Highly localized upper mantle deformation during plate boundary initiation near the Alpine fault, New Zealand

Peridotite xenoliths entrained in magmas near the Alpine fault (New Zealand) provide the first direct evidence of deformation associated with the propagation of the Australian-Pacific plate boundary through the region at ca. 25–20 Ma. Two of 11 sampled xenolith localities contain fine-grained (40–150 mm) rocks, indicating that deformation in the upper mantle was focused in highly sheared zones. To constrain the nature and conditions of deformation, we combine a flow law with a model linking recrystallized fraction to strain. Temperatures calculated from this new approach (625–970 °C) indicate that the observed deformation occurred at depths of 25–50 km. Calculated shear strains were between 1 and 100, which, given known plate offset rates (10–20 mm/yr) and an estimated interval during which deformation likely occurred (<1.8 m.y.), translate to a total shear zone width in the range 0.2–32 km. This narrow width and the position of mylonite-bearing localities amid mylonite-free sites suggest that early plate boundary deformation was distributed across at least ~60 km but localized in multiple fault strands. Such upper mantle deformation is best described by relatively rigid, plate-like domains separated by rapidly formed, narrow mylonite zones.

A new joint IAG-IASPEI sub-commission for Seismogeodesy

<p>Thanks to both technological evolution and analysis improvement in the past decades, space geodesy can now monitor crustal movements of a few millimeters over time opening new prospects for the study of earthquakes. However, fully exploiting the potential of geodetic measurements is subject to their further integration with seismological analysis and requires the development of a multidisciplinary approach. The new joint IAG-IASPEI sub-commission on Seismogeodesy aims to facilitate the cooperation between the geodetic and the seismological communities in order to improve our current understanding of the different processes leading to earthquakes. <br><br>The new Seismogeodesy sub-commission will focus on both observational challenges and theoretical aspects. Particular effort will be dedicated to identifying gaps of knowledge and opportunity for progress. Specifically, the sub-commission will:<br>* actively encourage the cooperation between all geoscientists studying the plate boundary deformation zones, by promoting the exploitation of synergies between different fields;<br>*  work to reinforce collocated and integrated geodetic and seismological monitoring of seismically active areas, inland and off-shore by increasing and/or developing infrastructures dedicated to broadband observations from the seismic wave band to the permanent displacement;<br>* be a reference group for the integration of the most advanced geodetic and seismological techniques by developing consistent methodologies for data reduction, analysis, integration, and interpretation;<br>* act as a forum for discussion and scientific support for international geoscientists investigating the kinematics and mechanics of the plate boundary deformation zone;<br>* promote the use of standard procedures for geodetic data acquisition, quality evaluation, and processing, particularly GNSS data and InSAR data;<br>* promote earthquake geodesy, the study of seismically active regions with large earthquake potential, and geodetic application to early warning system of earthquakes and tsunamis for hazard mitigation;<br>* promote the role of geodesy in tectonic studies for understanding the seismic cycle, transient and instantaneous deformation, and creeping versus seismic slip on faults.<br>* facilitate and stimulate the integrated exploitation of large data sets, using Machine Learning and Data Mining<br>* support the organization of periodic workshops, meetings, summer schools with special emphasis on interdisciplinary research and interpretation and modeling issues<br>* help to the emergence of a new generation of researchers in Seismogeodesy worldwide<br><br>We invite any researcher interested in Seismogeodesy to join us and have a fruitful discussion in front of our poster.  </p>

River patterns reveal two stages of landscape evolution at an oblique convergent margin, Marlborough Fault System, New Zealand

Here we examine the landscape of New Zealand's Marlborough Fault System (MFS), where the Australian and Pacific plates obliquely collide, in order to study landscape evolution and the controls on fluvial patterns at a long-lived plate boundary. We present maps of drainage anomalies and channel steepness, as well as an analysis of the plan-view orientations of rivers and faults, and we find abundant evidence of structurally controlled drainage that we relate to a history of drainage capture and rearrangement in response to mountain-building and strike-slip faulting. Despite clear evidence of recent rearrangement of the western MFS drainage network, rivers in this region still flow parallel to older faults, rather than along orthogonal traces of younger, active strike-slip faults. Such drainage patterns emphasize the importance of river entrenchment, showing that once rivers establish themselves along a structural grain, their capture or avulsion becomes difficult, even when exposed to new weakening and tectonic strain. Continued flow along older faults may also indicate that the younger faults have not yet generated a fault damage zone with the material weakening needed to focus erosion and reorient rivers. Channel steepness is highest in the eastern MFS, in a zone centered on the Kaikōura ranges, including within the low-elevation valleys of main stem rivers and at tributaries near the coast. This pattern is consistent with an increase in rock uplift rate toward a subduction front that is locked on its southern end. Based on these results and a wealth of previous geologic studies, we propose two broad stages of landscape evolution over the last 25 million years of orogenesis. In the eastern MFS, Miocene folding above blind thrust faults generated prominent mountain peaks and formed major transverse rivers early in the plate collision history. A transition to Pliocene dextral strike-slip faulting and widespread uplift led to cycles of river channel offset, deflection and capture of tributaries draining across active faults, and headward erosion and captures by major transverse rivers within the western MFS. We predict a similar landscape will evolve south of the Hope Fault, as the locus of plate boundary deformation migrates southward into this region with time.