STRUCTURAL STYLES OF TRANSPRESSIONAL DEFORMATION ALONG THE MARGIN OF THE NEW ZEALAND PLATE BOUNDARY: THE PORTERS PASS–AMBERLEY FAULT ZONE AND 2010-12 CANTERBURY EARTHQUAKE SEQUENCE

The Porters Pass-Amberley Fault Zone (PPAFZ) is a Quaternary hybrid dextral strike-slip fault zone accommodating complex transpressional deformation on the outer margin of the obliquely convergent Australia-Pacific plate boundary zone in the North Canterbury region of South Island. This ENE trending fault zone comprises an anastomosing left-stepping array of dextral strike-slip, oblique-slip and thrust fault segments overprinting and linking through an inherited E-W fabric of basement normal faults. Ongoing fault-zone deformation is forming the easternmost foothill ranges of the Southern Alps, which border the Canterbury Plains. The structural geomorphic transition from the plains to the foothills is marked by a series of segmented and thrust-faulted topographic steps reflecting upper crustal scale strain partitioning along the SE margin of the PPAFZ.

Active blind structures starting to daylight from beneath the north Canterbury Plains indicate plate boundary deformation extends SE of the PPAFZ. Here the structural pattern of deformation is defined by NE trending thrust faults, expressed by growing asymmetric anticlines, blind to emerging thrusts and back-thrusts, that are regularly segmented by, and interact with, E-W trending dextral strike-slip faults overprinting and reactivating an inherited network of inclined Cretaceous to mid-Tertiary normal faults. This relay of structures is accommodating transpressional deformation in response to the regional WNW oriented maximum horizontal compressional stress.

The active late Quaternary deformation from the north Canterbury Plains when combined with the distribution and seismological characteristics of the 2010-2012 Canterbury Earthquake sequence provide important new insights into the structural evolution of these fault systems as they progress from lower strain peripheries into the more actively deforming parts of the NZ orogen. The structural mesh beneath the Canterbury Plains is analogous in both scale and geometry to the more evolved PPAFZ to the west.