GEOPHYSICAL RESEARCH LETTERS, VOL. 33,, doi:10.1029/2005gl024320, 2006 Nature and characteristics of the flows that carved the Simud and Tiu outflow channels, Mars J. A. P. Rodriguez, 1,2 Kenneth L. Tanaka, 3 Hideaki Miyamoto, 4 and Sho Sasaki 5 Received 5 August 2005; revised 28 September 2005; accepted 12 October 2005; published 14 March 2006. [1] Geomorphic and topographic relations of higher and lower levels of dissection within the Simud and Tiu Valles outflow channels on Mars reveal new insights into their formational histories. We find that the water floods that carved the higher channel floors were primarily sourced from Hydaspis Chaos. The floods apparently branched into distributaries downstream that promoted rapid freezing and sublimation of water and limited discharge into the lowlands. In contrast, we suggest that the lower outflow channels were carved by debris flows from Hydraotes Chaos. Surges within individual debris flows possessed variable volatile contents and led to the deposition of smooth deposits marked by low relief longitudinal ridges. Lower outflow channel discharges resulted in widespread deposition within the Simud/Tiu Valles as well as within the northern plains of Mars. Citation: Rodriguez, J. A. P, K. L. Tanaka, H. Miyamoto, and S. Sasaki (2006), Nature and characteristics of the flows that carved the Simud and Tiu outflow channels, Mars, Geophys. Res. Lett., 33,, doi:10.1029/2005gl024320. 1. Introduction [2] It has been proposed that, mostly during the Late Hesperian [Tanaka et al., 2003], catastrophic emanations of subsurface fluids and sediments excavated the outflow channels [e.g., Baker and Milton, 1974; Carr, 1979], buried large expanses of the lowlands, and may have even formed oceans [e.g., Clifford and Parker, 2001]. Previous workers proposed that these catastrophic flows may have consisted primarily of water floods [e.g., Baker and Milton, 1974], debris flows [e.g., Nummedal and Prior, 1981; Tanaka, 1997, 1999], or a combination of debris flows and water floods [MacKinnon and Tanaka, 1989]. [3] Rotto and Tanaka [1995] observed two main topographic levels of outflow channel floor materials a higher floor level, which formed during the early to intermediate stages of catastrophic flooding, and a lower floor level, which formed during the latest stages of outflow activity (Figure 1). Our investigation examines the distinctive geomorphic and morphometric characteristics of the higher and lower outflow channel floors, which was largely ignored in previous work, as well as their respective distributary properties. Potential contrasts between higher and lower outflow channel activity include (1) the types of fluids and flow properties involved in their dissection, and (2) their roles in the depositional and inundation histories of the lowlands. 2. Geomorphology of the Simud and Tiu Valles Channel Floors [4] The higher outflow channel floors recognized within Tiu Vallis by Rotto and Tanaka [1995] are preserved as systems of truncated channels. Our mapping of their distribution indicates that they originated from Hydaspis Chaos (Figure 1). Channel floors that are proximal to their sources have topographically pronounced erosional morphologies such as scouring, streamlined islands (Figure 2a), and entrenched systems of deep canyons (Figure 2b). With increasing distance from Hydaspis Chaos, floor erosional morphologies are more subdued and the maximum depth of incision is shallower (Figures 1 and 2c 2e). In their most distal reaches, these channels form systems of shallow distributaries (with the exception of a single canyon that sources from a partly infilled impact crater 120 km in diameter (Figure 2f)), and lack distinct floor scouring. [5] The lower outflow channels consist of broad valley floors that appear to be sourced from Hydraotes Chaos (and possibly the chasmata north of this region) (Figure 1). Lower outflow channel floors include streamlined landforms and have distinct erosional scarps where unmodified by chaos development (Figure 3b). These channel floors generally appear to be covered by sedimentary deposits that embay mesas of older materials. The floors include knobs that may consist of large blocks transported by debris flows (Figure 3), and display rounded clasts and imbricated boulders at the Mars Pathfinder landing site, indicative of fluvial and debris-flow processes [Golombek et al., 1997; Tanaka, 1997, 1999]. These floors are also marked by faint longitudinal ridges, whose low relief does not appear to vary significantly with distance from Hydaspis Chaos or with channel width (Figure 3). 1 Department of Earth and Planetary Science, University of Tokyo, Tokyo, Japan. 2 Planetary Science Institute, Tucson, Arizona, USA. 3 Astrogeology Team, U.S. Geological Survey, Flagstaff, Arizona, USA. 4 Department of Geosystem Engineering, University of Tokyo, Tokyo, Japan. 5 National Astronomical Observatory, Mizusawa, Japan. Copyright 2006 by the American Geophysical Union. 0094-8276/06/2005GL024320 3. Higher Outflow Channel Activity: Fluids and Flow Properties [6] The dense scouring that characterizes the higher outflow channels proximal to their sources is consistent with catastrophic flooding [e.g., Baker and Milton, 1974]. However, the channels entrenchment (Figure 4a) into apparently well-indurated plateau materials (as indicated 1of5
