TECTONICS, VOL. 26, TC6010, doi:10.1029/2006TC001961, 2007 Click Here for Full Article Geomorphic expression of midcrustal extension in convergent orogens

Frank J. Pazzaglia,1 Jane Selverstone,2 Mousumi Roy,2 Kurt Steffen,2 Sarah Newland-Pearce,1 Will Knipscher,1 and Justin Pearce1 Received 28 February 2006; revised 17 July 2007; accepted 30 August 2007; published 19 December 2007. [1] Channel orientation, gradient, drainage basin spacing, 1. Introduction drainage patterns, and alluvial fan sedimentology define a set of geomorphic observables for fingerprinting modes of [2] The juxtaposition of crustal shortening and crustal extensional tectonism in convergent settings. Anomalous, extension in both time and space [e.g., Ruppel, 1995] is emerging as a characteristic of many active, convergent finely textured trellis drainage patterns in the Alps are orogens. Although long recognized as a local consequence spatially coincident with the low-angle Simplon and of crustal thickening and stored gravitational potential Brenner detachments. The detachment footwalls are energy in an overall convergent setting [e.g., Sander, characterized by numerous high-angle brittle faults, 1930; Bearth, 1956], extension is now recognized as which are etched out by low-order stream gullies and occurring on a regional scale and, perhaps more impor- channels. We quantify the relationship between these tantly, in a way that fundamentally shapes convergent faults and watershed-scale geomorphology using high- orogens. Structural and tectonic studies have documented resolution digital topography to extract channel and basin orogen-scale geologic and topographic fingerprints of metrics. Topographic data are complemented by a field extension in a wide range of tectonic settings [e.g., Wernicke data set of channel gradients and the integrated erosional et al., 1988; Mancktelow and Pavlis, 1994; Ruppel, 1995; response of the watersheds preserved in valley bottom Blackman et al., 1998]. For example, normal-fault-bound footwalls and sediment filled basins, many of which are alluvial fans. In footwall regions adjacent to the internally drained, in the Basin and Range province of the detachments, channels are steeper, more closely spaced, United States or throughout western and southern Italy have statistically coherent orientations, and feed coarse, clearly and unquestionably identify regional crustal stretch- steep alluvial fans. Drainage spacing increases in the ing. What is not so clear, however, in a casual study of footwall away from the detachment, and the trellis pattern topography is where major crustal stretching and extensional diminishes. In the detachment hanging walls, which exhumation of midcrustal rocks occurs in the hinterland of an typically lack high-angle brittle faults, there is a poor overall convergent orogen. correlation between channel steepness, density, or [3] Our goal in this paper is to demonstrate that extension orientation, and hanging wall alluvial fans tend to be accommodated by subvertical simple shear in the core of a relatively larger, gentler, and debris-flow dominated. convergent orogen leaves a characteristic fingerprint on the Comparisons are made to the Ticino dome region of the landscape. To do this we explore how the fracture/fault density acquired during extension and exhumation of Alps as well as to the Pioneer core complex of central midcrustal footwall rocks controls drainage patterns, drain- Idaho. In both cases, our analysis suggests high-angle age density, and surface processes of detachment fault brittle faulting of the footwall is particularly well systems in the Alps and western United States. We present preserved in the region adjacent to a detachment. These a suite of geomorphic characteristics that define the topo- results are consistent with a subvertical simple shear graphic fingerprint of subvertical simple shear and suggest model for footwall extrusion as well as annealing or a lack that broad application of our conceptual model can help of reactivation of brittle structures in the footwall distal to determine the relative importance of this mode of extension the rolling hinge. Citation: Pazzaglia, F. J., J. Selverstone, within orogenic systems. We recognize that other types of M. Roy, K. Steffen, S. Newland-Pearce, W. Knipscher, and J. Pearce deformation, perhaps those associated with large emergent (2007), Geomorphic expression of midcrustal extension in convergent thrust faults also impart subvertical simple shear that could orogens, Tectonics, 26, TC6010, doi:10.1029/2006TC001961. lead to a similar or dissimilar topographic fingerprint. For this study, we restrict ourselves to the fingerprint of exten- sional detachments because previous research has clearly established a correlation between subvertical simple shear and the deformation mechanism [Wawrzyniec et al., 2001]. 1Department of Earth and Environmental Science, Lehigh University, Bethlehem, Pennsylvania, USA. Further research beyond the scope of our study will be 2Department of Earth and Planetary Sciences, University of New necessary to evaluate the uniqueness of our results. Mexico, Albuquerque, New Mexico, USA. [4] Our approach is predicated on the well-established observation that fluvial and glacial erosion are influenced Copyright 2007 by the American Geophysical Union. 0278-7407/07/2006TC001961$12.00

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Figure 1. (a) Subvertical simple shear versus (b) flexural failure for a rolling hinge detachment in the Alps. Note the dense array of detachment-parallel faults in the footwall of the simple shear model.

by rock type and structure. Landscapes with mature, inte- patterns, channel metrics, and erosional characteristics are grated drainages develop drainage patterns subsequent to compared to similar and dissimilar tectonic settings in the rock type and structure that ultimately influence the size, Rocky and Appalachian mountains, respectively (Figure 2c). gradient, and orientation of both low- and high-order An unexpected outgrowth of the research is the recognition channels [Bloom, 1991]. The recent exposure of a midcrus- that, in many cases, extension may be accommodated by tal detachment at the surface introduces an anomalous and more numerous, difficult-to-identify detachments than pre- transient feature in the alpine landscape, namely a finely viously envisioned. The results of this study provide a textured trellis drainage pattern that qualitatively decays starting point, using geomorphology and topographic away from the detachment for both geomorphic and struc- analysis, for geologic studies where the role of extension tural reasons. Our goals and approach share some similarity in convergent orogens is unknown. to a recognized relationship between drainage patterns and the footwalls of metamorphic core complexes in the south- western United States [Spencer, 2000]; however we focus 2. Extension in Convergent Orogens specifically on the detachment and in particular, aim to [6] Syn- and post-orogenic extension has been recog- define the topographic signature of subvertical simple shear nized in many convergent orogens, including the Himalaya as the dominant process involved in the exhumation of [Burchfiel et al., 1992; Molnar et al., 1993], the Aegean footwall rocks (Figure 1). Cyclades [Lister et al., 1984], the Hellenic subduction [5] We first focus on two regions of the Swiss, Italian, wedge [Jolivet et al., 1996], the Central Range of Taiwan and Austrian Alps (Figures 2a and 2b) as a starting point for [Crespi et al., 1996], the European Alps [Mancktelow, 1985, contrasting field and topographic data with a large data set 1992; Behrmann, 1988; Selverstone, 1988; Ratschbacher et of existing structural measurements [Axen et al.,1995; al., 1989; Reddy et al., 1999; Sue et al., 1999; Frisch et al., Selverstone et al., 1995; Wawrzyniec et al., 2001]. The 2000; Wheeler et al., 2001], and the Apennines [Reutter, Alps offer the advantages of having well-studied extension- 1981; Malinverno and Ryan, 1986; Carmignani and al faults that were active during crustal shortening and Kligfield, 1990, Carmignani et al., 1994]. No single orogen unroofing [Behrmann, 1988; Mancktelow,1985; geodynamic model can account for the variety of tec- Selverstone, 1988; Steck and Hunziker, 1994; Axen et al., tonic styles associated with extension in convergent 1995], glacially scoured, well-exposed valley walls, and orogens [Waschbusch and Beaumont, 1996; Willett, high relief in low-order channels that have etched out 1999; Selverstone, 2005], but maintaining critical taper drainages in the topography. Regions of low-angle normal orogenic wedges [Platt, 1986] and coupling of crustal faulting and extension in the Alps are characterized by a detachments to stretching of the mantle lithosphere [Jolivet trellis drainage pattern, a geomorphic feature that we argue et al., 1998] play a role in many cases. below is anomalous for these settings. Alpine drainage

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Figure 2. (a) Location of the study areas in the Swiss, Italian, and Austrian Alps shown on a perspective digital shaded relief model constructed from the GTOPO30 topographic model. Abbreviations are as follows: Bn, Bern; Z, Zurich; To, Torino; M, Milano; Bo, Bolzano; I, Innsbruck; S, Simplon; O, Oberalp; T, Ticino; and B, Brenner. (b) Location of the Simplon and Brenner detachments (hatched white lines and tics in Figure 2a) in the regional, simplified geology of the Alps (modified from Axen et al. [1998]). (c) General locations of the Idaho (I) and Appalachian (A) study areas (shaded rectangles) in the United States. See Figures 9 and 12 for specific study area location maps.

