Continental-scale evolution of topography and river networks. Tectonics and climate shaping Eurasia

[This post is about our recent publication on PLOS ONE]

How much does the erosion and sedimentation at the Earth’s surface influence on the patterns and distribution of tectonic deformation? This question has been mainly addressed from a computer modelling perspective, at scales ranging from local to orogenic. In the PLOS ONE paper published today, we present a model that aims at understanding this phenomenon at the continental scale, looking at the feedbacks between continental enlargement and climate aridification during the collision of continents.

60-million-year indentation of a continent from the south 

(at 50 mm/yr). 

Left: Topography and areas with precipitation
higher than 400 mm/yr (red shading). Note the 

orographic rainfall developing at the southern flank 

of the growing plateau. Wind blows from the southeast 

(towards the upper left corner).

Right: erosion and sedimentation rates, and
contours of crustal thickening rate due to tectonics. 

By Garcia-Castellanos & Jimenez-Munt, PLOS ONE, 2015. 

+ info here.

We couple a thin-sheet viscous model of continental deformation with a stream-power surface transport model. The model also incorporates flexural isostatic compensation that permits the formation of large sedimentary basins and a precipitation model that reproduces basic climatic effects such as continentality and orographic rainfall and rain shadow. We calculate the feedbacks between these four processes acting at different scales in a synthetic scenario inspired and scaled by the India-Asia collision. The model reproduces first-order characteristics of the growth of the Tibetan Plateau as a result of the Indian indentation.

Note that the southern continent (the indenter, India)
is chosen fixed to our reference frame, whereas
 the northern continent (Asia) is moving southwards). 

The initial topography is flat with small random noise
forming a network of lakes. The tectonic indenter in
the southern boundary represents India, while a rigid
block fixed around x=2500km represents the Tarim Basin. 

Wind blowing from SE at 7 m/s (relative humidity=1).
Red shading indicates orographic precipitation.

The continental deformation adopts a thin-sheet tectonic
model. + info here.

What these simulations show is that, at large space and temporal scales, the climate dryness that develops in continental interiors triggers the trapping of sediment in closed basins within the continent, instead of exporting it to the continental margins. In the left panel you can see a large intramountain basin (comparable to the Tarim Basin) developing within Asia when a hard lithospheric region in predefined within the continent. The amount of sediment trapped in it is very sensitive to climatic parameters, particularly to evaporation, because it crucially determines its endorheic/exorheic drainage. We identify a feedback between erosion and crustal thickening leading locally to a <50 at="" climatically-enhanced="" concentrated.="" corners="" deformation="" flank="" growing="" in="" increase="" indenter="" is="" of="" orographic="" p="" place="" places="" plateau="" precipitation="" preferentially="" rates="" specially="" syntaxes="" takes="" the="" this="" upwind="" where="">

We hypothesize that this may provide clues for better understanding the mechanisms underlying the intriguing tectonic aneurysms documented in the Himalayas. At the continental scale, however, the overall distribution of topographic basins and ranges

seems insensitive to climatic factors, despite these do have important, sometimes counterintuitive effects on the amount of sediments trapped within the continent. The dry climatic conditions that naturally develop in the interior of the continent, for example, trigger large intra-continental sediment trapping at basins similar to the Tarim Basin because they determine its endorheic/exorheic drainage. These complex climatic-drainage-

tectonic interactions make the development of steady-state topography at the continental scale unlikely.

49 Open Challenges in Earth Science - The Known Unknowns

Mapping Ignorance

What keeps Earth scientists busy? These 49 open scientific questions aim at providing an updated, fully-referenced account of the main current scientific questions, disputes, and challenges in Geoscience.




