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Temporal and spatial variations of tectonic rock uplift are generally thought to be the main controls on long-term erosion rates in various landscapes. However, rivers continuously lengthen and capture drainages in strike-slip fault systems due to ongoing motion across the fault, which can induce changes in landscape forms, drainage networks, and local erosion rates. Located along the restraining bend of the San Andreas Fault, the San Bernardino Mountains provide a suitable location for assessing the influence of topographic disequilibrium from perturbations by tectonic forcing and channel reorganization on measured erosion rates. In this study, we measured 17 new basin-averaged erosion rates using cosmogenic 10Be in river sands (hereafter, 10Be-derived erosion rates) and compiled 31 10Be-derived erosion rates from previous work. We quantify the degree of topographic disequilibrium using topographic analysis by examining hillslope and channel decoupling, the areal extent of pre-uplift surface, and drainage divide asymmetry across various landscapes. Similar to previous work, we find that erosion rates generally increase from north to south across the San Bernardino Mountains, reflecting a southward increase in tectonic activity. However, a comparison between 10Be-derived erosion rates and various topographic metrics in the southern San Bernardino Mountains suggests that the presence of transient landscape features such as relict topography and drainage-divide migration may explain local variations in 10Be-derived erosion rates. Our work shows that coupled analysis of erosion rates and topographic metrics provides tools for assessing the influence of tectonic uplift and channel reorganization on landscape evolution and 10Be-derived erosion rates in an evolving strike-slip restraining bend.
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River patterns reveal two stages of landscape evolution at an oblique convergent margin, Marlborough Fault System, New Zealand
Abstract. Here we examine the landscape of New Zealand'sMarlborough Fault System (MFS), where the Australian and Pacific plates obliquelycollide, in order to study landscape evolution and the controls on fluvialpatterns at a long-lived plate boundary. We present maps of drainageanomalies and channel steepness, as well as an analysis of the plan-vieworientations of rivers and faults, and we find abundant evidence ofstructurally controlled drainage that we relate to a history of drainagecapture and rearrangement in response to mountain-building and strike-slipfaulting. Despite clear evidence of recent rearrangement of the western MFSdrainage 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 newweakening and tectonic strain. Continued flow along older faults may alsoindicate that the younger faults have not yet generated a fault damage zonewith the material weakening needed to focus erosion and reorient rivers.Channel steepness is highest in the eastern MFS, in a zone centered on theKaikōura ranges, including within the low-elevation valleys of main stemrivers and at tributaries near the coast. This pattern is consistent with anincrease in rock uplift rate toward a subduction front that is locked on itssouthern end. Based on these results and a wealth of previous geologicstudies, we propose two broad stages of landscape evolution over the last 25 million years of orogenesis. In the eastern MFS, Miocene folding above blindthrust faults generated prominent mountain peaks and formed major transverserivers early in the plate collision history. A transition to Pliocenedextral strike-slip faulting and widespread uplift led to cycles of riverchannel offset, deflection and capture of tributaries draining across activefaults, and headward erosion and captures by major transverse rivers withinthe western MFS. We predict a similar landscape will evolve south of theHope Fault, as the locus of plate boundary deformation migrates southwardinto this region with time.
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- Award ID(s):
- 1831623
- PAR ID:
- 10171089
- Date Published:
- Journal Name:
- Earth Surface Dynamics
- Volume:
- 8
- Issue:
- 1
- ISSN:
- 2196-632X
- Page Range / eLocation ID:
- 177 to 194
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.5194/esurf-8-177-2020