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[7] The Himalaya and the Alps represent young examples dynamics of crustal extension, particularly the exhumation of of syn-convergent extension in the core of the orogen midcrustal rocks, orthogonal to the direction of shortening. orthogonal to the direction of shortening. This extension is typically explained by out-of-plane motions associated with escape tectonics or oblique subduction. The scale of 3. Study Regions escape tectonics in the Himalaya is large, dominated by 3.1. Simplon and Brenner Detachments active strike-slip faults, and has been recognized in some of [9] Two major detachments that accommodated the earliest studies of the orogen [Molnar and Tapponnier, shortening-orthogonal extension in the Alps are the Simplon 1978]. Extrusion of the crust orthogonal to the direction of and Brenner Line low-angle normal faults. Both of these shortening and rotation of crustal blocks around syntaxes is detachments are characterized by 20 to 70 km of relative an important component of this type of extension [Zeitler et westward retreat of the hanging walls in the Neogene, with al., 2001]. Similarly, extension in the Alps involves extru- some speculation that a limited degree of movement may be sion along orogen-scale strike-slip faults [Hubbard and continuing today [Mancktelow, 1985, 1990, 1992; Selverstone, Mancktelow, 1992; Ratschbacher et al., 1991; Peresson 1988; Behrmann, 1988; Steck and Hunziker, 1994; Axen et al., and Decker, 1997; Bistacchi and Massironi, 2000; Eder 1995; Maurer et al., 1997; Wawrzyniec et al., 2001]. Average and Neubauer, 2000; Mancktelow et al., 2001], as well as rates of extension are estimated to have been on the order of development of orogen-perpendicular low-angle normal 2 km/Ma over 15 Ma [Grasemann and Mancktelow, 1993; faults [Steck, 1984; Mancktelow, 1985; Behrmann, 1988; Selverstone et al., 1995; Wawrzyniec et al., 2001]. Selverstone, 1988] and extensional reactivation of orogen- [10] A key structural feature of both Alpine detachment parallel thrust faults [Sue et al., 1999]. A range of river zones is that the hanging walls are generally unextended drainage patterns are expected to develop in these settings, and cut by few, widely spaced high-angle faults, whereas including elongate longitudinal systems following strike- the footwalls are highly extended, broadly folded, and cut slip faults as well as trellis patterns coincident with perva- by small, but pervasive high-angle normal faults, reflecting sively fractured regions around detachment faults footwall uplift dominated by a subvertical simple shear [Schlunegger et al., 1998; Spencer, 2000]. mechanism [Axen et al., 1995, 2001; Selverstone et al., [8] Field data on fault length, orientation, spacing, and 1995; Wawrzyniec et al., 2001]. These basic structural offset, together with estimates of extension rates, can be differences between the hanging wall and footwall have a used to constrain simple geodynamic models of footwall first-order geomorphic expression in the drainage pattern deformation. Insofar as geomorphic studies can document (Figure 3a) that might be considered anomalous because of the relationship between topographic metrics and the fault its isolated nature and no obvious correlation to rock type data, the general geodynamic behavior of crustal extension and location in a region of high relief. may also be inferred through geomorphology. For example, [11] The Simplon Line, located in south-central Switzerland extrusion of the footwall from beneath the hanging wall (Figure 2), juxtaposes Upper Pennine nappes in the hanging could be accomplished either by a rolling hinge undergoing wall against Lower Pennine nappes in the footwall [e.g., subvertical simple shear involving pervasive brittle faulting Mancktelow, 1990]. The footwall comprises basement-cored of the footwall (Figure 1a) or a rolling hinge detachment nappes with their metamorphic cover sequences [Steck, 1984; undergoing flexural failure involving little to no brittle Mancktelow, 1990], all of which were metamorphosed to faulting of the footwall (Figure 1b) [Axen and Bartley, middle amphibolite-facies conditions [e.g., Todd and Engi, 1997; Axen et al., 1998]. The latter mechanism has been 1997; Baxter and DePaolo, 2000] prior to extensional cited in several core complexes in the western U.S. Basin unroofing. The topography of the Simplon region (Figures 3b and Range [Axen and Bartley, 1997, and references therein] and 3c) contrasts with that of other parts of the Alps (Figure 3d) and the apparent lack of structural control on low-order in that there are numerous corrugated finely textured drain- drainages in the core complexes of southern Arizona sup- ages with orientations coincident with mapped faults. ports the flexural failure model [Spencer, 2000]. In contrast, [12] Bearth [1972, 1973] mapped numerous NNW-striking the Alps preserve good examples of footwalls that accom- high-angle faults in the southern portion of the Simplon modated extension via a subvertical simple shear mecha- footwall, and pointed out that many of them are associated nism [e.g., Axen et al., 1995; Wawrzyniec et al., 2001]. with deep gullies (Figure 4). Wawrzyniec [1999] and Pervasive brittle faulting in the footwalls of some Alpine Wawrzyniec et al. [1999, 2001] documented both west- detachments as well as the obvious corrugated hillslopes down and east-down gully-forming faults in the footwall and drainage patterns on large-scale topographic maps (Figure 4d). These structures are subparallel to the strike of (Figure 3) makes the Alps a good place to test the relation- the main Simplon detachment, and cut the mylonitic fabric ship between mesoscale faults and their topographic expres- associated with the detachment. Fluid inclusion analysis and sion, especially in low-order drainages. Fault spacing and rare crosscutting relationships indicate that the west-down the depth-temperature conditions at which the faults formed faults are older and formed at greater depths (15 25 km) are thought to be highly correlated with variables such as À than the east-down faults (3 7 km). Faults within the strain rate (rate of extension) and fluid pressure [e.g., Axen À hanging wall are rare in comparison to the footwall. Our et al., 2001]. If this is true, relatively straightforward own observations indicate many of the prominent gullies measurements linking fault populations to topographic (‘‘Schluchten,’’ Figure 4b) in the southern portion of the metrics could provide fundamental insights into the geo- footwall are localized within large west-down faults

