation distributes. Hence, the lower crust and the uppermost mantle can widen at a single place, whereas the brittle upper crust undergoes a more regional collapse. This deformation geometry causes a horizontal shear stress towards the necking area in the lower layers. Therefore, faults in the brittle upper crust consistently dip towards the rift center on both sides of the rift. The weak layer in the upper crust accommodates large strain, as it transfers the distributed deformation in the brittle layer horizontally into the necking area below. The opening gap in the center is filled with uprising mantle from below and with collapsing upper crust from above. Therefore, upper crustal rocks come to rest directly on top of mantle rocks with no lower crust in-between. Our model is symmetric at a lithospheric scale. The calculations reproduce the three main characteristics of non-volcanic margins such as the Iberian Margin: (1) At the tip of the continent upper crust rests directly on top of exhumed mantle on both sides of the rift with no substantial lower crust within some tens of kilometers of the distal margin. (2) The upper crust is defined by an oceanward dipping fault array and tilted blocks. (3) A prominent sub-horizontal ductile high strain zone, the s-reflector, is present at the base of the upper crust.
DE: 3210 Modeling
DE: 8105 Continental margins and sedimentary basins
DE: 8109 Continental tectonics--extensional (0905)
SC: Tectonophysics [T]
MN: 2003 Fall Meeting