A sedimentary sequence that was highly sensitive to fault rupture-driven changes in water level and sediment supply has been used to extract a continuous record of 22 large earthquakes on the Alpine fault, the fastest-slipping fault in New Zealand. At Hokuri Creek, in South Westland, an 18 m thickness of Holocene sediments accumulated against the Alpine fault scarp from ca. A.D. 800 to 6000 B.C. We used geomorphological mapping, sedimentology, and paleoenvironmental reconstruction to investigate the relationship between these sediments and Alpine fault rupture. We found that repeated fault rupture is the most convincing mechanism for explaining all the features of the alternating peat and silt sedimentary sequence. Climate has contributed to sedimentation but is unlikely to be the driver of these cyclical changes in sediment type and paleoenvironment. Other nontectonic causes for the sedimentary alternations do not produce the incremental increase in basin accommodation space necessary to maintain the shallow-water environment for 6800 yr. Our detailed documentation of this near-fault sedimentary basin sequence highlights the advantages of extracting paleoearthquake records from such sites-the continuity of sedimentation, abundance of dateable material, and pristine preservation of older events.
The~40 km long Vernon Fault, in the Marlborough Fault System of New Zealand, is characterized by dextral slip with subordinate reverse slip and exhibits abrupt variations in strike of up to 90°. Onshore fieldwork, paleoseismic trenching, and offshore high-frequency seismic reflection data are integrated together to identify the kinematics and paleoseismic history of three sections of the fault: (1) the NNE striking Vernon Hills section which branches off from the Awatere Fault; (2) the NE striking Big Lagoon section which borders Big Lagoon to the south and extends~9 km offshore to the east; and (3) the E-W striking Wairau Basin section, which is entirely submarine. The Vernon Fault can be shown to have a dextral slip rate of 0.8-4.9 mm/yr with a preferred estimate of 0.9 mm/yr (on the Big Lagoon section). We infer that a further unrecognized 3-4 mm/yr of dextral slip has been accommodated off fault as a result of accumulated slip on small and/or blind reverse faults adjacent to a 6 km wide restraining bend in the main fault. The onshore and offshore paleoseismic records are in good agreement. These indicate three to five events at eight sites and a mean recurrence interval of 3.9 ± 1.2 ka over the past~16 kyr, with the last event taking place at~3.3 ka. Earthquakes on the Vernon Fault are responsible for <25% of the Holocene subsidence rate of Big Lagoon over the last~13 ka. Most of the subsidence of this lagoon has been the result of surface deformation related with southern Hikurangi megathrust earthquakes.
<p>The coastal Awatere, Vernon, and Cloudy faults are bent and mutually intersecting, forming a complexly deforming dextral-oblique fault network. To try to explain the kinematic, paleoseismic and evolutionary complexities of this network, I present the results of an investigation into the rates, timing, and direction of slip on the faults within the network; which bifurcate eastwards from the central Awatere fault at the northeast end of the Marlborough Fault System. Displacements of dated and nondated late Quaternary features by the three faults were measured both onshore and offshore, constraining the kinematics of the fault network. The Vernon fault oddly maintains a dextral-reverse structure although it varies over 90° in strike and the Cloudy and coastal Awatere faults change from nearly pure strike slip to having a normal component eastwards. These data indicate that the fault-bounded blocks between the coastal Awatere, Vernon and Cloudy faults are rotating anticlockwise about a vertical axis relative to the block to the north of the fault system. Slip-rate data also indicate that of the 6 ± 1 mm/yr of slip on the central Awatere Fault, 1.1 ± 0.6 mm/yr has been partitioned ENE onto the coastal Awatere Fault and <4.9 mm/yr has been partitioned NNE onto the Vernon Fault. A slip-rate shortage in the splays of the Vernon Fault in the Vernon Hills is caused by a combination of unsighted faults and rotation of smaller splay-bounded blocks within the Vernon Hills. Paleoseismic records on the Vernon Fault were analysed onshore in a trench and offshore on seismic lines, with the records in good agreement. 3-5 earthquakes are recognised at different sites, with the last earthquake occurring 3.3 ka and a mean recurrence interval of 3-4 ka on the Vernon Fault. When combined with the paleseismic records from the Awatere and Cloudy faults I find that separate faults ruptured at similar times, suggesting a connectivity of the faults, as separate faults could mutually rupture during one earthquake or an earthquake could subsequently trigger an earthquake on a nearby fault. Finally I present the finite slip of geologic units and use these data as well as the late Quaternary slip data to describe the evolution of the fault network. I propose that the fault network at the NE end of the Awatere fault has stepped northwards into several splays, caused by clockwise rotation of the NE tips of the Marlborough faults.</p>
<p>The coastal Awatere, Vernon, and Cloudy faults are bent and mutually intersecting, forming a complexly deforming dextral-oblique fault network. To try to explain the kinematic, paleoseismic and evolutionary complexities of this network, I present the results of an investigation into the rates, timing, and direction of slip on the faults within the network; which bifurcate eastwards from the central Awatere fault at the northeast end of the Marlborough Fault System. Displacements of dated and nondated late Quaternary features by the three faults were measured both onshore and offshore, constraining the kinematics of the fault network. The Vernon fault oddly maintains a dextral-reverse structure although it varies over 90° in strike and the Cloudy and coastal Awatere faults change from nearly pure strike slip to having a normal component eastwards. These data indicate that the fault-bounded blocks between the coastal Awatere, Vernon and Cloudy faults are rotating anticlockwise about a vertical axis relative to the block to the north of the fault system. Slip-rate data also indicate that of the 6 ± 1 mm/yr of slip on the central Awatere Fault, 1.1 ± 0.6 mm/yr has been partitioned ENE onto the coastal Awatere Fault and <4.9 mm/yr has been partitioned NNE onto the Vernon Fault. A slip-rate shortage in the splays of the Vernon Fault in the Vernon Hills is caused by a combination of unsighted faults and rotation of smaller splay-bounded blocks within the Vernon Hills. Paleoseismic records on the Vernon Fault were analysed onshore in a trench and offshore on seismic lines, with the records in good agreement. 3-5 earthquakes are recognised at different sites, with the last earthquake occurring 3.3 ka and a mean recurrence interval of 3-4 ka on the Vernon Fault. When combined with the paleseismic records from the Awatere and Cloudy faults I find that separate faults ruptured at similar times, suggesting a connectivity of the faults, as separate faults could mutually rupture during one earthquake or an earthquake could subsequently trigger an earthquake on a nearby fault. Finally I present the finite slip of geologic units and use these data as well as the late Quaternary slip data to describe the evolution of the fault network. I propose that the fault network at the NE end of the Awatere fault has stepped northwards into several splays, caused by clockwise rotation of the NE tips of the Marlborough faults.</p>
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