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Figure 3

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(Figure 4a), particularly those with >1-m-wide central [16] We have also chosen comparison regions outside of zones of cataclasite. the Alps to evaluate the general applicability of our results [13] The Brenner Line is located 300 km east of the to other orogens (Figure 2c). The Pioneer Mountains core Simplon Line on the Austrian-Italian border south of complex of central Idaho [Dover, 1983; Wust, 1986; O’Neill Innsbruck (Figure 1). The detachment is represented by a and Pavlis, 1988; Silverberg, 1990] provides a geomorphic major mylonite zone that exposes deep-seated schists and comparison for a region where extension occurred in the gneisses of European and Neotethyan affinity beneath direction of shortening during the Sevier orogeny. In the lower grade rocks of the structurally higher Adriatic plate study area, the detachment has a general northwest-southeast [Behrmann, 1988; Selverstone, 1988]. orientation and dips to the northeast. The footwall of the [14] De Vecchi and Baggio [1982] and Baggio et al. Pioneer core complex is composed of Precambrian gneiss [1982] noted the existence of abundant high-angle, dip-slip and Tertiary quartz monzonite intrusives. The hanging wall faults within a region that corresponds to the Brenner is composed of the Mississippian Copper Basin Formation, footwall. These faults trend NNE and are subparallel to a mostly fine-grained quartzite and argillite. The footwall the trace of the Brenner Line. Axen et al. [1995] measured has detachment-parallel gullies reminiscent of the features the orientations and kinematics of several hundred of these noted in the Simplon footwall. Like the Alps study area, the faults, and noted that they can be subdivided into west- Pioneer core complex was extensively glaciated in the down and east-down categories. As in the Simplon region, Pleistocene and valley slopes expose fresh bedrock. the west-down faults formed earlier and deeper than the [17] Lastly, we investigate drainage patterns in a control east-down structures [Selverstone et al., 1995]. Prominent setting where there is no documented extension. The grass-filled gullies parallel to these faults are common Appalachian foreland of Pennsylvania (Figure 2c) exposes proximal to the Brenner Line (Figure 4c); in a few cases, folded and faulted Paleozoic strata of variable resistance we were able to establish that these gullies are associated where long, linear strike valleys have formed following the with cataclasites occupying west-down faults. outcrop of soft rock types, separated by linear ridges underlain by resistant rock types. The large-scale drainage 3.2. Comparison Regions pattern in the Ridge and Valley is trellis so channel orientations here give us some sense as to how well our [15] We have chosen two additional areas in the Alps for geomorphic comparison with the Simplon and Brenner method characterizes and quantifies this pattern in a setting study regions. The first of these areas, the Oberalp region, that has not been affected by extensional tectonics. lies within granitic basement of the heavily glaciated Aar massif north of the Rhone Valley fault zone (Figures 2a, 2b, 4. Methods and 3d). This area had no known extensional history during the Alpine orogeny and is today dominated by strike-slip [18] Our analysis is founded on the observation that faulting [e.g., Steck, 1984; Maurer et al., 1997]. This area mesoscale, high-angle brittle faults are associated with thus serves as a control on our interpretation of extension- prominent gullies in the detachment footwalls [Wawrzyniec, related geomorphic features from the Simplon and Brenner 1999; Wawrzyniec et al., 1999, 2001]. We assume that the regions. The second area encompasses the Val Leventina, gullies are the result of faults and proceed to try and which marks the western margin of the Ticino subdome quantify the differences in gully density, orientation, and within the Lepontine Alps (Figures 2a and 2b). Although erosion as a means to fingerprint the detachment footwall. few published data exist for this region, Wagner et al. [19] Rock type, climate, and distribution of glacial debris [1977], Merle et al. [1989] and Ring [1992] have docu- are all similar for the Simplon and Brenner field sites. Base mented top-west extensional motion leading to unroofing of level for these field sites, and base level for all of the sites the Ticino subdome prior to extension in the neighboring within and outside of the Alps is obviously different, but not Simplon region. Both the footwall and hanging wall of the primary to our investigation. We seek any erosive process, dome are localized within the same basement/cover nappe be it glacial scour, bedrock river scour, base-level driven sequence that comprises the footwall of the Simplon detach- fluvial incision, or debris flows that have the ability to etch ment. Given the general similarity between the Ticino and out the mesoscale brittle faults from the background topog- Simplon regions, we might expect to find similar geomorphic raphy. Obviously the more erosion, scour, and base level features developed in both. fall, the better the faults will stand out as gullies on topographic maps or DEMs. Since all sites have experi-

Figure 3. (a) Satellite image of the Brenner detachment (white line with ticks) with the hanging wall (left) and footwall (right) illustrating the dramatic difference in drainage texture and pattern on either side of the detachment. The trellis drainage pattern in the footwall is an important target of investigation to compare the alignment of low-order drainages to measured mesoscale fault populations. (b) Part of the Simplon 1:25,000 topographic map illustrating the pervasive hillslope corrugations and lineations linked to mesoscale brittle faulting in the Simplon lower plate. (c) Photograph of the Chesselchumma Ridge area showing the corrugated hillslope. Photo location is in the rectangle of Figure 3b. (d) Shaded DEM of the Oberalp region of the Alps where no extensional tectonics are suspected and which is used as a comparison topography to the Simplon and Ticino study areas (see Figure 2a).

6of21 TC6010 PAZZAGLIA ET AL.: GEOMORPHIC EXPRESSION OF EXTENSION TC6010 enced both extensive glaciation and local base level fall [20] Our field and DEM/map-based topographic data driven by overdeepening of trunk valleys by alpine glaciers, analysis focused on channels and hillslopes proximal to we assume that the degree of scour and etching, although the hanging wall and footwalls of detachment faults. Field not exactly equal, is severe enough at all sites such that our investigations visually established the relationship between results are not biased toward those sites where the faults mesoscale brittle faults and topography and the sedimentol- remain hidden in the hillslopes. ogy of deposits derived from watersheds where faults have been mapped. DEM/map-based analyses focused on estab- lishing channel orientation and gradient metrics for regions known to be affected by a major detachment as well as control areas. Both the field and lab investigations con- trolled for spatial scale through ordering of channels in the respective drainage networks [Strahler, 1957]. Channel ordering was the most objective way to compare watersheds of similar discharge across variable rock type, relief, and climatic setting. Ordering enjoys an advantage in this particular study over direct comparisons of watersheds of comparable areas because we have no independent data to know if discharge, a key controlling metric of channel form and process, is linearly dependent upon area. [21] Our initial field investigations included surveys of hillslope and channel gradients and measurements of rock mass strength [Selby, 1980] in the anticipation that the frequency of brittle faults would impart a dominant control on rock resistance, erosion rate, and subsequent mean hill- slope and channel declivity. What we found was that the deep, steep valleys, although helpful in their exposure of bedrock, made access impractical to obtain truly represen- tative in situ rock mass strength data. We do not present any of these rock mass strength data below. Rather, we turned the attention of the field studies to a more watershed-scale investigation including the alluvial fans developed at the mouths of third-order drainage basins (Figures 5 and 6). In the footwall of detachment faults, these fans lie at the confluence of drainages developed parallel or subparallel to the detachment and the master drainages that flow in broad corrugations orthogonal to the detachment. In the hanging wall, these fans lie at the mouth of third-order drainages where they spill out onto the valley floors of larger-order, relatively flat-bottomed river valleys. Our

Figure 4. Photograph of (a) fractures in a west-down mesoscale fault of the Simplon detachment, lens cap for scale is 4 cm across, (b) a gully developed in a fracture zone of the Simplon detachment, and (c) a prominent set of more widely spaced, third-order drainages and their associated alluvial fans, also coincident with a west-down mesoscale fault in the Brenner footwall. The faceted spurs at the mouths of these drainages are presumably the result of valley-side erosion by the trunk glacier formerly occupying the valley in the foreground. The transition from Figure 4a to 4b to 4c mirrors the topographic evolution of fractures, to gullies, to integrated watersheds moving away from the exposed detachment into the footwall of both the Brenner and Simplon lines. (d) Strike and rake of the population of mesoscale brittle normal faults measured in the gullies of the southern portion of the Simplon detachment. Contours are of density per 1% area of the graph and the contour interval is 2% (modified from Wawrzyniec et al. [2001]).

