Subsurface structure of the Canterbury region and implications for earthquake activity : interpretations from gravity and magnetic data

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Davy, B.W.; Stagpoole, V.M.; Barker, D.H.N.; Yu, J. 2012 Subsurface structure of the Canterbury region and implications for earthquake activity : interpretations from gravity and magnetic data. Lower Hutt, N.Z.: GNS Science. GNS Science report 2012/02 31 p.

Abstract: Gravity and aeromagnetic data were acquired over the Christchurch and adjacent area to map the subsurface structure and identify possible faults. This report describes the data acquisition and interpretation. The results presented here may be further refined in the future when they are compared with seismic reflection data collected in 2011. Interpretation of both gravity and magnetic data indicate that there are several distinct sets of lineaments that are postulated to be associated with basement structure and subsurface faults. Although the largest magnetic anomalies are related to Lyttelton Volcano, E-W and ENE oriented magnetic lineaments may correlate with faults in the northern Banks Peninsula region. A magnetic anomaly extends east from the Greendale Fault but it is not a dipolar anomaly, as might be expected for a fault. It may be caused by a subsurface volcanic feature, possibly an intrusion or a Miocene lava flow buried by younger sediments. Interpretation of gravity gradient data identified two principal sets of lineaments that have ESE to E and NNE orientations. The NNE set of lineaments may relate to NNE faults that formed in Cretaceous time which have been recently reactivated. The E-W to ESE-WNW lineaments are possibly associated with faults on the edges of Cretaceous grabens or half-grabens. Three E-W lineaments are interpreted to extend 20 km east of Rolleston, and there is a good correlation between the middle lineament and the 2010-2011 seismicity. Under Christchurch, seismicity jumps between E-W lineaments during the 2010-2011 earthquake activity. Interpretation of the datasets suggests that there is a boundary between separate crustal deformation domains in the Rolleston area. The domain between Rolleston and Lyttelton Volcano has E-W structural features segmented on 2-4 km length scale. West of Rolleston, lineaments are typically segmented at 5 km or more. Interpretation of the data suggests plate margin deformation is possibly being preferentially focussed in the Rolleston–Lyttelton Volcano domain by motion of higher rigidity crust associated with the adjacent Lyttelton Volcano. (auth)