Geomorphic Expression of Midcrustal Extension in Convergent Orogens

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 extensional 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 TC6010 1of21 TC6010 PAZZAGLIA ET AL.: GEOMORPHIC EXPRESSION OF EXTENSION TC6010 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 2of21 TC6010 PAZZAGLIA ET AL.: GEOMORPHIC EXPRESSION OF EXTENSION TC6010 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

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