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Figure 5. (a) Location of third-order watersheds and associated alluvial fans astride the Simplon detachment (thick, hatched, white line and ticks). Arrows point to prominent gullies in the footwall near the detachment. (b) Topography in the vicinity of the Brenner detachment. Arrows point to pervasive trellis tributaries in the detachment footwall. Brenner fan and drainage basin data come from the Val di Vizze (footwall) and large valleys west of Vipiteno (hanging wall). Basins studied are as follows: V, Vallettina; S, Sifinol; U, Uberwasser; and M, Masseria. approach was predicated on the well-established knowledge their role in controlling erosion and the liberation of detritus that the sedimentology and stratigraphy of alluvial fans by glacial, debris-flow, and fluvial processes. We chose fans reflects a watershed-integrated response to climate, tecton- and drainage basins with similar aspect, elevation, and ics, and rock type [Bull, 1991; Blair and McPherson, 1994]. relief. We measured grain size, channel slopes and hydraulic Given that the fans investigated are all Holocene (postgla- geometry, and made general maps of fan deposits keyed into cial) and in most cases late Holocene (postÀLittle Ice Age), relative age criteria based on stratigraphy and fan surface long-term tectonic influences are assumed to be subordinate soil stratigraphy and morphologic development [Birkeland, to local controls of rock type and prevailing climate and 1999].

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[23] Channel orientation, length, gradient, and density are extracted from the DEMs. The GIS computational method- ologies and associated programs to parse data are made publicly available from http://www.lehigh.edu/fjp3/ fjp3.html. In summary, we establish a common horizontal spacing of 25 m in DEMs that range in resolution of 10 m (control regions in Idaho) to 25 m (Simplon). Drainages are identified on the DEM using ArcInfo functions that route water to the lowest standing cell in an eight-cell region on a rasterized lattice. A channel is defined as a cell in that lattice containing at least 100 units of water. The resulting raster rendering of the stream network can be then ordered (Strahler ordering) and converted to line coverages. The channels are then extracted from the DEM by overlaying the line coverage and parsed through a FORTRAN program that calculates the straight-line orientation, length, and gradient from the channel head to its base. We use this strategy for measuring channel metrics in order to reduce Figure 6. (a) Representative alluvial fan and third-order the bias endemic to rasterized data where cells are arranged in drainage basin of a detachment hanging wall, the Vallettina discrete orientations 45 degrees from each other. Drainage basin in the Brenner hanging wall. (b) Representative density is the sum of the total length of streams of a given alluvial fan and third-order drainage basin of a detachment order per unit area. We use the hydrology extension in footwall, the Uberwasser basin in the Brenner footwall. ArcGIS v.9.1 to calculate drainage densities of first-order First-order streams are gray lines, second-order streams are channels astride the hanging walls and footwalls of thin black lines, and third-order streams are thick black detachments. lines. Note that the fans at the mouths of these basins are [24] Channel orientation data must be random samples nearly the same size, whereas the drainage basin areas are drawn from a uniform distribution, a requirement that we considerably different. Also not the more equant shape of test for using a Rayleigh test. Next, channel orientations the Vallettina basin compared to the elongate and incised were tested for statistically significant mean directions by shape of the Uberwasser basin. comparing against the (unimodal) von Mises distribution (the circular equivalent to the normal distribution). This test checks whether the data tend to cluster around any one [22] Topographic data were collated from paper 1:25,000 scale topographic maps as well as high horizontal resolution direction. If the data are nearly uniformly distributed, or if (10À30 m horizontal spacing) digital elevation models. In the data have multiple modes (nonunimodal) then this test the Brenner region only, we completed a detailed measure- will fail. The test assumes that the stream azimuths are ment of stream long profiles and stream profile concavity directional data (0À360 degrees) based on streamflow. The based on a normalized stream concavity index (SCI) van Mises test returns a measure of spread in the azimuthal [Demoulin, 1998; Zaprowski et al., 2005]. The stream data called k which is one of two free parameters in the concavity index provides a similar measure of profile distribution (the other free parameter is the mean). Both the concavity [Wells et al., 1988] as does linear regression Rayleigh test and von Mises test were performed with through a log-log plot of channel slope-upstream contrib- MATLAB codes modified from Jones [2006]. We report uting area [Snyder et al., 2000] and has been shown to k as a key statistical characteristic of the data in comparing closely covary with the slope (q) of log slope-log area plots channel orientations for the hanging walls and footwalls of [Zaprowski et al., 2005]. The investigated streams are detachments. oriented parallel to the detachment and reach from approx- imately 10 km into the hanging wall to approximately 14 km 5. Results into the footwall. In contrast, we use the DEMs and ArcInfo/Arcview GIS to make general qualitative observa- 5.1. Field Observations tions of the topographic texture as well as more precise [25] Generally speaking, the drainages on the hanging quantitative measures of drainages in the Simplon and walls of the Simplon and Brenner detachments assume a control areas. We follow the general techniques of Wise modified radial or dendritic pattern in the zone of highest [1982], Wise and McCrory [1982], and Wise et al. [1985] in topography. In contrast, the drainages on the footwall show artificial hillslope illumination to visually investigate the a strong linearity with the broadly warped detachment density and dominant orientation of linear features. The surface and the pervasive brittle faulting leading to a distinct illuminations are performed digitally in a GIS using trellis pattern. The hillslopes and ridges in the footwall also ArcInfo. The hillslope illuminations help identify regions contain corrugations (Figures 3b and 3c) typically not of contrasting topographic features that are subsequently observed in the hanging wall. We collect metrics on both investigated quantitatively. the alluvial fans and bedrock channels of these drainages.