[updated version in this link]

The Early Earth and the Solar System

Advances such as those occurred in the geochemistry of meteorites lead to new exciting hypotheses about the early stages of our planet, but as usual, answers are outnumbered by the new knowledge gaps: 

1. How did the Earth and other planets form? Were planets formed in situ? Or are orbital changes relatively frequent? What determined the different deep layering of the solar planets? [McKinnon, 2012, Science on Mercury] 
2. Was there ever a collision of the Earth with another planet Theia, giving birth to our satellite? [Canup, 2013, Science] There is compelling evidence, such as measures of a shorter duration of the Earth's rotation and lunar month in the past, pointing to a Moon much closer to Earth during the early stages of the Solar System. [Williams, CSPG Spec. Pubs., 1991]
3. What is the long-term heat balance of Earth? How did its internal temperature decay since it formed by accretion of chondrites? How abundant are radiogenic elements in the interior? Did a "faint young sun" ever warm a "snowball Earth"? [Wired; Marty et al., 2013, Science]
4. What made plate tectonics a dominant process only on Earth? [outreach paper] How did the planet cool down before plate tectonics?[Moore & Webb, 2013, Nature] Was the Earth's crust formed during the early stages of its evolution or is it the result of a gradual distillation of the mantle that continues today along with crustal recycling? Is the crust still growing or does its recycling compensate for crust formation at mid-ocean ridges and other volcanic areas?
5. How inherent to planetary evolution is the development of life conditions? [Zimmer, 2005, Science, Elkins-Tanton, 2013, Nature] Earth-like planets are now known to be abundant in our galaxy (two out of three stars may have one [e.g., Cassan et al., 2012, Nature]), but how many of them develop widespread durable water chemistry? How much of our water supplied by comets or asteroids? When and how did it reach the Earth? [outreach article]

Earth’s Interior

Our rock-sampling reach is limited to the upper 12 km of the Earth's crust, but the keys to extend our knowledge often lay far deeper than that. Indirect measurements such as seismic wave tomography, together with geodynamic and petrological modeling, become crucial: 

1. What are the chemical composition and mechanical properties of rocks in the Earth’s mantle at the extreme pressure and temperature they undergo? As planets age and cool off, their internal and surface processes coevolve, chemically and mechanically, shaping also the atmospheric composition. Therefore this question has direct implications for our understanding of the environmental evolution of the Earth. [Kerr, 2005, Science]
2. What are the dynamic processes in the Earth interior that accommodate and fuel plate tectonics? As seismometers spread more evenly over the planet's surface, the seismic imaging of the interior will rapidly improve, providing a detailed distribution of seismic wave velocity. Simultaneously, lab-based mineral physics must better constrain what these mechanical wave velocities tell us about the hot, deep rocks of uncertain composition in the mantle. Only then will computer models be able to test the proposed geodynamic models by trying to fit quantitatively those data and other geophysical observations such as gravity variations. [ref.3]
3. Sedimentary and volcanic rocks have recorded changes of the magnetic field throughout the evolution of the Earth. What causes the sudden reversals of the paleomagnetic field? How does the geomagnetic field link to the iron convection properties at the deep Earth? Or inversely, what can we learn about the mechanical behavior of the materials at those depths from the geomagnetic field? [more context in Nature] Are the magnetic reversals too fast to be related to core dynamics? [example.1] [ex.2] [ex.3] Could their frequency be related to the distribution of tectonic plates? [GRL article]. What causes superchrons (periods longer than 10 Myr without magnetic reversals)? Something internal to the core, or induced externally by the mantle/subducting slabs? Was the geomagnetic field always dipolar, or was it more asymmetric in the past? [introduction]
4. Are intraplate hotspots really made by deep sources of uprising materials (mantle plumes) coming from the deepest Earth's mantle? Or can they be explained by shallower convection? [e.g., Morgan, 1971; see also this recent Geology paper on Yellowstone].
5. What is the history of and what controls the excursions of the rotation pole relative to the surface geography, known as true polar wander? [ex]
6. What are the properties of deep rocks? How can we translate the heterogeneity in density, seismic wave velocity, or electromagnetic resistivity presently observed in the mantle and the lithosphere into variations of the mineralogical composition? And how do these measures relate to the dynamics of the Earth and to key mechanical properties such as the viscosity? [Faccenna & Becker, 210, Nature]. 
7. What are the causes for Large Igneous Provinces and massive flood basalts such as the Columbia River Basalts?

Tectonic-plate motion and deformation 

The successful adoption of the plate tectonics paradigm has lead to a myriad of new questions about its limits and about the lessons for risk mitigation.

Velocity of the earth's surface at the Indian-Asian collision,

from GPS data. Blue star indicates the 2008 China earthquake.

