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A piece of Active faults in New Zealand - 1: The Alpine Fault
The now yearly PATA (Paleoseismology Active Tectonics Archeoseismology) Days were held this year (13-19 November) in South Island of New Zealand. You can find the program of those 2 days of talks and posters here: http://www.earthquakegeology.com/.
The lucky guys who attended these 2 conference days could also spend 4 days in the field to admire the famous "Alpine Fault" and the recent ruptures that affected the North-eastern South Island during the M7.8 Kaikoura earthquake. Part of the field guide is available here: field_trip_guide_day1.
Many thanks to our guides and to the organizing committee!
Figure 1: Earthquake, tsunami, volcano eruption, flooding, landsliding... What else could happen in New Zeland? Here is the first set of pictures and explanations, dealing with the Alpine Fault. On the map below I pinpoint the 3 visited sites (1, 2 and 3) along that impressive fault.
Figure 2: Google Earth view. Red lines depict the active faults from the GNS Science database We started to drive along the Alpine Fault System (lato sensu) as soon as leaving Blenheim. The first stop along the Wairau Fault is the Dillon site where our Kiwis colleagues has dug 3 trench (see reference here: Zachariasen et al. 2006). The fault is there actually composed of two parallel strands and we focused on the northern one that cuts alluvial terraces of the Wairau River. 1 - The Wairau fault at Dillon trench site and Branch River further south The picture below -with the LiDAR DEM- show you the cumulative signature of (dominantly) right-lateral faulting along those two segments. The northern fault strand, marked by the 2 red arrows, creates a sag pond and a dextral offset of the northward-flowing stream (blue arrows). The Dillon trench was excavated in the pond area (between the 2 arrows). This trench and the two other ones nearby revealed that four surface-rupturing paleoearthquakes occurred in the last 6 ky. Considering both the age of the last event (1800 y BP), the mean recurrence time (1200 y) and the possible slip rate (3-5 mm/y), Zachariasen et al. 2006 suggest that the fault is close to the end of its interseismic period and has accumulated enough strain to produce a similar earthquake in a close future.
Figure 3; Picture shot during the PATA Days, looking to the West, of the northern segment. LiDAR image is from the Field trip Guide Book. The blue arrows on the picture and the DEM represent more or less the same features. LiDAR DEM from Upton et al. (2017) 40 km Southwest of Dillon trench site, the Wairau Fault intersects the Branch River Terrace system and this provides a stunning tectonic landscape and unique tectonic geomorphology example, which was described and analyzed as soon as the end of the 1960s (Lensen, 1968). There, strike-slip displacements increase with age, providing a ~4 mm/y slip rate. For instance, the W terrace riser -around ~17 ky old- is displaced by about ~50 m.
Figure 4: Illustration of the Wairau Fault scarp (bottom) and the offset (dextral + reverse) of the W-terrace riser of a former Branch River course. DEM map from Upton et al. (2017) 2 - Calf Paddock Site The northern spot along the Alpine Fault (sensu stricto) that we could visit is the "Calf Paddock" site (label 2 on Figure 1). It is located just south of the so-called "Big Bends" of the Alpine Fault, where the Awatere Fault, another big earthquake fault of South Island, branches to the Alpine Fault. Calf Paddock is an alluvial plain of the Maruia River that flows through the Alpine Fault. Like in the Branch River example described above, the interaction between the alluvial terrace system (including risers, aggradation surfaces and channels) provides a relevant picture to understand the fault slip history. The recent paper by Langridge et al. (2017) provides updated results based on a new topographic study and new datings, concluding that the Holocene slip rate is ~10 mm/y after dating at 12 ky the 12m dextrally displaced "T2" alluvial terrace. Note that the fault also has a slight reverse component (~1 mm/y). Another interest of the Calf Paddock site is that a wall has been built in 1964 to measure the potential aseismic/creep displacement on this section of the Alpine Fault. More than 50 years after the construction, no displacement can be seen by eye.
Figure 5: A view (to northeast) of the Holocene scarp of the Alpine Fault (along red arrows) across Calf Paddock site in the Maruia alluvial plain. DEM Map from Langridge et al. (2017), colored by elevation.
Figure 6: I am quite happy to present you the "Evison's Wall" that has been built in 1964 across the Alpine Fault Scarp to evidence the creep amount at the Calf Paddock site. As highlighted by the picture to the right, the creep is null ! 3 - Poerua Lake and Camp Creek site Moving 70 km to the southwest along the Alpine Fault (Figure 2), we there accessed to a faster section of the fault (~14 mm/y dextral and ~3 mm/y reverse: Langridge et al. 2012).
The investigations on the landcape, especially lake shoreline evolution with time through drowned trees, alluvial fan growth and related features resulted in a nice and convincing story relating those morphological changes to the earthquake sequence (Langridge et al., 2010). Below here is a picture showing the Poerua Lake with drowned trees, shot from on top of the fault, documenting a recent change in shoreline.
Figure 7: View of the Lake shoreline and some trunks of drowned trees There are several issues to study on-fault earthquake-related deformations along the Alpine Fault, often covered by "bush" (i.e. dense forest) on steep slopes or crossed by alluvial fans of coarse gravels with low preservation potentialities for paleoseismological reconstructions. During the fieldtrip, our leader guide (Rob Langridge) drove us to a potential site for further paleoseismological analyses, where there is potentially a good preservation of fine sediments ontop of the fault. This site is not far to the northeast of Poerua Lake (5 km), and is called Camp Creek. There, the Alpine Range front lines up with the Alpine Fault (see below Figure 8). The hillslope presents a knickpoint which actually marks the fault trace; in details, this knickpoint includes a counter-slope scarp and associated narrow depression which is locally closed (sagpond) and potentially contains fine sediments. Most of the attendees agreed that the site could be investigated for on-fault paleoseismological trenches.
Figure 8: Two views of the same Alpine Fault trace in the Poerua Lake area. Top: the Alpine Range front lines up with the Alpine Fault (red arrows). Below: the hillslope presents a knickpoint (red arrows) which actually marks the fault trace.
Figure 9: The sagpond is a kind of marsh with very soft sediments trapped between the fault scarp (to the left) and the mountain slope (to the right). The LiDAR DEM (Upton et al., 2017) (top-left) clearly shows the fault trace and the location of the sag pond. 4 - The increase of slip rate to the SouthMaybe you noticed that the fault slip rates increase from north to south, from 3-5 mm/y, 10 mm/y and 14 mm/y. And that's a true trend, which is confirmed further south along the Central segment of the Alpine Fault where slip rates up to 25 mm/y are documented (see figure 10 below from Langridge et al. 2017). The reason is that the ~25-30 mm/y slip rate of the central segment is progressively absorbed by the Marlborough System Faults (Hope fault, Clarence fault, Awatere fault, from south to north).
Figure 10: Map with slip rates along the Alpine Fault and Marlborough System Faults (from Langridge et al., 2017). The three sites here described are underlined in red.
Upton P., Langridge R. M., Stahl T., Van Dissen R. J., Howarth J., Berryman K., Clark K. J., Kelly K., Hammond K., 2017. 8th International PATA Days, Blenheim, New Zealand. Three-day post-conference fieldtrip; Northern South Island, Alpine Fault and Ruptures of the 2016 Kaikoura Earthquake, 17-19th November 2017. Lower Hutt (NZ): GNS Science. 64 pages.