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5.1.1. Hanging Wall Fans lack of fines in the deposits either reflects no production of [26] Fans of third-order watersheds in the hanging walls fines in the drainage basin or their subsequent efficient of both the Simplon and Brenner detachments (Figure 5) are transport by fan channels to the valley axis. The main relatively steep, generally poorly stratified, sand, gravel, and channel feeding the fan does not show any systematic cobble debris-flow dominated deposits associated with two downstream widening as is typical of alluvial channels with or more inset fan lobes (Figures 6a and 7). The fans are perennial flows. Footwall fans represent the more gentle entrenched with successively younger fan lobes telescoping subset of all measured fan declivities in comparison to basin forward onto the generally flat valley floors of the master area (Figure 7c). The drainage basins feeding footwall fans extension-parallel drainages. The main fan channels, have a relatively elongate shape and are filled by a trellis although deep (typically <2 m) are not incised in the drainage network (Figures 6 and 7c), a shape fostered by traditional sense because they stand on the topographically their relatively elongate straight-trunk channel. higher parts of the fan, bordered by constructional debris flow levies. Geomorphic surfaces of hanging wall fan lobes 5.2. Topographic Metrics have stabilized and been in the landscape long enough to be 5.2.1. Brenner Long Profiles modified by pedogenesis. Weak, rubified cambic B horizons [28] The footwall of the Brenner fault is deeply furrowed and weak spodic B horizons overlain by weak E horizons by elongate, northeast-trending, glacially scoured troughs attest to several centuries to millennia of soil formation. Soil hosting axial drainages. These furrows are parallel to the development is consistent with postÀLittle Ice Age depo- direction of extension and expressed as corrugation of the sition for valleys filled with ice during the late Holocene footwall surface. Tributaries flow down steep valley walls and possibly older, middle Holocene deposits in valleys that into the furrows and join the axial drainage with a nearly remained ice free during the Little Ice Age. The bases of the orthogonal confluence resulting in a particularly well- fan deposits are generally not well exposed but the deposits developed trellis drainage pattern. The tributary streams, are inferred to be at least 10 m thick in their distal portions most of which are third order with a lesser number of on the basis of the relative declivities of the fan surface and second-order channels, are oriented subparallel to a perva- underlying bedrock unconformity in the proximal portions sive mesoscale fault population that increases in density in where entrenchment of the fan and exposure of its base does the proximal region of the detachment. We have extracted occur. Texturally, the hanging wall fans are composed of the longitudinal (long) profiles of 30 tributary channels in a very poorly sorted and stratified, matrix-supported diamic- distribution oriented along the Pfitschtal (Val di Vizze) ton consistent with the type of deposits that result from stretching from the detachment to 10 km into the footwall. debris flow processes. Grain size is more variable in the Six long profiles were also collected on the hanging wall hanging wall fans with respect to the footwall fans (Figure 7a). block west of the Brenner detachment in drainages of equal We do not know if the significant presence of fines in these order. The spatial distribution of the data allows us to deposits reflects a large volumetric production of fines in compare long-profile characteristics on the hanging wall, the basin or a subsequent inefficiency of fan channels to to those proximal to the detachment on the footwall and move fines to the valley axis. Hanging wall fans represent distal to the detachment on the footwall. Stream long the steeper subset of all measured fan declivities in com- profiles are typically concave-up reflecting the downstream parison to basin area (Figure 7). The drainage basins feeding increase in discharge, fining of transported alluvium, and hanging wall fans have a relatively equant shape and are widening of the channel. Steep or convex long profiles filled by a dendritic, or modified dendritic drainage network indicate hydrologic or base level conditions that cause the (Figures 6 and 7c). channel to deviate from the typical concave form. The 5.1.2. Footwall Fans controls on profile concavity are generally known for [27] Fans of third-order footwall drainages in the footwall alluvial channels where downstream grain size fining, of the Brenner and Simplon detachments, in contrast, are increases in channel width, and increases in the peak annual generally narrow, steep, thin, mostly fluvial deposits of discharge all work to increase concavity [Sinha and Parker, angular boulders, cobbles, gravel, and lesser amounts of 1996; Gasparini et al., 2004]. Controls on profile concavity sand (Figures 6b and 7) with bedrock exposed at their base. for the bedrock-dominated streams in our study are less well Channels are generally not entrenched and tend to be known [Snyder et al., 2000; Duvall et al., 2004], but the broader than those found in the hanging walls. The fans downstream increase in peak annual discharge remains an are composed of one depositional lobe that rarely extends important consideration because it is linked to basin shape. onto the flat axial valley floor. Geomorphic surfaces of Elongate, narrow basins tend to not produce peaked flood footwall fans are either active or very young because they discharges as the tributaries converge upon the main chan- lack any appreciable pedogenesis except for a thin, organic- nel along its entire length; however, the converse is true for rich A horizon. Footwall fans are composed mostly of more elaborated, dendritic drainages as tributaries tend to poorly sorted (Figure 7a), but stratified, clast-supported converge on the medial portion of the main channel, alluvium that is consistent with the kinds of deposits that delivering runoff to the same place at the same time and typically result from fluvial, hyperconcentrated, or fine- driving up the peak discharge. Insofar as mesoscale brittle poor, granular debris flow depositional processes. Grain faults in the footwall block influence basin shape, long- size distributions are more narrow and consistent for the profile concavity should be correlated to the presence or footwall fans in comparison to the hanging wall fans. The absence of those faults.

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[29] Long profile shapes and concavity change systemat- that notion (Figure 9 and Table 1). The k values for the ically with respect to distance from the Brenner detachment Pennsylvania dendritic network show the lowest coherence (Figure 8). Generally, the profiles are convex (low concavity) (lowest k), particularly for the first- and second-order at or proximal to the detachment and they become progres- streams. In contrast, the rectangular and in particular the sively more concave farther east of the detachment into the trellis drainage patterns inherit a strong control from under- footwall block. The concavities of all hanging wall channels lying structure and the channel orientations extracted from are similar to those of the footwall drainages far to the east the Ridge and Valley do have preferred orientations, noted of the detachment. The general increase in concavity into as the relatively higher k values (Table 1). The southeast- the footwall block follows the qualitative observation that northwest oriented channels of first, second, and third order the drainage patterns are also dendritic farther from the in the Ridge and Valley illustrate the distinct trellis pattern detachment. formed by tributary streams flowing at nearly right angles [30] The noted drop in concavity approximately 4À6km into higher-order strike valleys oriented parallel to the east of the Brenner detachment correlates to a known northeast-southwest structural grain of the range. We note change in bedrock structure. Here channels abruptly become that the highest coherence in channel orientations occurs in convex before returning to a steady eastward increase in the first-order streams in the trellis network, but in the third- concavity. These convex tributaries are coincident with a order streams for the rectangular and dendritic networks. major bedrock-cored landslide that once dammed the Despite the significant differences in rock type, relief, Pfitsch valley, creating a lake that failed catastrophically climate, and geologic histories between the Alps and in 1100 A.D. [Lammerer, 1990]. Immediately north of the Appalachian Mountains, dendritic drainages in both settings landslide deposit, early folded mylonites related to the are an indicator of little to no underlying structural control, Brenner detachment are truncated by younger, shallowly whereas preferred channel orientations are correlated with west-dipping mylonites [Axen et al., 1995; G. J. Axen and specific drainage patterns, such as trellis, and/or a known J. Selverstone, unpublished data, 1995]. These latter underlying rock type or structural control. mylonites reflect development of a new, more favorably [32] The Oberalp region in the central Swiss Alps oriented detachment during synchronous shortening and (Figure 2) provides context for the coherency of channel extension east of the Brenner line. Apatite fission track orientations astride the Simplon detachment (Tables 1 and 2 samples from the area yield ages ranging from 7 to 10 Ma and Figure 10). Channel orientations in the Oberalp region [Grundmann and Morteani, 1985; Fu¨genschuh et al., 1997] vary among first, second, or third orders, but there is a weak but are not sufficiently closely spaced to resolve an age preferred orientation to the northeast-southwest for second- discontinuity across the young mylonites. It is probable that order channels (Figure 10a). Hillshade illuminations do not rocks between this younger detachment and the main reveal any obvious change in topographic texture across the Brenner Line were transferred from the footwall to the Oberalp region and we have no basis for subdividing the hanging wall during Miocene extension. Hence the location area to test for different channel orientations on suspected of the abrupt decrease in stream concavity east of the main footwall or hanging wall blocks. There is little variation in Brenner detachment coincides with more recently exposed the k-value for the Oberalp region which is reflected in the brittle faults and the development of elongate, fault- wide range of channel orientations seen in the rose diagrams controlled channels. These systems undergo elaboration (Figure 10a). The channel orientations in the Oberalp region processes farther to the east of the detachment and stream may ultimately be controlled by underlying rock type or concavities return to their eastward-increasing trend. We structure, but there is no coherency to that structure such note that one of our footwall drainages, the Sifinol water- that the channels exhibit a preferred orientation. shed, shares a fan gradient more in line with the hanging 5.2.3. Alps Channel Orientation and Drainage Density wall basins than the footwall basins (Figure 7b). The Sifinol [33] Rose diagrams are used to compare stream orienta- watershed is located close to the projected location of the tions for the Oberalp, Simplon, and Ticino regions (Figure 10 hanging wall of the more easterly, younger portion of the and Tables 1 and 2). The topography straddling the Simplon Brenner detachment. detachment reveals differences in drainage orientation and 5.2.2. Coherence of Reference Network Stream density between the hanging wall and footwall. In the Azimuths hanging wall, first-order channels have no preferred orien- [31] As an independent check on the ability of our tation, but second- and third-order channels have preferred methods to distinguish and quantify channel orientations orientations to the southeast and northwest-northeast respec- we collected data from an old decaying orogen where tively (Figure 10b). In contrast, first-, second-, and third- fluvial processes have had a long time to adjust to rock order footwall channels all show a preferred northwest and type and structure. The Appalachian mountains of central southwest orientation that is particularly well developed for Pennsylvania have at least three distinct drainage patterns, third-order channels (Figure 10b). The footwall third-order dendritic, trellis, and rectangular, which occur on the channels have a greater k than the Oberalp reference. Appalachian Plateau, Ridge and Valley, and Piedmont Similarly, faults astride the detachment have a strong, physiographic provinces, respectively. Dendritic drainage predominantly northwest orientation [Wawrzyniec et al., patterns are characterized by little to no control by a 2001] (Figure 4d) that matches the channel orientations prevailing underlying structure and the channel orientations from the footwall. The gullies that expose the faults corre- extracted from the Appalachian Plateau are consistent with spond to all three channel orders we investigate, but most of

