Styles and scales of structural inheritance throughout continental rifting; Examples from the Great South Basin, New Zealand

Abstract

<p>Distinct crustal terranes and intruded igneous plutonic material are repeatedly brought together and deformed throughout multiple tectonic events. Accordingly, the resulting continental crust is highly heterogeneous and exhibits widespread lateral lithological and rheological variability that exerts fundamental controls on the structural style of rifting and eventual continental breakup across multiple scales of observation.</p><p>Lateral variations in crustal rheology and strength, such as those posed by igneous intrusions or distinct crustal terranes, may cause certain areas to be less prone to rifting and localise activity in adjacent areas, influencing rift structural style and the geometry of individual faults. Furthermore, the boundaries between different terranes often represent crustal-to-lithospheric-scale heterogeneities that may localise strain and reactivate during subsequent tectonic events, acting to partition the rift into distinct structural domains.  </p><p>Using borehole-constrained 2D and 3D seismic reflection data, we showcase a number of styles of structural inheritance in the Great South Basin, offshore of the South Island of New Zealand. Pre-rift basement in this area comprises terranes corresponding to a relict Island arc system, including the dominantly plutonic Median Batholith and the dominantly sedimentary Murihiku Terrane, a former forearc basin. We find the spacing, structural style and geometry of faults varies greatly between the relative ‘strong’ and ‘weak’ crustal terranes, whilst the boundaries between individual terranes are often associated with complex plan-view fault geometries. In particular, the southern margin of the Median Batholith is reactivated as a large-scale shear zone and upper crustal fault system that is oriented at a high angle to, and thereby segments, the Great South Basin. Furthermore, these terrane boundaries also appear to exhibit some controls over the locations of Cenozoic intraplate volcanic systems and fracture zones associated with breakup along the Pacific-Antarctic ridge. </p><p>Within the rift itself we identify a series of granitic laccoliths along, and potentially exploiting, the southern boundary of the Median Batholith. The presence of this ‘strong’ granitic material inhibits fault nucleation and retards the propagation of approaching rift-related faults, causing them to splay and eventually terminate as they approach the granitic material. Through quantitative fault analyses, we find that individual fault segments maintain kinematic, and to some degree, geometric coherence across the system before terminating at the margin of the stronger crustal material. </p><p>The multi-scale complexity and variability of continental crust exerts a strong influence over multiple aspects of rift geometry from inception to its eventual breakup. We showcase a range of different styles of structural inheritance that are applicable to other rift systems worldwide. We show how different styles of structural inheritance, ultimately related to lateral and vertical variations in crustal strength and rheology, may dictate the location, geometry and structural style of different aspects of rift physiography from the whole rift scale to that of individual faults and fractures.</p>