Topographic Characteristics of the Submarine Taiwan Orogen L

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 111, F02009, doi:10.1029/2005JF000314, 2006 Topographic characteristics of the submarine Taiwan orogen L. A. Ramsey,1 N. Hovius,2 D. Lague,3 and C.-S. Liu4 Received 21 March 2005; revised 15 December 2005; accepted 6 January 2006; published 18 May 2006. [1] A complete digital elevation and bathymetry model of Taiwan provides the opportunity to characterize the topography of an emerging mountain belt. The orogen appears to form a continuous wedge of constant slope extending from the subaerial peaks to the submarine basin. We compare submarine channel systems from the east coast of Taiwan with their subaerial counterparts and document a number of fundamental similarities between the two environments. The submarine channel systems form a dendritic network with distinct hillslopes and channels. There is minimal sediment input from the subaerial landscape and sea level changes are insignificant, suggesting that the submarine topography is sculpted by offshore processes alone. We implement a range of geomorphic criteria, widely applied to subaerial digital elevation models, and explore the erosional processes responsible for sculpting the submarine and subaerial environments. The headwaters of the submarine channels have steep, straight slopes and a low slope-area scaling exponent, reminiscent of subaerial headwaters that are dominated by bedrock landslides. The main trunk streams offshore have concave-up longitudinal profiles, extensive knickpoints, and a slope-area scaling exponent similar in form to the onshore fluvial domain. We compare the driving mechanisms of the likely offshore erosional processes, primarily debris flows and turbidity currents, with subaerial fluvial incision. The results have important implications for reading the geomorphic signals of the submarine and subaerial landscapes, for understanding the links between the onshore and offshore environments, and, more widely, for focusing the future research of the submarine slope. Citation: Ramsey, L. A., N. Hovius, D. Lague, and C.-S. Liu (2006), Topographic characteristics of the submarine Taiwan orogen, J. Geophys. Res., 111, F02009, doi:10.1029/2005JF000314. 1. Introduction [3] The aim of this paper is to apply existing protocols of topographic analysis to both submerged and subaerial parts [2] Many mountain belts start below sea level and remain of an active mountain belt in order to identify the key partly submerged throughout their existence. Topography topographic attributes of both and to deduce from them sculpted by submarine processes is subsequently uplifted constraints on formative erosional processes. We use Tai- above sea level and forms a template on which subaerial wan as our example. Taiwan is a young collisional orogen relief develops. In turn, the products of subaerial erosion with similar total relief above and below sea level and a are transported into the submarine landscape and may good coverage of topographic and bathymetric data. We drive its topographic evolution by erosion and/or deposi- also have good constraints on onshore erosion rates and tion. The subaerial and submarine landscapes are intrinsi- fluvial sediment supply to the submarine slope [Dadson et cally linked, and it is important to match advanced al., 2003, 2004]. We begin with a brief discussion of the knowledge of subaerial erosion and landscape evolution submarine slope in general and the important erosional with equivalent knowledge of submarine topography and features. erosion. Given the limitations on access, initial insights [4] Many active continental margins have partially sub- can be gained by applying terrain analysis techniques merged mountain belts. Common characteristics of such common in subaerial geomorphology to bathymetric data margins include a narrow shelf, a relatively steep continen- sets. tal slope with an overall gradient of 3°–4°, punctuated by structural complications, and a continental rise dominated 1Bullard Laboratories, Department of Earth Sciences, University of by mass flow deposition [O’Grady et al., 2000; Pratson and Cambridge, Cambridge, UK. Haxby, 1996]. The overall shape of the continental slope is 2Department of Earth Sciences, University of Cambridge, Cambridge, controlled by the nature and intensity of slope erosion and UK. 3 transport processes, and the maximum angle of repose of UMR 6118, Ge´osciences Rennes, CNRS, Rennes, France. the substrate. Some continental slopes are straight; others 4Oceanography, National Taiwan University, Taipei, Taiwan. grade exponentially toward the rise and/or recline toward Copyright 2006 by the American Geophysical Union. the shelf edge, giving rise to a sigmoidal cross-sectional 0148-0227/06/2005JF000314$09.00 shape [Schlager and Adams, 