Collaborative Research: Erosional Forcing of Late Quaternary Compressive Strain, West Central Taiwan

The concept of steady-state growth of compressive orogens is one of the most accepted ideas in continental tectonics developed in the last two decades, yet surprisingly few direct data exist to test the models that have been advanced. Testing and refining the concept of steady-state mountain building requires identification of cases of transient response and dynamic feedback in compressive orogens, and documentation of the spatial and temporal scales at which crustal deformation responds to erosion. This project takes advantage of a unique natural experiment in the active fold-thrust belt of west-central Taiwan where enough modern seismologic, geodetic, structural and geochronologic data exist to assess how an orogen responds to rapid and abrupt changes in wedge volume at scales of tens of kilometers. The Puli embayment in the west-central Taiwan lies approximately 1.5 km lower than neighboring parts of the fold-thrust belt. Geologic and geodetic evidence suggests that thrusting occurs over the width of the topographic embayment, rather than being concentrated at the front of the thrust belt, as is the case in the adjacent part of the thrust belt to the south where the topography maintains a classic wedge shape. The hypothesized connection between strain and topographic gradients is tested by (a) development of a database of late Quaternary slip rates and deformation patterns using a combination of structural, geochronologic and geomorphic methods, and (b) use of a 3D finite element model to determine the necessary and sufficient conditions required to produce the observed topography and deformation patterns. Fault-slip rates are obtained via dating of deformed stream terrace and basin fill deposits. Age dating also enables testing and calibration of long-term stream incision laws. The calibrated laws provide a quantitative basis for topographically derived estimates of strain patterns in inaccessible or poorly constrained areas of the belt such as the Alishan Range

COLLABORATIVE RESEARCH: St. Elias Erosion and Tectonics Project (STEEP)

This project provides funds for a two-year renewal of the St. Elias Erosion-tectonics Project (STEEP). STEEP is a 9 institution, multidisciplinary study of the St. Elias orogen in southern Alaska that involves researchers examining the system from the outcrop to lithosphere scale. To date, STEEP has produced 17 papers with another 9 submitted or nearing submission, sponsored 71 abstracts, will have matriculated 5 masters and 4 Doctoral students by Spring 2010, and fundamentally changed our understanding of Alaskan tectonics and the interaction of tectonics and climate in mountain building. The renewal funds will be used for: 1) final processing and interpretation of some key datasets that were not acquired until year 5 of the project including the marine seismic survey (ship delays) and reoccupation of key GPS sites (weather problems in 2008); and 2) a complete integration of results which was not possible until now due to these delays. A complete integration and synthesis of these superb datasets has the potential to be transformative in our understanding of how crustal structure and tectonic forces interact with Earth surface processes of glacial erosion and sedimentary transport to grow a mountain range and a massive continental shelf

Collaborative Research: St. Elias Erosion and Tectonics Project (STEEP)

1) Refinement of a regional scale model to include an approximation of the true 3D geometry of the orogen. 2) Develop a new local-scale model that incorporates topography, GPS data, and glacial erosion processes to refine the initial results. 3) Develop a modeling experiment to test the hypothesis that the rise and fall of ice masses during glacial cycles might influence where deformation is focused at any given time

Numerical Facility in Geodynamics

Support from this grant will contribute to the construction of a numerical facility for Geodynamical Modeling at the University of Maine to investigate mechanical and thermal problems arising in lithosphere and mantle deformation. Specifically, the PI\u27s will examine the degree of coupling among atmospheric and tectonic processes through construction of three-dimensional models conditioned by observations from the active tectonic regions of eastern Tibet, New Zealand and southeast Alaska. In related research, the PI\u27s are examining the necessary conditions for formation and exhumation of ultra-high pressure metamorphism terrains during plate convergence. The overriding objective of the numerical facility is to provide tools for application of continuum mechanics to geological processes for those with an intuitive feel for the earth, but not necessarily a strong grasp on continuum mechanics. The facility is designed to remove some of the difficulties encountered by earth scientists in solution of mechanical equations, thereby allowing concentration upon the causative processes and the relevant observations. An important component of research at the numerical facility will be in applying modern visualization techniques to numerical results to enhance communication among modelers and geologists, and to allow dissemination of results to a wider educational audience

Extension and Partitioning in an Oblique Subduction Zone, New Zealand: Constraints from Three-Dimensional Numerical Modeling

Contraction, strike slip, and extension displacements along the Hikurangi margin northeast of the North Island of New Zealand coincide with large lateral gradients in material properties. We use a finite- difference code utilizing elastic and elastic-plastic rheologies to build large- scale, three-dimensional numerical models which investigate the influence of material properties on velocity partitioning within oblique subduction zones. Rheological variation in the oblique models is constrained by seismic velocity and attenuation information available for the Hikurangi margin. We compare the effect of weakly versus strongly coupled subduction interfaces on the development of extension and the partitioning of velocity components for orthogonal and oblique convergence and include the effect of ponded sediments beneath the Raukumara Peninsula. Extension and velocity partitioning occur if the subduction interface is weak, but neither develops if the subduction interface is strong. The simple mechanical model incorporating rheological variation based on seismic observations produces kinematics that closely match those published from the Hikurangi margin. These include extension within the Taupo Volcanic Zone, uplift over ponded sediments, and dextral contraction to the south

