Abstract. 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.
Abstract Strike‐slip landscapes are often associated with a suite of characteristic geomorphic features that provide primary evidence for interpreting fault slip histories. Here we explore the role of shutter ridges, areas of relief advected laterally along faults, in generating two classic strike‐slip processes: progressive lateral offset of channels and stream capture. Landscape models and comparative analysis of the Marlborough Fault System, NZ, show that the length of channel offsets observable in a landscape is primarily controlled by the length of shutter ridges. In our simple landscape model, this scale is controlled by the drainage spacing, and therefore by the geometry of the mountain range. In a more complex landscape, this scale may be controlled by lithologic or structural contrasts. We also find that shutter ridge relief inhibits stream capture, especially at slow fault slip rates relative to hillslope erosion rates. In this case, lateral drainage advection enables streams to “outrun” capture.
<p>The landscape at the NE end of the South Island, New Zealand, records oblique plate collision over the last 25 million years. Using low-temperature thermochronology, geomorphic analyses, and cosmogenic <sup>10</sup>Be data, we document the landscape response to tectonics over long (10<sup>6</sup>) and short (10<sup>2</sup> &#8211; 10<sup>3</sup>) timescales in the Marlborough Fault System (MFS) and related Kaik&#333;ura Mountains. Our results indicate two broad stages of landscape evolution that reflect a changing plate boundary through time. In the eastern MFS, Miocene folding above blind thrust faults generated prominent Kaik&#333;ura Mountain peaks and formed major transverse rivers early in the plate collision history. By the Pliocene, rotation of the plate boundary led to a transition to dextral strike-slip faulting and widespread uplift that 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. 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.&#160;Over short timescales (hundreds to thousands of years), apparent catchment-wide average erosion rates derived from <sup>10</sup>Be data show an increase from SW to NE, along strike of the Seaward Kaik&#333;ura Range. These rates mirror spatial increases in elevation, slope, channel steepness, and coseismic landslides, demonstrating that both landscape and geochronology patterns are consistent with an increase in rock uplift rate toward a subduction front that is presently locked on its southern end. Remarkably, the form of the topography, hillslopes, and rivers across much of the MFS appears to faithfully record the complex and changing tectonic history of a long-lived, oblique convergent plate boundary.</p>
Abstract. Here we examine the landscape of New Zealand's Marlborough Fault System, where the Australian and Pacific plates obliquely collide, in order to consider landscape evolution and the controls on fluvial patterns at complicated plate tectonic boundaries. Based on topographic patterns, we divide the study area into two geomorphic domains, the Kaikōura and Inland Marlborough regions. We present maps of drainage anomalies and channel steepness, as well as an analysis of the plan view orientations of rivers and faults, and find abundant evidence of structurally-controlled drainage and a history of capture and rearrangement. Channel steepness is highest in a zone centered on the Kaikōura domain, 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 Kaikōura domain, Miocene folding above blind thrust or reverse 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 Inland Marlborough domain. Despite clear evidence of recent rearrangement of the Inland Marlborough drainage network, rivers in this domain still flow parallel to the older faults, rather than along orthogonal traces of younger, active faults. Continued flow in the established drainage pattern may indicate that younger faults are not yet mature enough to generate the damage and weakening needed to reorient rivers. We conclude that faulting, uplift, river capture and drainage network entrenchment all dictate drainage patterns and that each factor should be considered when assessing tectonic strain from landscapes, particularly at long-lived and complex tectonic boundaries.
