Presentation on theme: "Dynamics of sediment routing systems in tectonically-active mountain ranges OR Control of margin dynamics by sediments, part 4: erosion and transport Alex."— Presentation transcript:
Dynamics of sediment routing systems in tectonically-active mountain ranges OR Control of margin dynamics by sediments, part 4: erosion and transport Alex Densmore Department of Geography & Institute of Hazard, Risk and Resilience Durham University
Sanjeev discussed sedimentary basins as bathtubs that evolve and fill over geological time scales, and (importantly) preserve a record of the detritus from margin evolution The taps in this case are the sediment routing systems which (1) liberate sediment from bedrock and (2) feed it into the developing basin What role do those systems play? “…the Steering Committee envisions a successor program that will investigate the coupled geodynamic, surficial, and climatic processes that build and modify continental margins over a wide range of timescales (from s to My).”
Gawthorpe et al. Basin Research (1994) Densmore et al. JGR-Solid Earth (2003) Why we should care: 1) We’ve talked about the fault framework setting the entry points for sediment, but we need a sediment source as well, and that source is intimately linked to the deformation field
Why we should care: 2) Good theoretical reasons to expect that erosion and mass transfer can affect tectonic deformation rates and patterns along active convergent margins Not so much work done in extensional settings – see Dorsey et al. white paper Willett et al. Geology (2006)
Two key concepts have shaped our understanding of sediment delivery in tectonically active areas: 1)The growth of fault arrays 2)Coupled sediment routing systems Lost River Range and Borah Peak, Idaho
Gupta and Scholz JSG (2000) Schlische et al. Geology (1996) Faults grow by a combination of lateral tip propagation and linkage of existing segments, preserving a roughly linear displacement-length relationship
This systematic pattern invites a space-for-time substitution that gives us a powerful framework in which to interpret spatial variations in topography, erosion and sediment flux Foster et al. Geomorph. (2008)
Evolution of topography In many fault-bounded ranges, relief is decoupled from fault displacement and slip rate, due to 1)Strength-limited hillslopes and efficient surface processes (e.g. landsliding) and/or 2)Glacial erosion This sets up challenges for the tectonic ‘fidelity’ of landscapes (= none?) and for paleo- topographic reconstruction from sediment records (= possible?) uniform relief Densmore et al. JGR-Solid Earth (2004)
Koppes & Montgomery Nature Geosci. (2009) This should not be surprising – we know that both rivers and glaciers are capable of eroding at rates that equal or exceed even rapid rock uplift rates (>1-10 mm/y)...
Koppes & Montgomery Nature Geosci. (2009)...and that these high rates can be sustained over geological time scales We now have a wide range of tools at our disposal to measure these rates (at scales from 10 0 to 10 8 y)
Stock et al. Lithosphere (2009) In detail, however, the relationships between rates of rock uplift and erosion remain a challenge to unpick Much depends on the time scale over which they are measured; short-term rates are perturbed by non-steady erosion processes and by (long) EQ recurrence intervals So what actually sets the rates that we measure?
The second major conceptual advance is to examine source-to-sink relationships in small, tractable systems that can be easily characterized over a range of temporal scales – such as catchments and their associated fans Key question: how sensitive are these systems to changes in tectonic or climatic boundary conditions, and over what time scales? Dolomite Canyon, California The answer to this determines their utility as a recorder of those changes (sedimentology, but also geometry, geomorphology...)
We have moved from prescriptive sediment routing models in which sediment flux q s is set arbitrarily (LEFT), to those in which the catchment and fan interact (RIGHT): fan sets base level for catchment, modulating erosion and sed supply sediment plus subsidence rate sets base level and filling pattern, etc. Marr et al. Basin Research (2000)Allen & Densmore Basin Research (2000)
These models typically suggest long (10 5 to 10 6 y) system-scale response times to perturbations in tectonic forcing. The landscape is thus perpetually ‘catching up’ to tectonic inputs Raises a set of questions: 1) How and where can we look for the signal of individual events if response times are so long? 2) What are the implications for building stratigraphy (Mohrig, Gupta talks)? Carretier & Lucazeau Basin Research (2005)
Where do we go from here? 1. How can we better read the record? Grain size of sedimentary deposits is a promising potential ‘fingerprint’ of past tectonic subsidence rates and sediment discharge (e.g. Fedele & Paola, 2007), if we can decipher it Duller et al. JGR-Earth Surf. (2010)
Bell et al. Basin Research (2009) Where do we go from here? 2a. What does the tectonic driver look like? Can we get the 3d tectonic defm field through the life span of an orogen? This absolutely requires the stratigraphic record (and a wide range of settings...)
2b. Can we move beyond single fault blocks to consider sediment pathways when many faults and sources are involved? (we’d better try!) Where do we go from here?
3. Can we apply our MARGINS-type joined-up thinking to a better characterization of hazards, given that they are focused in many of the geographical locations of interest to MARGINS research Where do we go from here? Non-seismic fatal landslides Dave Petley (Intl Landslide Centre/Durham University)
The co-occurrence of major geological hazards (especially the earthquake- landslide-tsunami trinity) remains a major challenge, both for 1) our physical understanding of long- term mountain building and sediment transfer (e.g., what was the net mass in the Sichuan earthquake?), and 2) disaster resilience and response This need not involve compromise: we must understand the magnitude- frequency distribution BOTH for sediment routing and for hazard, and hazard research will move (is moving) toward longer time scales to deal with the data gap
~50 MtC yr -1 eroded from mountain islands to oceans Compare to terrestrial biosphere re-growth after Last Glacial Maxium: ~ MtC yr -1 Silicate weathering atm CO 2 consumption: ~110 MtC yr -1 Oblique view of Taiwan (LANDSAT 7 - NASA Worldwind) 4. What about the carbon cycle? Marginal rivers are a potentially(?) large (~50 MtC yr -1 ? Recall Lou Derry’s ‘error bars’) source of carbon to the oceans; compare with Terrestrial biosphere re-growth after LGM: ~ MtC yr -1 Silicate weathering atmos CO 2 consumption: ~110 MtC yr -1 Where do we go from here? (Ittekkot, 1988; Meybeck, 1993; Gaillardet et al. 1999; Schlünz & Schneider, 2000; Broecker et al. 2001)
Western Southern Alps New Zealand 1 km Hilton et al. Nature Geosci. (2008) Very large floods dominate POC erosion and export Do earthquakes ‘liberate’ carbon from bedrock and vegetation and deliver it to the river network via landsliding... and if so, do the big events matter more?