Source: CALTECH

1. What is the relative importance of the forces driving plate tectonics: slab pull, slab suction, mantle drag, and ridge push? [e.g., Conrad & Lithgow-Bertelloni, 2004; Negredo et al., GRL, 2004, vs. van Benthem & Govers, JGR, 2010]. What is the force balance and the geochemical cycle in subduction zones? [Emry et al., 2014, JGR] How much water (and how deep) penetrates into the mantle? [Ranero et al., 2003, Nature] How much subcontinental erosion takes place under subduction areas? [Ranero et al., 2000, Nature]
2. What happens after the collision of two continents? Does continental collision diminish the rate of plate subduction, as suggested by the slab-pull paradigm? [Alvarez, EPSL, 2010] How frequent are the processes of mantle delamination and slab break-off? What determines their occurrence? [Magni et al., GRL, 2013; Durezt & Gerya, Tectonoph., 2013]
3. Why are orogens curved when seen from space? [Weil & Sussman, 2004, GSASP 383]
4. How does the long-term deformation derived from paleomagnetism and structural geology link quantitatively to the present-day motions derived from GPS and from neotectonic patterns of crustal deformation? [Calais et al., EPSL, 2003] How do these last two relate to each other? [ref] Can we learn from regional structure of the crust/lithosphere from that link (or viceversa)? 
5. Are plate interiors moving in steady-state linear motion? How rigid are these and why/when did they deform? [Davis et al., (2005) doi:10.1038/nature04781, and Wernicke & Davis, (2010) doi:10.1785/gssrl.81.5.694]. 
6. How is the relative motion between continents accommodated in diffuse plate boundaries? (eg., the Iberian/African plate boundary). What determines the (a)seismicity of a plate contact? 
7. How/when does deformation propagate from the plate boundaries into plate interiors? [e.g., Cloetingh et al., 2005, QSR] 
8. What is the rheological stratification of the lithosphere: like a jelly sandwich? Or rather like a creme brulée? [Burov & Watts, 2006]. Is the lower crust ductile? Is strength concentrated at the uppermost mantle? Or just the other way around? [e.g., McKenzie et al., 2000, JGR; Jackson, 2002, GSA Today; Handy & Brun, 2004; and a nice recent post]. 

9. 

   Add caption

   Does the climate-controlled erosion and surface transport of sediment modify the patterns of tectonic deformation? Does vigorous erosion cause localized deformation in the core of mountain belts and prevent the propagation of tectonic shortening into the undeformed forelands? Does the deposition of sediment on the flank of mountains stop the frontal advance of the orogen? Is there any field evidence for these effects predicted from computer models? [Philip Allen's blog] [Willett, 1999, JGR; Whipple, 2009; Garcia-Castellanos, EPSL, 2007]
10. Can earthquakes be predicted? [Heki, 2011, GRL; Freed, 2012, Nat.Geosc.]. How far away can they be mechanically triggered? [Tibi et al., 2003, Nature]. Little is known about how faults form and when do they reactivate [ex.6], and even worse, there seems to be no clear pathway as to solve this problem in the near future. Unexpected breakthroughs needed. 
11. How can the prediction of volcanic eruptions be improved? What determines the rates of magma accumulation in the chamber and what mechanisms make magmas eruptible? [ex.7][ex.7b]
12. In many regions, the elevation of the continents does not match the predictions from the classical principle of isostasy for the Earth's outer rigid layer (the lithosphere). This deviation is known as dynamic topography, by opposition to isostatic topography. But what are the mechanisms responsible? Can we learn about the mantle dynamics by estimating dynamic topography? [ref.1] Can the hidden loads needed to explain the accumulation of sediment next to orogens (foreland basins) be linked to these dynamic forces? [Busby & Azor, 2012, Wiley]
13. How do land-forming processes react to climate change at a variety of scales, ranging from the Milankovitch cycles to the late Cenozoic cooling of the Earth? Is there a feedback from erosion into climate at these time scales, through the Carbon cycle and the weathering of silicates, for example? What is the role of the surface uplift and erosion of Tibet on the drawdown of atmospheric CO2 over the Cenozoic? [Garzione, 2008, Geology]

