Landscape evolution: the interactions of tectonics and surface processes

The last decade has witnessed a resurrection of historical decadal geodetic rates provide a reasonable proxy for the long-term rates of plate motions (Donnellan et al., 1993; concepts and controversies concerning the evolution of landscapes in active tectonic regimes. New insights into Abdrakhmatov et al., 1996). If true, geodetic measurements provide an accurate snapshot of the tectonic forcing and quantification of spatial patterns of deformation and of surface processes have enlivened this debate and function at regional scales that drives long-term landscape development. Combined with geological indicators of provided an impetus to address some long-standing questions. Under a regime of persistent tectonic forctectonism, geodesy helps to reveal considerably more than previously possible about the detailed spatial distriing, are there predictable stages of topographic form through which a landscape will pass? What are landscape bution and magnitude of deformation over broad regions and over a wide range of time-scales. response times to changes in the rate and duration of tectonic forcing? How does the topographic response In addition to tectonism, the second major ingredient in shaping actively deforming landscapes is the role vary if tectonic forcing is steady over millions of years or is highly pulsed and separated by long intervals of played by surface processes. This set of processes not only modifies tectonically generated landforms, but quiescence? To what extent does the topographic form vary as a function of climate and changes in climate? erosion and deposition may actively influence patterns of deformation (Pinter & Brandon, 1997). The debate conHow reliably can we read the record of past tectonic and climatic events in the landscape, and how far back in cerning landscape evolution has been enlivened and advanced by new approaches and the application of new time can this record be extended? Spurred by improved measurement of deformation at time-scales ranging from technologies to geomorphic problems. During this debate, several major themes have recurred: co-seismic (Hager et al., 1991) to several millions of years (DeCelles et al., 1998) and by an enhanced understanding $ Numerical models of diverse surface processes can be coupled to create synthetic landscapes whose properties of the rates and controls of surface processes, answers to some of these questions are emerging. Although none is mimic key attributes of real-world landscapes (Willgoose et al., 1991; Howard, 1994; Tucker & Slingerland, 1994; likely to be completely answered by individual studies, the papers in this special volume provide a sampling of Braun & Sambridge, 1997). When compared with landscapes in which topography, age and geomorphology the range of approaches presently being employed in studies of regional topography and tectonics. have been quantified, these models can provide insights concerning the relative importance of specific processes One major advance in studies of active tectonics and topography is that the strain fields associated with indiin shaping regional topography. For example, in a surfaceprocess model, it is possible to systematically test how vidual seismic events are being documented with unprecedented accuracy and breadth. Ground-based surveys changes in variables such as water discharge, sediment flux, rock strength, glacial erosion rates or stable hillslope have measured the patterns of co-seismic deformation on normal and reverse faults in two dimensions (Stein et al., angle affect the spatial distribution of topography, erosion and deposition (Tucker & Slingerland, 1996; Densmore 1988), and these measurements underpin attempts to understand co-seismic strains in the upper crust using et al., 1998). $ In actively deforming mountain belts, interactions and elastic half-space models (King & Ellis, 1990). For the first time, radar interferometric studies are documenting feedback between tectonics, climate and surface processes influence not only the geomorphology but also may co-seismic displacements over 1000s of km2 at cm-scale resolution (Massonnet et al., 1993), such that regional control patterns and rates of strain in orogens (Beaumont et al., 1992). Coupled tectonic–geomorphic models often patterns of displacement are now being delineated, even at long distances from the active fault. Geodetic measuresuggest that high strain rates are spatially associated with high erosion rates. For example, the presence of a large, ments, and particularly GPS campaigns, have recently characterized regional interseismic strain fields in many underloaded river in an active mountain belt may permit very rapid erosion, thereby accelerating rock uplift in actively deforming areas (e.g. Norabuena et al., 1998). The similarity of geodetically determined strain rates at that area (Koons, 1998). More surprisingly, perhaps, is the prediction that, given identical conditions of tectonic decadal time-scales to the long-term rates derived from sea-floor spreading rates and plate motions (DeMets forcing, the direction from which moisture is advected toward an orogen will exert a fundamental control on et al., 1990) has suggested to many investigators that the