HR: 09:45h
AN: T51E-08    [Abstracts]
TI: Controls on Strain Localization in Continental Rifts: Importance of Reworking Interval and Basement Reactivation
AU: * Imber, J
EM: jonathan.imber@durham.ac.uk
AF: Reactivation Research Group, Department of Earth Sciences University of Durham, Durham, DH1 3LE United Kingdom
AU: Holdsworth, R E
EM: r.e.holdsworth@durham.ac.uk
AF: Reactivation Research Group, Department of Earth Sciences University of Durham, Durham, DH1 3LE United Kingdom
AU: Marsh, N
EM: nicola.marsh@durham.ac.uk
AF: Reactivation Research Group, Department of Earth Sciences University of Durham, Durham, DH1 3LE United Kingdom
AB: Many rift basins are characterized by an initial period of slow tectonic subsidence - rift initiation - followed by a rapid increase in subsidence rates during rift climax. Numerical modeling studies predict slow tectonic subsidence during an initial period of distributed extension on isolated faults, followed by a rapid increase in subsidence as strain becomes localized on a few large rift-bounding faults. These thin-plate models suggest that the transition from rift initiation to rift climax (RI to RC) - that is, the time taken for strain to localize on major faults - occurs within ca. 5 Myr following the onset of rifting. The aim of this presentation is to constrain the duration of the RI to RC transition in natural rifts in order to discuss the controls on strain localization during crustal extension. We have compiled data from 27 continental rift basins worldwide in which there are reasonable constraints on the ages of rift initiation and rift climax, crustal stretching factor and regional tectonic history. In each case, we have identified the RI-RC transition using one or more of the following criteria: (1) an abrupt increase in tectonic subsidence; (2) a rapid increase in fault displacement rate; or (3) direct observation of fault linkage. These criteria provide consistent estimates for the duration of the RI to RC transition in basins where tectonic subsidence rates, fault displacement rates and structural observations are available. The data indicate that the mean duration of the RI to RC transition is 7.9 Myr, but are scattered with minimum and maximum durations of 0.1 and 40 Myr, respectively. First-order estimates using the McKenzie uniform stretching model show that crustal stretching factors at the time of the RI to RC transition range from 1.01 to 2.1, with a mean value of 1.1. Analysis of the current limited dataset suggests there is no correlation between the duration of the RI to RC transition and crustal stretching factor at the time of the RI to RC transition. In contrast, there is a weak but statistically significant positive correlation between the duration of the RI to RC transition and the 'reworking interval' - that is, the period of time between the onset of rifting and the previous major tectonic event (rifting, orogenesis etc). Again, the data are scattered but some of the variation can be accounted for by basement fault reactivation. In basins where we can demonstrate unequivocally that fault reactivation has taken place during rifting, the mean duration of the RI to RC transition is 2.9 Myr, but is 8.8 Myr in basins where the rift-related structures cross-cut basement fabrics or the rift faults are oriented parallel to the basement anisotropy but there is no direct evidence for fault reactivation. Making the reasonable assumption that the RI to RC transition is a manifestation of fault linkage, we suggest that strain localization in continental rifts is influenced by both the reworking interval and by fault reactivation. Several published studies show that basement reactivation gives rise to rapid propagation of large displacement rift-bounding faults. We speculate that strain localization occurs more rapidly in recently reworked crust where the ratio between the strengths of the brittle upper and viscous lower crust may be greater than in 'older' regions with cooler, higher viscosity lower crust or upper mantle.
DE: 8010 Fractures and faults
DE: 8109 Continental tectonics: extensional (0905)
SC: Tectonophysics [T]
MN: Fall Meeting 2005