The Development of Topography above Subduction Zones

Convergent plate motion at subduction zones builds topography through the accretion of lower plate crust and sediments into the overriding plate. Over long timescales, deformation and crustal thickening of this material results in a large wedge-shaped cross-section of upper plate crust. The shape of this crustal wedge is controlled primarily by the material properties of the wedge (e.g., friction, cohesion, pore fluid pressure) and the subduction interface (e.g., friction, basal shear traction) (Davis et al., 1983; Dahlen, 1984). While these considerations neatly describe the first-order characteristics of subduction wedges, many details of their growth and the processes driving changes in rock uplift rate and surficial fault kinematics and slip rates remain poorly constrained in natural settings. Since my Ph.D., I have worked with collaborators to bridge this knowledge gap. Most of this research on has focused on Crete, Greece, a forearc high above the Hellenic Subduction Zone in the Eastern Mediterranean where we have worked to quantify rates of rock uplift, fault slip, and denudation to constrain different kinematic and geodynamic models.

Alluvial fan overlying 27 m high marine terrace, south central coast of Crete, Greece.

Collaborators: Richard Ott (ETH), Sean Willett (ETH), Karl Wegmann (NCSU), Mark Brandon (Yale), Frank Pazzaglia (Lehigh), Charalampos “Babbis” Fassoulas (Univ. of Crete), Elena Bruni (ETH), Francesco Pavano (Lehigh)

Funding: CAREER: Data Driven Inversion of Subduction Zone Topography (NSF EAR-2041910), SubiTop European Union ITN to Willett (Grant Agreement No. 674899, 2016 – present), American Chemical Society Petroleum Research Fund to Wegmann (Grant No. 50792-DNI8, 2010-2013), GSA Graduate Student Research Grant to Gallen (Grant No. 9596-11, 2011), Sigma Xi Grants-in-Aid of Research to Gallen (2012)

Relevant publications: Pavano and Gallen (2021), Ott et al. (2021), Ott et al., (2019a,b), Gallen and Wegmann (2017), Gallen et al. (2014).

Landscape Response to “Extreme Events”

Large earthquakes and extreme precipitation events impart an impressive forcing on landscapes. When such “extreme events” occur in high-relief mountains they often result in widespread mass-wasting, which represents an important natural hazard in the immediately aftermath of an event, but also acts to alter geomorphic processes for years following the initial catastrophe. While such events are relatively rare on the human timescale, they are quite common on the geologic timescale and might be an important mechanism driving long-term landscape evolution. For the past several years I have been working with colleagues to study the impact of extreme events, mostly large continental earthquakes, on erosion and sediment transport in high relief settings over a range of time scales. My colleagues and I have also worked to exploit these events to learn about learn about physical properties of landscapes, specifically near-surface material strength, at unprecedented spatial scales.

Landslides in the Trisuli valley triggered by the 2015 Gorkha Earthquake, Nepal. (photo by Maarten Lupker)

Collaborators: Maarten Lupker (ETH), Marin Clark (Univ. of Michigan), Katie Schide (ETH), Dorran Howell (ETH), Marius Huber (ETH), Kirk Townsend (Univ. of Michigan) Josh West (USC), Ananta Gajurel (Tribhuvan University), Elena Bruni (ETH)

Funding: ETH-Zurich Research Council (2015 – 2019), University of Michigan Turner Postdoctoral Fellowship (2013-2014)

Relevant publications: Bruni et al. (2021); Gallen et al. (2017); Roback et al. (2017a, b); Gallen et al. (2015a)

Landscape evolution in decaying mountain belts

I have a long-standing interest in understanding evolution of landscapes in ancient mountain belts. Traditionally, these settings are thought of as slow, steadily eroding and rather boring; however, recent studies by myself and others have demonstrated that landscape evolution in postorogenic regions is surprisingly unsteady. Despite the growing consensus surrounding the unexpected dynamics of ancient mountain landscapes, vigorous debate continues as to the mechanisms driving such activity. My research on this topic has focused on better understanding the drivers of unsteady landscape evolution in postorogenic settings and developing new and innovative techniques to address questions regarding landscape evolution in slowly eroding environments. I am also interested in understand the impact of long-term landscape dynamics on the diversity and evolution of aquatic species that call these regions home.

Cullasaja River, NC.

Collaborators: Ryan Thigpen (Univ. of Kentucky), Karl Wegmann (NCSU)

Relevant Publications: Gallen (2018), Gallen and Thigpen (2018), Gallen and Wegmann (2015), Gallen et al. (2013), Gallen et al. (2011)

Signal Preservation in Geomorphic Systems

This research aims to better understand if and how geomorphic markers (e.g. river and marine terraces) record tectonic and climate signals so that we can appropriately exploit these features for meaningful information.

Fluvial terrace ~90 m above modern river bed. Emparos River, Crete, Greece.

Relevant Publications: Gallen and Fernández-Blanco (2021), Gallen et al. (2015b)