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Figure 7. Comparison of footwall to hanging wall alluvial fan data. Abbreviations are as follows: SHW, Simplon hanging wall; SFW, Simplon footwall; BHW, Brenner hanging wall; and BFW, Brenner footwall. (a) Grain size of largest clasts in the alluvial fan trunk channel, with measurements made every 1 m along channel thalweg. (b) Fan gradient: basin area plot. (c) Fan gradient: drainage basin shape. The lines in Figures 7b and 7c are drawn only as a guide to help separate the hanging wall fans from the footwall fans. the larger gullies roughly correspond to our third-order topographic texture oriented northwest-southeast and sub- channels. parallel to the main river valley orientation amenable to [34] In contrast to the Oberalp region, hillshade illumi- testing channel orientation populations in the context of a nations of the Ticino area reveal a marked contrast in suspected detachment (Figure 11). Relatively low k values,

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Figure 8. Plot of the Pfitsch valley tributary concavities with respect to distance from the Brenner detachment into the footwall block. The concavities of similar-sized basins in the hanging wall block are plotted with the open square symbols for comparison. Open triangles are drainages directly on the Brenner detachment. The queried fault indicates the likely location of a smaller detachment synthetic to the main Brenner detachment. This fault projects to the surface at 4000 m along our line of section. all less than the Oberalp reference region indicate no fall. There is no obvious preferred orientation of channels preferred channel orientation for the suspected hanging wall for the hanging wall of the Pioneer detachment, but a block which lies to the southwest of the main river valley, favored orientation of first- and second-order channels but the suspected footwall block to the northeast of the main exists in the footwall (Figure 12). Channel orientation river valley has nearly orthogonal channels in southwest- coherency is greater for all three stream orders in the northeast and southeast-northwest orientations for first, footwall than in the hanging wall (Table 2). The same second, and third orders (Tables 1 and 2 and Figure 10c). telltale gullying (Figure 12c) is prominent in the footwall Stream orientation coherency of the suspected Ticino foot- proximal to the Pioneer detachment and exerts a major wall is greater for all three orders with respect to the control on channel orientation. Furthermore, the orientation Oberalp reference and the suspected Ticino hanging wall of these footwall channels is nearly east-west, consistent (Tables 1 and 2). with the expected orientation of a set of high-angle, meso- [35] The differences in topographic texture that the hill- scale brittle faults that would be expected to form parallel to shade illuminations reveal in the Simplon (Figure 4) and the detachment, as observed for the Simplon fault. Ticino (Figure 11) areas, but which are lacking in the Oberalp region (Figure 3d), are rooted in differences in the relative drainage density (Table 3) as much as they are in 6. Discussion the drainage orientations. The drainage density in the 6.1. Tectonic Significance of Oriented Gullies in the footwall of the Simplon detachment, especially in the area Footwalls of Detachment Faults proximal to the detachment, is higher than in comparable [37] The concept of rock type and structural control on areas of the hanging wall. Similarly, a marked contrast of channel orientation and drainage pattern is well established drainage density is observed on either side of the Ticino in the geomorphologic literature [Braun, 1983; Bloom, area again with higher density of channels in the suspected 1991] but it is less common to be able to propose a link footwall. Drainage densities in the Simplon footwall and the between a specific topographic feature and a specific suspected Ticino footwall are higher than the drainage structure, reflecting a specific geomorphic response to a density of the Oberalp region. specific tectonic process. Our study proposes a model 5.2.4. Channel Orientation and Drainage Density of linking tectonic process to geomorphic form and process the Pioneer Core Complex based on a synthesis of the channel shape, channel orien- [36] Channel orientations astride the Pioneer core com- tation, drainage density, fault orientation, and alluvial fan plex of central Idaho share many similarities to the data from stratigraphy for regions where major crustal-scale detach- the Simplon and suspected Ticino detachments (Table 2). The ments have been identified (Table 4). Pioneer core complex offers a good comparison to the Alps [38] The footwalls of the Simplon and Brenner detach- study sites because of similar footwall rock type, climate, ment faults in the Alps are notable for their pervasive, relief, intensity of glacial scour, and syn-glacial base level

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Figure 9. (a) Stream orientations for the drainage patterns of the Appalachian foreland in Pennsylvania. (b) Digital shaded topography of central and eastern Pennsylvania from 90-m SRTM data showing location of topography sampled (at 30-m resolution) to study stream orientations in the dendritic Appalachian Plateau (D), trellis Appalachian Ridge and Valley (T), and rectangular Piedmont (R). distinct gullies on steep slopes flanking long, linear valleys parallel or subparallel to the detachment (Figures 10 oriented in the direction of extension. At the small scale, and 11). At the larger scale, the drainage pattern developed gullies on the footwall typically correspond to first, second, on the footwall is a trellis pattern. The preferred gully and third Strahler-order channels when resolved at the scale orientations and trellis pattern are anomalous for highly of a high-resolution (25 m) digital elevation model. The dissected, alpine settings and belie an important underlying gullies (channels) exhibit a preferred orientation that is structural control. Mesoscale, brittle, high-angle faults are

