Patterns of denudation through time in the San Bernardino Mountains, California: Implications for early-stage orogenesis Steven A. Binnie a, ⁎, William M. Phillips a, 1 , Michael A. Summerfield a , L. Keith Fifield b , James A. Spotila c a Institute of Geography, University of Edinburgh, Drummond St., Edinburgh EH8 9XP, UK b Department of Nuclear Physics, Research School of Physical Sciences and Engineering, The Australian National University, Canberra ACT 0200, Australia c Department of Geosciences, Virginia Polytechnic Institute and State University, 4044 Derring Hall, Blacksburg, Virginia 24061, USA abstract article info Article history: Received 21 January 2008 Received in revised form 4 September 2008 Accepted 5 September 2008 Available online 18 October 2008 Editor: R.W. Carlson Keywords: denudation rates topographic evolution cosmogenic nuclides orogenesis San Bernardino Mountains Basin-averaged cosmogenic 10 Be concentrations, apatite (U–Th)/He thermochronometry and incision into a dated palaeosurface constrain spatial and temporal variations in the rates of denudation experienced during the early-stages of orogenesis in the San Bernardino Mountains, California. Cosmogenic 10 Be analysis measures denudation over intermediate (∼ 10 2 –10 4 years) time scales and records rates which decrease from a maximum of 2700±500 mm ka - 1 in the south to a minimum of 52±5 mm ka - 1 in northern catchments. Corresponding rates from (U–Th)/He and incision into a dated palaeosurface measure long-term (∼ 10 6 years) denudation rates which decrease from between 1200 ± 400 mm ka - 1 in the south to a minimum of 30± 20 mm ka - 1 in the north. The temporal consistency observed in the broad-scale patterns of denudation rates probably results from the persistent imprint of the initial crustal architecture and drainage network. These have maintained an influence on slope distributions, and are thus fundamental factors controlling the gross patterns of denudation throughout the early stages of orogenesis. Where variations between the denudation rates measured over different time scales are apparent the intermediate-term rates are found to be consistently greater than the long-term, with the increase being more pronounced around the fault bounded peripheries of crustal blocks relative to quiescent block interiors. This provides empirical support for a model of mountain building whereby topographic development is dictated by the headward retreat of drainage systems that propagate away from zones of displacement. Our findings indicate that recent localised increases in denudation rates in young fault block orogens may be explained by a progressive denudational response to prior tectonic uplift, rather than a consequence of climatic change. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Tracing the variability, or consistency, in denudation rates through time is necessary for a detailed empirical description of topographic development and offers insights into the tectonic and climatic mechanisms influencing such rates. By utilising various techniques able to derive denudation rates over a range of time scales previous studies have provided evidence for the existence of an approximate steady-state between rates of crustal uplift and denudation (e.g. Burbank et al., 1996; Matmon et al., 2003; Vance et al., 2003), the episodic nature of sediment transport due to rare storm events (e.g. Kirchner et al., 2001; Tomkins et al., 2007) and constrained numerical simulations of landscape evolution (e.g. Barnes and Pelletier, 2006; van der Beek et al., 2002). These studies have typically relied on apatite and zircon fission-track thermochronometry to quantify long- term denudation rates, but these techniques have a relatively low spatial resolution. A detailed understanding of orogenic development requires denudation rate data over a range of temporal scales and at a spatial resolution sufficiently high to reveal relationships between tectonics, denudation and topographic development. In this study we provide a detailed analysis of denudation rates in the San Bernardino Mountains (SBM) over different time scales by combining cosmogenic 10 Be data and (U–Th)/He-thermochronometry with measurements of incision from a dated palaeosurface. Our findings elucidate the topographic evolution of the early stages of orogenesis and show that increases in denudation rates during this period may be a typical response to topographic development, and are not necessarily indicative of climatic change. 2. Field area The SBM are the major topographic component of the Transverse Ranges northeast of the San Andreas Fault (Fig. 1). They extend roughly 80 km from north to south and 100 km east to west, and Earth and Planetary Science Letters 276 (2008) 62–72 ⁎ Corresponding author. Tel.: +44 131 650 9140; fax: +44 131 650 2524. E-mail addresses: sbinnie@geo.ed.ac.uk (S.A. Binnie), phillips@uidaho.edu (W.M. Phillips), mas@geo.ed.ac.uk (M.A. Summerfield), keith.fifield@anu.edu.au (L.K. Fifield), spotila@vt.edu (J.A. Spotila). 1 Present address: Idaho Geological Survey, University of Idaho, Moscow, Idaho, USA 83844, US. 0012-821X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2008.09.008 Contents lists available at ScienceDirect Earth and Planetary Science Letters journal homepage: www.elsevier.com/locate/epsl