Tectonic Geomorphology and Paleoseismology of theLake Heron Fault, New Zealand

Abstract

New Zealand's South Island is actively deforming as a result of oblique continental collision between the Pacific and Australian Plates. Within the central portion of the island, this results in reverse faulting, including earthquake rupture on the previously unreseached Lake Heron Fault. Because few of these faults have been studied, there is a major gap in knowledge on the paleoseismicity of the region. In this thesis, I use structural mapping, deformation analysis, geophysics, topo- graphic contouring, fault measurements, Monte Carlo simulations, and paleoseismic trenching, in conjuction with geochronologic approaches including Schmidt hammer exposure age dating, and radiocarbon dating to characterize the earthquake history and behavior of the Lake Heron Fault. Measurements of Pleistocene-Holocene displacement of discrete and distributed de- formation indicate that along the Lake Heron Fault, total vertical deformation is approx- imately 20 m since the Last Glacial Maximum. This deformation can be constrained, using Schmidt hammer exposure age dating, to the last 10:15±2:95 ka, and is consistent across surfaces which could differ in age by up to 15 ka. This suggests that a period of quiescence was followed by increased activity, and using Monte Carlo simulations, a vertical slip rate of 2:25 ± 1:05 mm/yr was calculated. This is more than double pub- lished vertical uplift rates, though when the fault dip within bedrock (60°) is integrated, net slip falls within published geodetic slip rates. While the fault was seen to dip 60° in bedrock, near surface dip measurements indicate a de ection and shallowing of fault dip by up to 80% in some locations in Late Quaternary sediments. Further structural calculations indicate that while the Lake Heron Fault has a strike (200°), preferential to pure dip-slip motion, which is seen on discrete fault scarps, mi- crotectonic measurements from crestal grabens show that along the fault, near-surface stress and localized surface rupture is not representative of the regional stress or the structure at depth. From paleoseismic trenching, evidence of 2-3 earthquakes was discovered, and using radiocarbon dating, the timing of the last events is constrained to the last 2.6 ka, with the most recent event occurring approximately 0:6 ± 0:2 ka. Large single event displacements (2:75 ± 0:25 m), earthquake magnitudes (Mw 7:4 ± 0:3), and a short recurrence interval (1:45±0:42 ka) indicate that the Lake Heron Fault has been highly active in the Holocene. Results of this study in conjunction with others also show there is a possibility the Lake Heron and Forest Creek fault represent an 80 km long, segmented reverse fault, capable of generating Mw 7.7 earthquakes. However, because of variability in the timing of paleoearthquakes, the structures more frequently ruptures independently in magnitude 7.0+ earthquakes. Lastly, I propose that changing crustal stress provides a possible mechanism for apparent temporal variability in the earthquake recurrence on the Lake Heron Fault.