RODRIGUEZ ET AL.: SIMUD AND TIU OUTFLOW CHANNELS, MARS Figure 1. MOLA DEM of southern circum-chryse. The lower outflow channel floors form wide and extensive valleys shown in the DEM in different tones of blue. The higher outflow channel floors are outlined in white. The approximate maximum extent of dissection by the surface flow that resulted in the formation of the higher outflow channel floors is shown by the dashed yellow lines. Also shown are different panels given in Figures 2 and 3. The dotted and the dashed white rectangles show the locations of Figures 4a and 4b, respectively. by their commonly steep, modestly degraded margins), suggest that there was a later stage of prolonged noncatastrophic surface flows [Rodriguez et al., 2005]. Noncatastrophic flows are also consistent with the lack of pronounced scouring in distal reaches of these channels and the significant topographic control on the flows as evidenced by complex systems of relatively shallow channel distributaries (Figure 4b). The apparent predominant and sustained non-catastrophic surface flow, and the extensive and voluminous subsurface erosion proposed in the plateaus around Hydaspis Chaos [Rodriguez et al., 2005], suggest that these channels were carved by water and/or other low viscosity fluids that emanated from confined aquifers [Carr, 1979]. [7] The floor elevation of the most deeply incised higher outflow channels is the same as the elevation of the adjacent lower outflow channel floors (Figure 1). This observation suggests that the latest stages of surface flow in the higher outflow channels occurred over predominantly low surface gradients ( 0.07 ), and that most of the vertical development in the topography of the Simud and Tiu Valles was produced by higher outflow channel activity, whereas lower outflow channel Figure 2. (a) Subframe of THEMIS image V03883004. Shown is part of a higher outflow channel floor adjacent to the fluids source region. Notice the dense scouring on the floor of the channel. (b) Subframe of THEMIS image V10324009. Shown is part of a higher outflow channel floor adjacent to its source region. Notice the various topographic levels of canyon incision into the densely scoured floors (see Figure 4 for regional topography). (c) Subframe of THEMIS image V05144009. Shown is part of a higher outflow channel floor about 220 km from its source region. Notice the more moderate degree of floor scouring and the shallower depth of incision into the plateau. It seems that surface flow into the impact crater at the top of the image resulted in its sedimentary infilling. (d) Subframe of THEMIS image V11672009. Shown is part of a higher outflow channel floor about 450 km from its source region. Notice the fainter erosional markings on the channel floors and the much shallower depth of incision into the plateau (see Figure 4 for regional topography). (e) Diagram of image area shown in Figure 2d. Surface flow in this region resulted in two distinct levels of incision (black arrows, long dashed arrow) forming a relatively shallow channel that branches up into various distributaries toward the northwest over just a few kilometers. (f) Subframe of THEMIS image V05219008. Shown is a meander that forms part of a higher outflow channel floor about 550 km from the Hydaspis Chaos. Notice the general lack of floor scouring. The maximum depth of this channel is 350 m (as obtained from MOLA data), which could be related to the surface flow over friable (thus easily erodable) materials that likely form the periphery of the impact crater from where it sources (see Figure 1). 2 of 5
from Hydraotes Chaos. Coleman [2005] shows that basal erosion in the outflow channels was not related to glacier flow. [10] Thus, a new model is needed that can explain both the pervasive sedimentary deposits in the lower outflow channels and the scouring that produced the deposits lowrelief ridges. Figure 3. (a) Subframe of THEMIS image V08527020. Lower outflow channel floor located in a 50 km wide channel that sources directly from the Hydraotes Chaos. Notice low relief longitudinal ridges at lower right (white arrows) and knobs (black arrows). (b) Subframe of THEMIS image V05082024. Sample of a 500-km-wide outflow channel floor about 800 km from Hydraotes Chaos. Notice low-relief longitudinal ridges (white arrows) and knobs (black arrows). activity resulted in predominantly lateral erosion and channel widening. 4. Lower Outflow Channel Activity: Fluids and Flow Properties [8] The lower outflow channels source from Hydraotes Chaos. They have greater widths than higher outflow channels, which could be related to deeper water floods produced by higher turbulence and/or by higher discharge rates [Miyamoto and Sasaki, 1998]. The imbricate boulder trains at the Mars Pathfinder landing site suggest that the flows were dense, favoring thick flows resulting from high viscosity. In this sense, the greater widths of the lower outflow channel are consistent with high-viscosity debris flows [e.g., Nummedal and Prior, 1981; Tanaka, 1997, 1999]. [9] Longitudinal ridges can be produced by basal erosion in catastrophic floods, debris flows and glaciers. On Earth, the relief of those produced by high-velocity flows is typically more accentuated in narrower channels close to the flood source regions [e.g., Baker and Milton, 1974], yet this is not observed in the narrower channels that emanate Figure 4. (a) Oblique view of a hill-shade construction from a MOLA-derived DEM. Shown are higher outflow channels