Internally-drained basins, Ebro Basin

Post-tectonic erosional rebound of captured basins

[Published as Garcia-Castellanos & Larrasoaña, Geology, 2015]
Estimating the paleo-elevation and isostatic rebound of intramountain basins since after drainage opening. 

The animation to the right shows the calculated paleotopography of the Ebro basin since the time of drainage opening (endorheic-exorheic transition) until today. Blue lines indicate isostatic rebound (meters) in response to sediment removal by erosion. Lower panel shows a section along the dated places located in red. 

Basins formed within mountainous regions often become perfect sedimentary traps that do not drain to the sea but to internal evaporitic lakes. When they do, their sediment layers ideally record the climatic, topographic, and tectonic history of the surroundings. And when these basins eventually overtop or overfill with sediment, they are rapidly excavated by the new outflowing fluvial network, exposing excellent stratigraphic outcrops. However, this erosion often removes the uppermost basin infill, and essential information about the late basin history is lost. We have estimated the timing and elevation of the maximum infill of the Ebro basin (NE Spain) by computing the rebound of the basin in response to erosion, adopting the common idea that the Earth's rigid outer shell (the lithosphere) rests on a fluid magmatic asthenosphere in an Archimedes-type equilibrium (isostasy). We combine these calculations with existing paleomagnetic ages of the sediment basin infill. The results show that the basin became overfilled between 12 and 7.5 million years ago, and that it reached a maximum elevation of up to 750 m above present sea level. The basin has been ever since incised at a rate close to 0.1 mm/yr and has been isostatically uplifted by up to 630 m at its center. This uplift may explain why the Ebro River, opposite to other large Mediterranean rivers, does not present a deep gorge excavated within its own basin during the desiccation of the Mediterranean (Messinian salinity crisis, 5.5 million years ago).
Present and past Endorheic basins in the world. 

Drainage evolution of the Ebro Basin, TISC model

[related scientific publications: García-Castellanos et al., 2003, JGRUrgeles et al., 2010, Basin Res.; Garcia-Castellanos & Cloetingh, 2012]

The Ebro foreland basin (Pyrenees, NE Iberia) started forming about 65 million years ago, during the Palaeocene, as a result of the tectonic collision and subduction of the Iberian plate underneath the European plate. The basin became landlocked in late Eocene (37 Ma) by the Pyrenees, the Catalan Coastal Ranges (CCR) and the Iberian Range, and was subsequently filled with alluvial, fluvial and lacustrine sediments. During the Miocene, the Catalan Coastal Ranges underwent a tectonic extension that opened the Western Mediterranean. Although the topographic barrier constituted by the CCR was significantly reduced, the Ebro Basin remained closed at least until late Miocene times, when the endorheic fluvial system opened through the present Ebro River to the Mediterranean initiating the formation of the delta.

Ebro Basin numerical model

The causes for this major drainage changes are the interplay between erosion and climatic processes, since there is no evidence for tectonism coeval in space and time with the breakthrough of the Ebro River across the CCR. Accerelated CCR erosion due to the lowering of the sea level in the Mediterranean during the Messinian Salinity Crisis had been proposed as a possible triggering mechanism, but regional isostatic behaviour of the lithosphere cannot be dismissed, since it controls the vertical movements during both crustal compression and extension.

We found that basin overfilling with sediments and the changes in the hydrological balance in terms of precipitation/evaporation (which controlled the water level in the closed basin) were equally relevant for the final overspilling of the basin. We linked quantitative approaches to these processes in a 3D numerical model of drainage evolution constrained by well logs, geological cross sections and fission track data.

The results suggest that the capture of the endorheic drainage was driven by sediment overfilling of the basin and the erosion of up to 2-3 km at the Catalan Coastal Ranges rift flank, which flexural uplift significantly 

delayed the drainage opening.

Model evolution

The models shown here and in the publications are the result of developing and using the software TISC to quantitatively relate lithospheric, crustal and surface processes. The numerical model links quantitatively the drainage evolution with the tectonic evolution, using as a constraint the erosion deduced from isotopic composition and the sediment accumulation dferived from seismic and borehole surveys.

This animation shows a top view of the calculated drainage network and topography (top left), sediment thickness distribution (top right), and cross section as predicted by the computer simulation. Areas dotted in dark blue are lakes. Time is indicated on the top left corner. The horizontal movement of tectonic blocks is contrained from structural geology studies of the three mountain ranges sorrounding the basin.

Note that the overspill of the lacustrine basin is predicted at 10 Myr before present, approximately coinciding with the youngest lacustrine sediments found in the basin.