Earth's landscape history and present environment

The shape of the planet's solid surface, its topography, is the key feature that connects many of the disciplines within Earth science, probably because it is the feature that most affects our daily life. It is today common wisdom that landscape forms from a complex interplay between tectonics and climate, through a list of mechanical, chemical, and biological processes acting at the surface of the Solid Earth. Topographic data is becoming now available at resolutions finer than a few meters, and the sedimentary record is also being archived at unprecedented rates. But:

Drainage patterns in Yarlung Tsangpo River, China (NASA)

1. Can we use these data to derive past tectonic and climatic conditions? Will we ever know enough about the erosion and transport processes? Was also the stocasticity of meteorological and tectonic events relevant in the resulting landscape? And how much has life contributed to shape the Earth's surface? 
2. Can classical geomorphological concepts such as 'peneplanation' or 'retrogressive erosion' be understood quantitatively? Old mountain ranges such as the Appalachian or the Urals seem to retain relief for > 10^8 years, while fluvial valleys under the Antarctica are preserved under moving ice of kilometric thickness since the Neogene. What controls the time-scale of topographic decay? [Egholm, 2013, Nature]
3. What are the erosion and transport laws governing the evolution of the Earth’s Surface?[Willenbring et al., Geology, 2013] Rivers transport sediment particles that are at the same time the tools for erosion but also the shield protecting the bedrock. How important is this double role of sediment for the evolution of landscapes? [Sklar & Dietrich, Geology, 2001 (tools and cover effect); Cowie et al., Geology, 2008 (a field example)].
4. Can we predict sediment production and transport for hazard and scientific purposes? [NAS SP report, 2010; Geology, 2013] 



Smaller-scale patterns at the limit
between river channels and hillslopes.
Credit: Perron Group, MIT

5. What do the preserved 4D patterns of sediment flow tell us from the past of the Earth? Is it possible to quantitatively link past climatic and tectonic records to the present landforms? Is it possible to separate the signals of both processes? [e.g., Armitage et al., 2011, Nature Geosc]. 
6. Can we differentiate changes in the tectonic and climate regimes as recorded in sediment stratigraphy? Some think both signals are indeed distinguishable [Armitage et al., 2011, Nat.Geo.]. Others (Jerolmack & Paola, 2010, GRL], argue that the dynamics intrinsic to the sediment transport system can be 'noisy' enough to drown out any signal of an external forcing. 
7. Does surface erosion draw hot rock towards the Earth’s surface? Do tectonic folds grow preferentially where rivers cut down through them, causing them to look like up-turned boats with a deep transverse incision? [Simpson, 2004, Geology]
8. How resilient is the ocean to chemical perturbations? What caused the huge salt deposition in the Mediterranean known as the Messinian Salinity Crisis? Was the Mediterranean truly desiccated? What were the effects on climate and biology, and what can we learn from extreme salt giants like this? [e.g., Hsu, 1983; Clauzon et al., Geology, 1996; Krijgsman et al, 1999, Nature; Garcia-Castellanos & Villaseñor, Nature, 2011]. Were the normal marine conditions truly reestablished by the largest flood documented on Earth, 5.3 million years ago? [Garcia-Castellanos et al., 2009, Nature]

   

   Artistic view of the refilll of the

   Mediterranean after the Messinian salinity

   crisis. Authors: Pibernat & Garcia-Castellanos.

   Downloadable here.

9. How do the patterns of river networks form? [eg. Devauchelle et al., 2012, PNAS; Perron et al., 2012, Nature]. And what information about the past do these patterns contain? Can we quantitatively reconstruct past ecology or climate from old river patterns? [e.g., Hartley et al., 2010, J. Sedim. Res.]
10. Do we need a new geological epoch called Anthropocene? When do the Homo Sapiens start to have a significant impact on the Earth System? 8000 BP?[Ruddiman, 2003, Climatic Change]; 2000 BP? [Scalenghe, 2011, The Holocene]; 1850 AD? [Crutzen & Steffen, 2003]

Climate, Life, and Earth

Source: R.E..Rhode, Wikipedia

The geological record shows that climate is relatively stable over tectonic time-scales whereas it undergoes abrupt changes in periods ranging from decades to hundreds of thousand years. Past periods when the planet underwent extreme climate conditions may help to understand the mechanisms behind that behavior and its significance for the evolution of the Solid Earth.

