Potentially active faults in the rapidly eroding landscape adjacent to the Alpine Fault, central Southern Alps, New Zealand

Abstract

Potentially active faults are exposed in the steep glaciated topography of the central Southern Alps, New Zealand, immediately adjacent to the Alpine Fault plate boundary. Four major faults exposed along the flanks of three of the highest mountain ranges strike 10–23 km (potentially 40 km) NNE oblique to the Alpine Fault, dipping 57° ± 12° NW in the opposite direction. Youngest discernable motions were reverse dip‐slip, accommodating both margin‐perpendicular shortening and dextral margin‐parallel components of plate motion. Kinematic analysis yields a compression axis (295/10° ± 9° trend or plunge) equivalent to the contemporary shortening determined from seismological and geodetic studies, suggesting the faults may be active, although definitive evidence for recent movement or single event displacements is lacking. There are 106 other potentially active faults mapped in central Southern Alps with strike lengths 4–73 km. Earthquake parameters were assigned from fault trace lengths and historical earthquake statistics, indicating potential for MW 5.5–7.4 earthquakes at recurrence intervals of 1000–10,000 years. Such long recurrence intervals are consistent with the faults having little surface expression, with rapid erosion of these seismically agitated mountains erasing any evidence of surface rupture during periods between earthquakes. The central Southern Alps faults exemplify the difficulty in fully deciphering long‐term (e.g., Holocene or Quaternary) records of seismicity in tectonically active regions with rapidly evolving landscapes. Although there may be little evidence of surface ruptures remaining in the landscape, the faults are still an important potential source of earthquakes and seismic hazard.

Structural heterogeneity of the midcrust adjacent to the central Alpine Fault, New Zealand: Inferences from seismic tomography and seismicity between Harihari and Ross

“…In particular, some of the clusters have an apparent NW dip (Figure d) and appear to be associated with faults of the Main Divide Fault Zone [ Cox and Findlay , ] that strike obliquely to the Alpine Fault. We infer that these earthquakes confirm present‐day seismic activity on the deep extensions of structures previously suspected to be active [ Cox et al ., ]. Faults in this region, beneath the Southern Alps and east of the Alpine Fault, have been suggested to accommodate some of the remainder (10–15%) of the relative plate motion not already accommodated by coseismic slip on the Alpine Fault [ Wallace et al ., ].…”

Coupled numerical–analytical approach to landscape evolution modeling

“…Higher uplift rates in the northwest are supported by estimation of erosion rate through suspended sediments in rivers, showing an increase across the divide with greater erosion rate on the northwest flank by a factor of 3-5 with respect to the southeast flank (Hicks et al, 1996;Cox et al, 2012). Higher uplift rates in the northwest are supported by estimation of erosion rate through suspended sediments in rivers, showing an increase across the divide with greater erosion rate on the northwest flank by a factor of 3-5 with respect to the southeast flank (Hicks et al, 1996;Cox et al, 2012).…”

“…Higher uplift rates in the northwest are supported by estimation of erosion rate through suspended sediments in rivers, showing an increase across the divide with greater erosion rate on the northwest flank by a factor of 3-5 with respect to the southeast flank (Hicks et al, 1996;Cox et al, 2012). We use this ratio in the simulation while choosing U r = 10 mm yr À 1 following the erosion rate estimation in Cox et al (2012), which leads to U p = 2.7 mm yr À 1 . To test the contribution of spatially variable uplift we performed another DAC simulation, where this forcing was added to the previously studied mechanisms.…”

Changes in hot spring temperature and hydrogeology of the Alpine Fault hanging wall, New Zealand, induced by distal South Island earthquakes

“…The event ruptured the 30-km-long Greendale Fault (Quigley et al 2010), previously concealed beneath the Canterbury alluvial outwash plains, within a network of faults that accommodates distributed deformation east of the Alpine Fault (Cox et al 2012a;Litchfield et al 2013). Right-lateral strike-slip displacement reached 5.3 m at the surface, averaging 2.5 AE 0.1 m (Quigley et al 2012), initiating a prolonged datapoints, collected by hammering a steel rod into the ground, quickly removing it and inserting a fibre glass probe with a thermistor on its tip, then reading the temperature once the probe had thermally equilibrated.…”