What is the cause of the present-day uplift across the central Southern Alps, New Zealand?

Surprisingly fast bedrock uplift of ~5 mm/yr is observed by both GNSS and InSAR measurements across the central Southern Alps where oblique convergence is accommodated by the Alpine fault and other tectonic structures. This mountain range also features anomalously heavy precipitation and high erosion on the western side of the main divide. Since vertical motion is sensitive to perturbations from surface processes, we first evaluate how surface processes operating at different time scales (e.g., erosion, (de)glaciation, and hydrological loading) impact the present-day GNSS measurements. Erosion-induced rebound might be significant, but with an upper limit below 2 mm/yr uplift across the central Southern Alps, whereas elastic rebound caused by modern glacier melting and hydrological loading are secondary, as the magnitude is below 0.3 mm/yr at the locations of GNSS stations. Next, we use elastic dislocation models to explore the geometry and kinematics of a ramp-décollement system consistent with the GNSS-derived shortening and remaining vertical motions. Our best-fit model has 5.5-6.5 mm/yr reverse slip on the 40-50°SE-dipping Alpine fault locked above ~9 km depth, connected to a flat décollement accommodating 7.5-11.0 mm/yr of shortening. The remaining far-field shortening might be taken up by folding near the ramp-décollement junction and/or reverse slip along a NW-dipping backthrust in the hinterland. Our results present how climate-related surface and endogenic tectonic processes modulate the present-day vertical deformation across the central Southern Alps, giving new insights into active mountain building for this mountain range.