Along-Strike Differences in the Southern Alps of New Zealand: Consequences of Inherited Variation in Rheology

Long- and short-term strain variations along the Australian-Pacific plate boundary through the South Island of New Zealand, including a 300% increase in orogen width, coexistence of oblique thrusting on orthogonal structures, and variability in the locus of orogenic gold deposits, coincide with rheologically relevant geological variation. Our model investigates the consequences of thin, strong lower crust in the north and thick, weak lower crust in the south. Solution of the full 3-D mechanical equations reproduces the larger wavelength strain patterns of the orogen. A 3-D perturbation-based analytical solution leads to the identification of the sensitivity of displacement type to minor stress changes. Transition from boundary-normal thrusting to boundary-parallel thrusting occurs at the transition from strong to weak lower crust and is related to an increase in either tau(yz) (shear stress in the yz plane) or the ratio of the coordinate normal stresses, (sigma(yy)/sigma(xx)), where x and y are in the horizontal and z is vertical. Both mechanisms are compatible with the geologically dependent rheological variation employed in our model. Citation: Upton, P., P. O. Koons, D. Craw, C. M. Henderson, and R. Enlow (2009), Along-strike differences in the Southern Alps of New Zealand: Consequences of inherited variation in rheology, Tectonics, 28, TC2007, doi:10.1029/2008TC002353

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.</p

The Alpine Fault Hangingwall Viewed From Within: Structural Analysis of Ultrasonic Image Logs in the DFDP-2B Borehole, New Zealand

International audienceUltrasonic image logs acquired in the DFDP‐2B borehole yield the first continuous, subsurface description of the transition from schist to mylonite in the hangingwall of the Alpine Fault, New Zealand, to a depth of 818 m below surface. Three feature sets are delineated. One set, comprising foliation and foliation‐parallel veins and fractures, has a constant orientation. The average dip direction of 145° is subparallel to the dip direction of the Alpine Fault, and the average dip magnitude of 60° is similar to nearby outcrop observations of foliation in the Alpine mylonites that occur immediately above the Alpine Fault. We suggest that this foliation orientation is similar to the Alpine Fault plane at ∼1 km depth in the Whataroa valley. The other two auxiliary feature sets are interpreted as joints based on their morphology and orientation. Subvertical joints with NW‐SE (137°) strike occurring dominantly above ∼500 m are interpreted as being formed during the exhumation and unloading of the Alpine Fault's hangingwall. Gently dipping joints, predominantly observed below ∼500 m, are interpreted as inherited hydrofractures exhumed from their depth of formation. These three fracture sets, combined with subsidiary brecciated fault zones, define the fluid pathways and anisotropic permeability directions. In addition, high topographic relief, which perturbs the stress tensor, likely enhances the slip potential and thus permeability of subvertical fractures below the ridges, and of gently dipping fractures below the valleys. Thus, DFDP‐2B borehole observations support the inference of a large zone of enhanced permeability in the hangingwall of the Alpine Fault

Surface rupture of multiple crustal faults in the 2016 Mw 7.8 Kaikōura, New Zealand, earthquake

Multiple (>20 >20 ) crustal faults ruptured to the ground surface and seafloor in the 14 November 2016 M w Mw 7.8 Kaikōura earthquake, and many have been documented in detail, providing an opportunity to understand the factors controlling multifault ruptures, including the role of the subduction interface. We present a summary of the surface ruptures, as well as previous knowledge including paleoseismic data, and use these data and a 3D geological model to calculate cumulative geological moment magnitudes (M G w MwG ) and seismic moments for comparison with those from geophysical datasets. The earthquake ruptured faults with a wide range of orientations, sense of movement, slip rates, and recurrence intervals, and crossed a tectonic domain boundary, the Hope fault. The maximum net surface displacement was ∼12  m ∼12  m on the Kekerengu and the Papatea faults, and average displacements for the major faults were 0.7–1.5 m south of the Hope fault, and 5.5–6.4 m to the north. M G w MwG using two different methods are M G w MwG 7.7 +0.3 −0.2 7.7−0.2+0.3 and the seismic moment is 33%–67% of geophysical datasets. However, these are minimum values and a best estimate M G w MwG incorporating probable larger slip at depth, a 20 km seismogenic depth, and likely listric geometry is M G w MwG 7.8±0.2 7.8±0.2 , suggests ≤32% ≤32% of the moment may be attributed to slip on the subduction interface and/or a midcrustal detachment. Likely factors contributing to multifault rupture in the Kaikōura earthquake include (1) the presence of the subduction interface, (2) physical linkages between faults, (3) rupture of geologically immature faults in the south, and (4) inherited geological structure. The estimated recurrence interval for the Kaikōura earthquake is ≥5,000–10,000  yrs ≥5,000–10,000  yrs , and so it is a relatively rare event. Nevertheless, these findings support the need for continued advances in seismic hazard modeling to ensure that they incorporate multifault ruptures that cross tectonic domain boundaries