Three-dimensional lateral crustal thickening in continental oblique collision: an example from the Southern Alps, New Zealand

Abstract

SUMMARY Crustal thickening in an oblique continental collision, such as in the South Island of New Zealand, necessarily involves deformation processes in three dimensions (3-D). We have investigated the role played by the strength of the lower crust using simplified 3-D mechanical models. These models show that crustal thickening occurs away from the area of maximum compression, along an axis inclined to the plate boundary (about 10 ◦ ‐20 ◦ to the plate boundary in the case of the South Island), and perpendicular to the convergence direction. Furthermore, the specific geometry of the relatively old and strong Australian lithosphere versus the Pacific lithosphere also controls the location of crustal thickening. These conditions could explain the observed mismatch between the locations of maximum elevation and minimum gravity in South Island, New Zealand, as a consequence of decoupled deformation owing to low-viscosity lower crust. 1 I NTR O DUCTION Two-dimensional modelling approaches for continental oblique collision implicitly assume that convergence occurs perpendicular to the plate boundary, and that deformation is identical in sections perpendicular to the plate boundary. However, crustal and lithospheric thickening under oblique collision necessarily involve three dimensions: the extra degree of freedom allows a component of lateral movement to occur. The lithospheric deformation leading to the formation of the Southern Alps of South Island, New Zealand demand a 3-D modelling approach. Here, continental oblique convergence has been accommodated since about 6 Ma, and the topography and gravity data indicate that surface deformation and thickening at depth are geometrically offset. We study the lithospheric scale deformation of the region using simple mechanical models based on the collision of two lithospheric blocks, one being mechanically weaker than the other. The weakest block will deform in one of two ways: it can thicken vertically, producing a ‘crustal’ root and mountain ranges, or it can spread laterally in the horizontal direction, towards regions of lower compression. This lateral flow away from the area of maximum compression is favoured if the weakest block is unable, mechanically, to support the weight of thickened material. The question that we pose is if the convergence is oblique, will the direction of flow remain oriented parallel to the margins of the lithospheric blocks that represent the plate boundary?