1. What caused the largest carbon isotope changes in Earth? [Grotzinger et al., 2011, Nat. Geosc.] How does Earth’s climate system respond to elevated levels of atmospheric CO2? 
2. Was there ever a snow-ball Earth during the earliest stages of Life on Earth? 
3. Were there also rivers and lakes on Mars? [Hans, 2012] Were there large outburst floods similar to those on Earth? 
4. What were the causes and what shaped the recovery from mass extinctions as those at the K-T boundary, the Permian-Triassic or the Late Triassic? Massive volcanism? Meteorites? Microbes? [recent papers: ex.8, ex.9, ex.10, Rothman et al., 2014, PNAS]
5. What triggered the extreme climatic variability during the Quaternary and the roughly coeval acceleration in continental erosion and sediment delivery to the margins? [Peizhen, Molnar et al., 2001, Nature; Herman et al., 2013, Nature] Was this related to the tectonic closure of the Central American Seaway? How do these climate events translate quantitatively into sea level changes?
6. How do climate changes translate quantitatively into sea level changes? How do ice sheets and sea level respond to a warming climate? What controls regional patterns of precipitation, such as those associated with monsoons or El Niño?
7. What caused the Quaternary extinction(s)? Human expansion? Climate Change? How sensitive are ecosystems and biodiversity to environmental change? Was the large fauna extinction ~13,000 yr ago a result of the Younger Dryas climatic event? Was this caused by an extraterrestrial impact? [ex.11, ex.12] Or may it be linked to the outburst of Lake Agassiz? 
8. How relevant are subsurface microorganisms to earth dynamics by controlling soil formation and the methane cycle? What are the origin, composition, and global significance of deep subseafloor communities? What are the limits of life in the subseafloor realm?  
9. The atmosphere is shaped by the presence of life, a powerful chemical force. The Earth’s evolution has seems to affect the evolution of life [see the Cambrian explosion of animal life, for instance; plus this recent paper on that]. To what extent? And how much control has life on climate? [another recent one]. Is it possible to quantify these links to make reliable predictions that allow filling the data gaps or assessing the chances for extraterrestrial life?
10. How much of the present climate change is anthropogenic? How will growing emissions from a growing global population with a growing consumption impact on climate? 

Broader open questions

1. Many of the questions above are related to the extreme diversity of spatial scales of Earth processes. Direct observation (by sampling or remotely) is mostly limited to a thin layer around the solid surface of the Earth, and physical experimentation is limited to the pressures of the uppermost layers of the planet. Many processes including plate tectonics are known to be driven by the nature of the materials that make up the planet interiors, down to the smallest atomic scales, as thought for instance for the trigger of earthquakes. Answers may arrive via new devices and analytical tools working at the high pressures and temperatures of Earth’s interior.
2. Time scales also pose a problem to know the mechanical and chemical properties of Earth's materials. Partly because we deal with time scales in very different orders of magnitude while we are limited to make observations from the present. But also because scaling the rates of lab experiments (e.g., mineral physics) or analogue models (e.g., sandbox experiments) with the corresponding geological scenarios is not always convincing. 
3. Implementing Episodicity in Gradualism: For historical reasons, geology has generally underestimated the role of episodicity in nature. However, there is a growing view that exceptional events and stochasticity have a relevant role in many of Earth's subsystems. An example for this is the preeminence of flooding events (larger than average) in erosion and surface sediment transport and during the evolution of landscape, and the importance of upscaling flood stochasticity into sediment transport models [eg., Lague, 2010, JGR]. Climate variability at all time-scales has been already mentioned above. Even plate tectonics may have been episodic (during the Archean at least, [ref]).  4D hyperscale data sets in geomorphology are increasingly showing the limits of smooth-process approaches. Future understanding of the Earth will benefit from incorporating the full frequency spectrum (the episodicity) in modeling natural phenomena, rather than systematically approaching these as gradual processes. 
4. Computer models tell whether the complexity of nature can be explained by the interplay between simple processes, but: how can we further model the Earth as a complex system of complex systems? And when can we expect ‘compact’ explanations? 