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Table 1. Statistical Coherence of Reference Network Stream [41] These interpretations are subject to complexities, Azimuths however, particularly as one moves farther from the main

a detachment. In the Simplon and Brenner regions, gully Reference Network Pattern Stream Order k drainage densities are greatest near the detachment systems Oberalp modified rectangular 1 1.63 (reflecting closely spaced faults) and decrease farther to the 2 1.792 east. This reduction in drainage density, and hence in 3 1.716 inferred fault spacing, may be due to the exhumation Pennsylvania dendritic 1 1.425 mechanism, and may not necessarily reflect originally 2 1.384 3 1.801 mechanically weak footwall rocks. For example, fluid trellis 1 1.826 inclusion studies have identified two distinct fault popula- 2 1.742 tions (deeper, west-down and shallower east-down faults) as 3 1.69 footwall rocks moved through a lower and upper hinge zone rectangular 1 1.414 [Selverstone et al., 1995]. While footwall drainages proxi- 2 1.498 3 1.637 mal to the detachment suggest dense fault spacing in rocks that passed through both hinge zones, the decreased drain- aLargest coherence value is boldface. age density farther east may reflect rocks that had been exhumed from initial positions along a detachment-ramp exposed in the gullies, but not in the intervening interfluves structure, and would thus have only traveled through the (to the extent that our detailed field investigations of a few upper hinge zone. In the Brenner region, additional com- interfluves are representative of the region). The spatial plexities in interpreting footwall drainage/fault spacing correlation of the gullies with the mapped faults strongly distal to the detachment arise from the possible effects of suggests that channel development and orientation is pri- east-down detachment faulting at the eastern boundary of marily controlled by these faults [Wawrzyniec et al., 2001] the Tauern window. Generally, footwall rocks distal to the (Figure 4d). detachment would have been exhumed earlier than those [39] The fault-controlled drainage patterns we document proximal, but the entire region surrounding the detachment are predicted to develop in the footwalls of detachment would have been exposed to erosion at roughly the same systems where footwall exhumation is accommodated by a time, presumably since the Messinian as the Alps were subvertical simple shear mechanism (Figure 1a) [Axen and unroofed at an accelerated pace because of Messinian base Bartley, 1997]. In contrast, we do not expect these same level fall and subsequent glacial-interglacial climates [Willett gullies in footwalls that underwent primarily ductile exten- et al., 2006]. Geomorphic characteristics including drainage sion or flexural failure [Axen and Bartley, 1997] where the density, drainage basin shape, and channel concavity that footwall block bends without significant faulting (Figure 1b). vary away from the detachment are likely controlled by how In these cases, the footwall rocks will lack the contrast in the mesoscale faults anneal or reorganize to local stresses erodibility represented by narrow cataclasite-filled zones within largely unfaulted volumes, and are thus unlikely to Table 2. Statistical Coherence of Detachment Fault Stream develop a distinct gully pattern. Azimuths [40] The suite of geomorphic characteristics we investi- gated thus provides an important discriminant of the process Greater Than of subvertical simple shear during midcrustal exhumation at Detachment Stream Oberalp detachment faults. Application of our conceptual model of Network Location Order k Reference?a the tectonic significance of fault-controlled drainage pat- Simplon footwall 1 1.488 no terns may allow identification of previously unrecognized 2 1.469 no extension in active, convergent orogens. In regions of 3 1.831 yes subvertical simple shear, the drainage network proximal to hanging wall 1 1.672 no the detachment thus provides rough constraints on local 2 1.906 yes 3 1.66 no brittle fault orientation and density (Figures 4d, 10, 11, and Ticino footwall 1 2.363 yes 12 and Table 3). The spacing of these high-angle faults 2 2.605 yes serves as a proxy for the degree of brittle failure in the upper 3 1.819 yes part of the footwall and thus provides clues to the strength hanging wall 1 1.475 no 21.711no properties (rigidity) of the exhumed footwall block. Our 3 1.633 no model is easily tested elsewhere and, if verified, could result Pioneer Mountains, footwall 1 2.234 yes in determination of the relative importance of subvertical Idahob simple shear versus flexural failure or other distributed 2 2.581 yes mechanisms in accommodating exhumation of midcrustal 3 1.767 yes hanging wall 1 1.787 yes rocks. On the basis of our observations and conceptual 2 1.97 yes model, we would conclude that the suspected Ticino 3 1.172 no detachment is in fact a detachment that accommodated a orogen orthogonal extension similar to the Simplon and Comparison is based on three significant figures. bComparison to an Alps reference is considered in the context of obvious Brenner structures. discrepancies in rock type, climate, and orogenic history.

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Figure 10. Stream orientation plots for the (a) Oberalp region, (b) Simplon detachment, and (c) Ticino regions. Note the correspondence in channel orientations for the Simplon footwall and the mesoscale brittle normal fault strike of Figure 4d. Half-rose plots show the mean channel orientations for first-order (arrow), second-order (circle), and third-order (square) streams; gray shading is the azimuth confidence angles based on the data standard error and coherence (k). Abbreviations are as follows: SHW, Simplon hanging wall; SFW, Simplon footwall; THW, Ticino hanging wall; and TFW, Ticino footwall. while still deeply buried, rather than by more erosion and channels), but which are also a bit ambiguous in terms of geomorphic modification of structures distal to the detachment. the coherency of the other channel orders (Tables 1 and 2). [42] Channel orientation data in the Simplon region The differences in coherency in channel orientations among indicate that the hanging wall may impart a structural the hanging walls and footwalls of the Ticino detachment control related to high-angle faults propagating up from and the Pioneer detachment (see below) are much more the detachment. The research by Wawrzyniec et al. [2001] clear than what we discovered for the Simplon detachment. argues for three domains astride the Simplon detachment, a [43] A test of the conceptual model we propose for the footwall domain dominated by the high-angle faults, a Alps is provided by analyzing channel orientations in the transitional domain straddling the footwall and proximal Pioneer core complex of central Idaho. As in the Alps, hanging wall with a lower density of the high-angle faults, the low-order channels in the footwall rocks of the Pioneer and a hanging wall domain distal to the detachment with detachment have a preferred orientation parallel to the little to no high-angle faults. These domains are consistent detachment that is not shared by the drainages in the with our channel orientation data which argue for better hanging wall rocks. The Pioneer footwall is cut by meso- orientation coherency in the footwall (for third-order gulley/ scale faults that promote gullying proximal to the detach-

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Figure 11. Comparative shaded DEM images of the Ticino region with illumination from the (a) northwest and (b) northeast. Suspected top to the west detachment fault is shown as the thick hatched line. White arrows in Figure 11a point to gullies in a proposed Ticino footwall. ment (Figure 12). Drainage orientation coherency is rela- be used to focus attention on a region as a potential location tively high in the Pioneer Mountains footwall in comparison of a detachment. Particularly in glaciated landscapes, long to its hanging wall (Table 2), and greater than the reference overdeepened valleys will have short, steep tributaries Oberalp k values for all drainage orders, but we caution oriented roughly orthogonal to the axial streams forming a against using an Alps reference frame for comparing to the trellis pattern. This pattern is very obvious for both the different geologic, tectonic, and climatic setting of central Brenner and Simplon footwalls, but curiously, does not Idaho. In contrast to the Pioneer detachment, the detach- continue into the hanging walls (Figures 3 and 4). The ment faults of core complexes in the American southwest do Brenner and Simplon detachments themselves lie in deep not appear to have gullies developed parallel to the detach- valleys that separate the trellis pattern in the footwall from a ment in the footwall rocks [Spencer, 2000]. One possibility modified dendritic pattern in the hanging walls. Perhaps the is that the climatic, geologic, and erosional setting is appropriate question to ponder is if there is any underlying sufficiently different such that the gullies do not form, but tectonic reason for large glaciers to furrow the footwalls we suspect that the more likely explanation is that the footwalls there were exhumed by flexural failure. There is no lack of erosion and gullying in the American southwest Table 3. Drainage Density of First-Order Channels Adjacent to and regoliths are even more uncommon here than our alpine Detachment Faultsa study sites. For these reasons, we prefer the tectonic interpretation for the presence or absence of gullies. Detachment Footwall Hanging Wall

b 6.2. Criticisms of the Significance of Trellis Drainage Oberalp 1221 1221 Simplon 1325 1048 Patterns Ticino 1926 1350 Pioneer Mts 986 848 [44] The presence of a trellis drainage pattern alone should not be viewed as the key indicator of a detachment aDrainage density is in m/km2. First-order channels are defined by fault that has undergone subvertical simple shear. Rather, minimum drainage area of 0.1 km2. the trellis pattern is the first large-scale observation that may bThere is no known or inferred detachment fault in the Oberalp region.

