Abstract
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.
Highlights
Tectonics and deformation impart a lasting impression on landscapes and often act as primary drivers of surface processes (e.g., Wobus et al, 2006; Whittaker et al, 2008; Cowie et al, 2008, Kirby and Whipple, 2012; Whipple et al, 2013)
Though most of the rivers within the western Marlborough Fault System (MFS) do not presently flow along the active faults, we suggest that strikeslip fault motion still plays a fundamental role in ongoing drainage network evolution
Seaward translation and overthrusting of crust atop the downgoing subducted slab (Little and Roberts, 1997; Walcott, 1998) are supported by geophysical data that show a zone of crustal thickening in the overlying plate (EberhartPhillips and Bannister, 2010). This pattern of widespread uplift is supported by the ksn map, which shows a hotspot of steep rivers across the Kaikoura Range highlands and lowlands (Fig. 5a). During this second stage of landscape evolution, we propose that the combination of strike-slip fault motion along major faults, regional uplift, and rapid incision of large rivers maintains relief in the Kaikoura ranges and explains the triangular facets bounding the MFS faults in this region, as well as lateral offsets of rivers flowing over strike-slip faults (Fig. 6d)
Summary
Tectonics and deformation impart a lasting impression on landscapes and often act as primary drivers of surface processes (e.g., Wobus et al, 2006; Whittaker et al, 2008; Cowie et al, 2008, Kirby and Whipple, 2012; Whipple et al, 2013). Planform patterns of drainage networks, from offset channels (Wallace, 1968) to rivers flowing around or through folds (Keller et al, 1998), can yield valuable insights into the tectonics underlying the landscape response. Drainage network morphometry may yield additional evidence of reorganization of catchments by ridge migration (Pelletier, 2004; Willet et al, 2014) or by wholesale river capture (Craw et al, 2003, 2013; Clark et al, 2004) and flow reversal (Benowitz et al, 2019), often in response to tectonics. Drainage anomalies or unusual patterns in river planform can serve as recorders of these past river captures and drainage reorganizations (e.g., Brookfield, 1998; Hallet and Molnar, 2001; Burrato et al, 2003; Clark et al, 2004; Delcaillau et al, 2006; Willett et al, 2014), though not all anomalies necessarily link to river captures, even in active tectonic settings (Bishop, 1995)
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