General background:

Note that the specific references given above for each open question are sometimes just examples and may not always be the best representative. Furthermore, the list is surely biased towards Solid Earth Science, my own field. The following general references can give you an alternative perspective on the subject.

• BESR (2008). Origin and Evolution of Earth: Research Questions for a Changing Planet The National Academies Press DOI: 10.1002/gj.1188
• Science 125th anniv. compilation.
• NAS report Origin and evolution of Earth [pdf] [html]
• NAS report on surface processes
• Earth Magazine 2012 article.

Please send additions/suggestions to d.g.c@csic.es

Acknowledgements:
For various inputs/criticisms to this list: Brian Romans, Umberto Lombardo, Jean Braun, Mikael Attal, Alexandra Witze, Michael Klaas, Matt Hall, Chris Rowan.

PD: I published a shorter version of this post in Mapping Ignorance. -Daniel

Steady-state topography during orogenesis: Erosion vs. Tectonics competition

I uploaded to Youtube a simple but interesting numerical model. It computes a constant tectonic uplift with the competing river erosion along a cross section, allowing enough time to reach two successive topographic steady states (one during uplift, another one after uplift):

Steady state topography development, and other parameters of the model.

Uplift occurs between x=-50 and x=+50 km. The dashed line indicates 

the topographic profile that would develop in absence of erosion. 

Equilibrium topography is reached at ~4 and at ~8 Myr.

This is calculated assuming constant uplift rate at the center (x=-50 to +50 km) and a 1D stream power law erosion model. Uplift rate is 1 mm/yr and stops at t=5 Myr. River erosion is proportional to slope and water discharge. Precipitation rate is constant over the entire profile. Calculations are performed under Linux with the program tAo (Garcia-Castellanos, 2007, EPSL). +info and software download here: https://sites.google.com/site/daniggcc/software/tao

As you can see, topographic growth goes on until a first steady state (with a maximum topography of ~3000 m) is reached before 5 Myr. If you look at the numbers, you'll see an equilibrium between erosion rates and uplift rates at that time. At 5 Myr uplift stops and then erosion leads to the new equilibrium: a flat topography (at 8 Myr).

Now, the question is: does steady-state topography exist in nature? And if it does, can we recognize it? In real Earth, neither climate nor tectonics are constant through time. The questions are probably too big for this small blog, but you can find some hints in this article by Willett and Brandon (2001).

Nevertheless, the notion of steady-state topography is useful to understand some basic principles of orogenesis, as Whipple (2009) showed in a very simple and elegant way. Consider these two end-member types of orogen:

Evolution of for parameters (orogen width, erosion, topography, and rock uplift) for two simple models of orogenic growth. Left: fixed width orogen; Right: Self-similar growth. At t=0, the erosion coefficient is set to a double value (red) and to half of the reference value (green). Erosion is assumed proportional to elevation. The parameters are shown normalized. Redrawn from Whipple (2009).  

Assume both orogens grow in response to the convergence of two tectonic plates, producing a constant tectonic flow Fa, and that they are eroded at a rate proportional to elevation. The red lines in the figures above correspond to a change to double erosion efficiency. With such a simple representation, it becomes clear that if erosion mechanisms become more efficient, both orogen types initially undergo an increase in erosion rate, but this will gradually decrease back to the initial erosion value (the one compensating the imposed tectonic flow, as in the animation above). The way the orogen returns to the original low erosion rate is by decreasing its elevation R.
One interesting thing is that, whereas for a fixed width, rock uplift rates return to normal after some time, the self-similar growth predicts a permanent increase in uplift rates.
And the other interesting conclusion is that the time response is controlled mainly by the erosion efficiency itself (within the approaches of the model, of course).

Simple models are generally more inspiring than the most complex ones.

References:

Whipple, K. (2009). The influence of climate on the tectonic evolution of mountain belts Nature Geoscience, 2 (2), 97-104 DOI: 10.1038/ngeo413

Willett, S., and Brandon, M. (2001). On steady states in mountain belts Geology, 30, 175-178