2001]. Superposed on this F02009 1of21 F02009 RAMSEY ET AL.: SUBMARINE TAIWAN OROGEN F02009 general shape is a range of erosional and aggradational a dendritic pattern directly comparable to subaerial drainage relief, including submarine rill and gully systems, canyons, networks. They are thought to grow by retrogressive slope and landslide scars. failure but are relatively short (0.5–5 km) and narrow [5] The relief offshore is created by a hierarchy of (<250 m wide) and have a depth of up to 40 m [Buffington erosional and depositional processes, as it is on land. In and Moore, 1963]. Rills are sets of low-relief, subparallel, both environments, the top of this hierarchy is likely to linear gullies cut purely by sediment flows [Pratson et al., consist of a small set of processes lowering the channel 1994]. They can be up to 300 m wide and 40 m deep, form thalweg, and the adjacent valley sides. In active, subaerial on low-gradient slopes and are possibly a precursor to mountain belts, rivers and debris flows cut uplifting bedrock canyon formation [Pratson and Coakley, 1996]. [e.g., Hartshorn et al., 2002; Stock and Dietrich, 2003] and [8] Submarine erosional features have been described in landslides limit the relief of interfluves [e.g., Burbank et al., direct analogy with subaerial landforms as far back as 1976 1996; Schmidt and Montgomery, 1995]. The result is a [Chough and Hesse, 1976], implying close parallels in their ridge-and-valley landscape with straight hillslopes, and a formation and evolution despite the important differences well-connected, dendritic and concave-up channel network between the densities and viscosities of air and water capable of evacuating erosion products over the long term. [Orange et al., 1994; Peakall et al., 2000]. A logical next Offshore, landslides, debris flows, and turbidity currents are step in submarine geomorphology is to gain a full quanti- three of the most important erosional processes in sculpting tative understanding of the similarities and differences the landscape. They occur at a range of spatial scales, from between submarine and subaerial topography. initiating small rill and gully systems to obliterating sub- [9] In the following sections, we introduce the methods marine canyons (e.g., the Albermale slide [Driscoll et al., used for characterization of subaerial and submarine topog- 2000]), and can be triggered by progressive sediment raphy. We go on to show that, although the onshore and accumulation and undercutting, and events such as earth- offshore landscapes of Taiwan are superficially very dif- quakes, storms, gas disassociation or sea level changes. ferent, their quantitative topographic characteristics are [6] Offshore erosional processes have been documented similar. This raises the compelling question as to whether and modeled in some considerable detail, with emphasis similarities in topographic form can originate from simi- on the origin and evolution of submarine canyons [e.g., larities in formative erosional process, despite obvious Fulthorpe et al., 2000; Hampton et al., 1996; Mohrig et differences in boundary conditions. Given that the current al., 1998; Parker et al., 1986; Pratson and Coakley, 1996; metrics for distinguishing landscapes are diagnostic, we Spinelli and Field, 2001]. Submarine canyons sculpt attempt an exploratory interpretation of our results, fully hundreds to thousands of meters of offshore relief and aware that the subject of submarine geomorphology is in can be several tens of kilometers wide. They vary in form its infancy. from straight, steep-sided, V-shaped channels to gentle, meandering U-shaped valleys and generally have concave- up long profiles. Initially, submarine canyons were thought 2. Topographic Analysis to originate at sea level lowstands where rivers were able [10] The aim of terrain analysis is to identify the key to deliver sediment well below the shelf break [Daly, attributes of topography and spatial organization of the 1936], and turbidity currents were thought to drive their landscape, and to derive constraints on the mechanics of formation [Heezen and Ewing, 1952; Kuenen, 1937]. the principal erosional processes. Mountain topography can However, the abundance of submarine canyons with steep be considered on three spatial scales: the mountain belt head scarps well below the shelf break [Twichell and scale, the catchment scale and the subcatchment scale. Roberts, 1982] indicates that canyon initiation by spring These length scales determine the type of topographic sapping [e.g., Dunne, 1980; Orange et al., 1994] and analysis undertaken and the inferences made. propagation by retrogressive slope failure may also occur [11] At the mountain belt scale, topographic analysis [Farre et al., 1983]. Orange et al. [1994] highlighted

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