adjacent to Hydaspis Chaos. A-A 0 indicates the elevation profile track shown by the red line. The system of higher outflow channels in these regions dissects the plateau materials to depths of about 1000 m. The top of the plateau is indicated by the thick black line in the elevation profile. (b) Oblique view of a hillshade construction from a MOLAderived DEM. Shown are higher outflow channels distal to Hydaspis Chaos. B-B indicates the elevation profile track shown by the red line. The systems of higher outflow channels in these regions dissect the plateau materials to depths of just 200 m. The top of the plateau is indicated by the thick black line in the elevation profile. The black pointers show other plateau regions where surface flow has eroded pre-existing landforms such as impact craters. The white pointer shows the location of a partly infilled impact crater (see also Figure 2f). 3of5
Figure 5. Conceptual sketch illustrating how (a) a debris flow may break up into (b) multiple surges, which (c) subsequently undergo differentiation into low and high volatile pulses. [11] Terrestrial debris flows sometimes display multiple flow pulses due to flow instability produced by irregularities in the release of materials at the source [e.g., Baloga and Bruno, 2005]. We therefore propose that, within individual debris flows that carved the lower outflow channels, instabilities may have resulted in multiple surges. The subsequent differentiation of the debris flows led to the formation of (a) volatile-depleted pulses, which being more viscous, would have had thicker frontal zones, thereby leading to longer flow distances, and (b) highly turbulent volatile-enriched pulses, which being less viscous, would have had much thinner frontal zones capable of eroding surfaces but not necessarily traveling lengthy distances (Figure 5). [12] Longitudinal ridges may have been formed by roller vortices [Thompson, 1979] within volatile-enriched pulses, and their similar relief throughout the lower outflow channels is consistent with these pulses having similar dimensions and volatile content. Their low relief, when compared to those in the higher outflow channels, could be related to the fact that they are likely made up of sedimentary materials, whereas the latter seem to have been the result of flow erosion into bedrock. [13] Thus, we conclude that multistage debris flows resulted in extensive sedimentary deposits within the lower channel floors of the Simud and Tiu Valles, and perhaps in the northern lowlands. 5. Hydrogeologic Implications of Higher Channel Distributary Properties [14] Much of the work concerning the hydrologic characteristics of the floods that carved the Martian outflow channels has been done by analogy to the geomorphology of terrestrial catastrophic flood channels, in particular that of the Channeled Scablands in Washington State, USA [e.g., Baker and Milton, 1974]. The distributary development of the Channeled Scablands was at least partly controlled by the topography of pre-existing valley systems. For example, the catastrophic floods that carved the Channeled Scablands were blocked by the Horse Heaven Hills in the west and the Blue Mountains to the south as the water raced to the Walula Gap where the Columbia River heads west to the Pacific Ocean. Unable to completely flow out through the narrow gap, the flood waters were pushed back and reversed the flow of the Snake River all the way past Lewiston, Idaho. [15] On Mars, however, the higher outflow channel floor activity produced water discharges over Noachian and Hesperian cratered terrains. In these terrains, impact craters with rims higher than the floods would promote branching of the flow; otherwise inundation and infilling of impact craters would likely occur (e.g., Figure 2c; white pointer in Figure 4b). The formation of transient crater lakes along the course of the flow may have also resulted in significant reduction of flood discharges. [16] The branching lengthens the overall wetting perimeter (which is the length of the trace of the channel floor and walls in cross-section normal to the flow direction). The average flow velocity depends on both the flow depth and the slope angle; for example, a velocity of a steady-state laminar viscous flow is proportional to the square of the depth, whereas a turbulent flow depends on the square root of the depth if it follows the Manning equation. In this sense, if the slope angle does not vary greatly, the flow velocity depends primarily on the depth. If the total wetting perimeter increases, the flow rate per unit width decreases due to the mass balance and ultimately decreases the depth of the flow. [17] Therefore, we propose that the flow depth decreases with progressive branching and increasing distance to the Hydaspis Chaos, and that as a consequence, smaller topographic obstacles to the flow, such as abundant small impact craters, will result in dramatic flow dispersion and thinning, thereby promoting rapid water freezing and sublimation. This proposition is consistent with the higher outflow channels proximal to the Hydaspis Chaos being much deeper and confined than their distal distributaries (Figure 4). Thus, we propose that higher outflow channel activity, unlike lower outflow channel activity, did not result in significant discharges into the lowlands. References Baker, V. R., and D. J. 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