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Right-lateral Faults(A) Range Front and FoothillsNW23%10%5%5%10%15%20%14%Reverse FaultsEBouguer gravity anomalies (Hicks 1989; Bennett et al. 2000)together with information from exploration wells and seismiclines. These data have been used to contour the top of basementbelow the cover sequence (Figure 1). Two major depocenters(Pegasus-Rangiora basin to the north and Rakaia-Hinds basinto the south) are identified elongated in an easterly orientationand separated by an intervening structural high coincidentwith Banks Peninsula, where uplifted basement graywackesare exposed beneath the Miocene volcanics. Exploration wellshave penetrated late syn-rift sequences infilling these basins.Discontinuous E-W fault traces mapped in the Quaternarygravels of the Canterbury Plains along the Ashley, Rakaia, andHinds fault systems (Figure 1) are thus interpreted as surfacetraces of buried basement faults belonging to the structuraldomain of the Chatham Rise.STRUCTURAL ANALYSISW24%S(B) Canterbury Plains, Banks Peninsula andAdjacent Offshore40%30%20%10%NSOffshore, the northwestern edge of the Chatham Rise preservesa strong extensional fabric defined by closely spacedE-W striking, S-dipping normal faults that bound half-grabensinfilled with up to 2 km of inferred Late Cretaceous syn-riftsediments (Barnes 1994). Projection of these structures westwardbelow the Canterbury Plains is conjectural but is based on5%10%15%20%▲▲Figure 5. Rose diagrams of fault strike azimuths within theCanterbury region covered by Figure 1, weighted for mappedlength: A) Area of the Southern Alps rangefront and foothillswhere Torlesse basement is exposed; B) Area of the CanterburyPlains, Banks Peninsula, and offshore where basement is largelyconcealed. Top half of each plot is for faults where reverse-slipis dominant; bottom half is for faults with predominantly rightlateraland/or normal slip.Right-lateraland Normal Faults Reverse Faults42%EObserved slip senses on the three major ruptures within theearthquake sequence are consistent with the inferred pattern ofσ 1 stress trajectories (Figure 3). However, some stress heterogeneityis evident, especially near rupture terminations and faultintersections (Figure 4). In Anderson’s (1905, 1951) applicationof the Coulomb criterion for brittle shear failure to the initiationof faults within intact isotropic crust, strike-slip faultsforming in a wrench stress regime (σ v = σ 2 ) should be verticaland lie at ± ~30° to the σ 1 orientation. In contrast, large-displacementstrike-slip faults commonly lie at far higher angles(often >45°) to regional σ 1 trajectories and are distinctly “non-Andersonian” (Mount and Suppe 1987; Balfour et al. 2005).It follows that vertical, low-displacement strike-slip faults atAndersonian orientations are possibly newly formed structuresin the contemporary stress field, but they could also be inheritedfaults that happen to be optimally oriented for frictionalreactivation. Following the same argument, oblique-slip rupturesmost likely result from the reactivation of inherited faultsin the prevailing stress field.The principal ruptures of the Canterbury earthquakesequence can be viewed with these considerations in mind(Figure 3). First, the subvertical Greendale fault rupture lyingat 25°–35° to regional σ 1 is at optimal Andersonian orientation,implying that it is either a comparatively newly formed strikeslipfault or an inherited structure that is optimally orientedfor reactivation in the present stress field. It should be borne inmind that most of the inherited dip-slip faults within the basementare likely to have dips that are substantially less than vertical,though the Greendale rupture could occupy an amalgamof opposite-dipping structures. Note further, however, that atits western termination, the Greendale rupture trace curvesinto parallelism with the σ 1 stress trajectories (Figure 3), a propagationcharacteristic of low-displacement shear fractures andconsistent with the local existence of CMT mechanisms withcomponents of normal slip. Moreover, total strike-slip displacementon the Greendale fault appears not to be large becauseit has not been recognized to continue along strike into thebedrock geology of the Southern Alps foothills. In fact, at itsFigure 5Seismological Research Letters Volume 82, Number 6 November/December 2011 829