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Figure 12. (a) Channel orientations and (b) shaded DEM image (30-m resolution) of the Pioneer Mountains, Idaho, core complex. The detachment fault is shown in black with ticks in Figure 12b. The arrow points to hillslope gullies shown in Figure 12c. Half-rose plots show the mean channel orientations for first-order (arrow), second-order (circle), and third-order (square) streams; gray shading is the azimuth confidence angles based on the data standard error and coherence (kappa). orthogonal to the detachment. Clearly the core complex footwalls of the American southwest are furrowed [Spencer, 2000], and these features may represent some primary Table 4. Summary of Fan and Topographic Metrics corrugation beneath the detachment molded into the top Hanging of the footwall. Observation Footwall Wall [45] Unlike the Alps study areas, the large elongate valleys of the Pioneer Mountains footwall do not terminate Fan lobes single, unentrenched multiple, entrenched Fan facies fluvial and fine-poor fine-rich debris at the detachment but rather project well into the hanging debris flows flows wall, carrying a trellis drainage pattern along. Nevertheless, Drainage basin shape narrow, elongate equant at the scale of first-, second-, and third-order drainages the Trunk channel profile convex concave footwall imparts a preferred orientation whereas the hanging Orientation of parallel to detachment no preferred low-order channels orientation wall does not (Figure 12). The hanging wall argillites of the Density of relatively high relatively low Pioneer detachment differ from the footwall crystalline low-order channels rocks but clearly some tectonic process, in our opinion

18 of 21 TC6010 PAZZAGLIA ET AL.: GEOMORPHIC EXPRESSION OF EXTENSION TC6010 subvertical simple shear, has been able to impart a texture in ment. The Simplon and Brenner detachments in the Alps the footwall that is lacking in the hanging wall so the result provide the natural laboratory where we observed the is the same: low-order channels oriented parallel to the drainage basin and alluvial fans form and process in the detachment in the footwall only. context of the detachment, the exhumed footwall, and the hanging wall. The model that has emerged suggests 6.3. Effects of Faulting on Erosional Processes that pervasive mesoscale faults in the footwall and parallel to the detachment generate erodible cataclasites that, when [46] The influence of the mesoscale brittle faulting is not limited to channel orientations. The density of faults in the exposed, lead to numerous parallel hillslope gullies. The footwall increases with proximity to the detachment, a drainage basins that evolve around these gullies are closely characteristic shared by the density of low-order channels spaced, elongate, and narrow. They constitute a family of (Table 3). High channel densities result from numerous, low-order (first-, second-, and third-order) tributaries that closely spaced, elongate watersheds, as opposed to broad, enter the trunk axial stream drainage at right angles, result- more oval shaped watersheds where trunk channels have ing in an overall trellis drainage network. Peak discharges branching tributaries. Small, elongate watersheds generate are suppressed by the narrow, elongate drainage shapes relatively low discharge, but more importantly, the dis- leading to convex long profiles for the footwall drainages, charge hydrographs of elongate watersheds are less flashy especially those proximal to the detachment. Similarly, and have lower peaks than more equant watersheds of these drainages are production limited leading to small fans comparable area. This difference in discharge generation of coarse material at their mouths. These geomorphic arises because the tributaries of equant-shaped watersheds features are not as pronounced for footwall drainages farther deliver runoff to the trunk channel at more or less the same from the detachment. Because the entire footwall was time, to more or less the same place during basin-wide exposed and sculpted at the same time, changes in the precipitation. Flashier, more peaked hydrographs are asso- spacing of brittle faults that cut the footwall likely reflect ciated with channels that can generate high shear stresses annealing processes during extension while the footwall during high flows, resulting in less steep, more concave was still deeply buried in the crust. long profiles. The concavity data associated with low-order [49] In contrast, the hanging walls of the Alpine detach- channels on the Brenner detachment footwall are consistent ments are typically not cut by a pervasive set of high-angle with the influence of basin shape on profile concavity. brittle faults with similar orientation; however, the Simplon Elongate watersheds strongly controlled by the brittle faults detachment is apparently an example of how the detachment are smaller, have a low concavity (they are convex), and are occupies a fault zone with high-angle faulting extending clustered proximal to the detachment. Higher concavities into the hanging wall proximal to the detachment. Hanging are associated with more equant watersheds located distal to wall channels typically do not follow a preferred orientation the detachment or in the hanging wall where the high-angle and drainage basins exhibit a relatively wider spacing and brittle faults are lacking. equant shapes. The overall drainage pattern is dendritic or modified dendritic. The drainage basins are not production [47] The fault-influenced watershed shape relationship has a further implication in terms of the geomorphic limited, generating fines and coarse material that accumu- processes that operate in those watersheds and the resulting late in relatively large debris flow dominated fans at the sedimentologic record of denudation from the footwall basin mouths. versus hanging wall setting. Footwall watersheds have little [50] Our study has demonstrated a link between easily regolith on narrow hillslopes, are dominated by fluvial identifiable geomorphic features in alpine topography with processes, and generate a coarse, angular detritus. Geo- extensional tectonics of low-angle detachments. The meth- morphically, these watersheds are production limited as the odology is supported by channel orientation and drainage pervasive brittle faulting in the footwall limits the normal density studies in landscapes like the Appalachians where growth and development of narrow, elongate basins into drainage patterns are well-known to be closely adjusted to larger, more equant basins. The steep gradient and high rock type and structure. Furthermore, our observations in drainage density of these drainages contribute to the pro- the Alps have been demonstrated to be portable and duction-limited character. Hanging wall watersheds, in corroborated by other regions known or suspected to be contrast, have broad hillslopes mantled with regolith, are dominated by low-angle detachments, including the Pioneer dominated by debris flow processes, and generate a fine- Core Complex of central Idaho and possibly the Ticino grained detritus in addition to coarse clasts. The alluvial dome in the Alps. In both cases, our analysis suggests that fans deposited by such diverse watersheds reflect the high-angle brittle faulting of the footwall is preserved in the different erosional and hydrologic processes that are direct region adjacent to the detachment. These results are con- consequence of basin shape and the presence or absence of sistent with a subvertical simple shear model for footwall high-angle brittle faults. extrusion as well as annealing of brittle structures in the footwall distal to the detachment rolling hinge. Low-order channel orientation, channel gradient, drainage basin spac- 7. Conclusion ing, drainage patterns, and valley bottom fan development define a set of geomorphic observations for fingerprinting [48] Our findings highlight a connection between drain- age evolution and underlying structural factors associated extension tectonics in contractional orogens. with crustal extension and subaerial exposure of the detach-

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[51] Acknowledgments. Funding was provided by NSF grants EAR- Kirby, Alex Densmore, Fritz Schlunegger, and Noah Finnegan. We wish to 9902828 to F. J. P. and EAR-9909102 to J. S. and M. R. We thank Tim express a special thanks to the Associate Editor and to Editor Hodges for Wawrzyniec for discussions. D. T. M. of Switzerland was purchased and their patience in granting revision extensions. licensed from Bundesamt fu¨r Landestopographie, CH-3084, Wabern. The manuscript has been significantly improved by careful reviews from Eric

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