Insights From Low Temperature Thermochronometry Biology Essay

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Sections 1.1. and 1.2. have been largely taken from the precursor proposal. Section 1.3. highlights the contributions from this project to our understanding of the orogenic growth of the Atlas Mountains.

The high elevation of active convergent mountain belts represents a balance between the tectonic processes that create topography and erosional surface processes that destroy it. Foremost among these feedback mechanisms is the dependence of erosion rate on relief and elevation. Increased relief in mountain belts leads to enhanced erosion through the relief dependence of major erosional mechanisms such as fluvial incision and mass wasting (Willet and Brandon, 2002). However, the understanding how internal (tectonic) forces act to shape the Earth's surface requires knowledge of long-term changes (Willet 1999; Koons 1989; Montgomery and Brandon, 2002). Burbank et al. (2003) for example deny that precipitation has a major role in erosion in the Himalaya and suggest that tectonically forced upward movement is the most important factor affecting erosion across a region of such different rainfall.

Low-temperature thermochronometric methods, e.g. fission-track on apatite and zircon, (U-Th)/He on apatite (see review in Reiners et al. 2005) are commonly used to quantify denudation rates from rock now exposed at the surface from active mountain belts. Temperature increases with depth in the Earth and, at high temperatures, noble gases emitted in radioactive decay diffuse away; defects (tracks) in crystals, produced by the charged particles expelled in nuclear fission, anneal. By measuring the concentrations of such gases or tracks, one can date when the sample cooled below a temperature at which diffusion or annealing becomes very slow. The cooling ages provide an estimate of the average exhumation rate if combined with a geothermal gradient, which in these cases equals the rate at which material above the sampled rock has been eroded. Such estimates apply to periods as short as 500,000 years to as long as several million years. But in all cases they span several glacial and interglacial cycles, smoothing out the effects of large climatic changes (Molnar 2003). This approach on bedrocks from the hinterland is sometimes limited since erosion has often removed the record of earlier stages of orogenic growth and the methods employed are not sensitive to detect more recent short-term changes (<106 yrs).

To overcome this shortcoming, researchers have increasingly begun to study orogenic sedimentary records (see review in Ruiz et al. 2004) and use exposure age dating to surface rocks and soils to monitor erosion rates over timescales of 104 - 106 yrs (Lal 1991; Anderson et al. 1996; Repka et al. 1997; Gosse and Phillips, 2001; Bierman and Nichols, 2004), and at the scale of entire river catchments on detrital quartz (Schaller et al. 2001; von Blanckenburg, 2005). A possible alternative is to combine these two approaches in one to constrain long and short term denudation rates. Such novel approach can be rapidly developed and is consequently proposed for this project to rapidly constrain past and present differential denudational history across the Atlas Mountains in SW Morocco.

1.2. Geological setting

The Moroccan Rif-Atlas system is composed of four different mountains ranges: Rif, Middle-Atlas, High-Atlas (HA) and Anti-Atlas (AA). The Rif is part of the Mediterranean Alpine system, it is the southwest expression of the collision between the African and European plates due to the closure of the Tethys Ocean. The Atlas chains developed from the inversion of a Triassic-Jurassic rift or transtensional basins installed on a thinned continental crust as a consequence of continental convergence between Africa and Europe during the Cenozoic (Mattauer et al., 1977; Laville 1985; Jacobshagen et al., 1988; Giese and Jacobshagen, 1992; Laville and Piqué, 1992; Beauchamp et al., 1996; Gomez et al., 2000; Frizon de Lamotte et al., 2000; Piqué et al., 2002; Teixell et al., 2003; Arboleya et al., 2004). In this chain, Palaeozoic basement massifs crop out in the highest portions of the chain, where they reach altitudes of more than 4000m a.s.l. The AA adjacent to the south of the HA is a deeply eroded remnant of a late Variscan fold belt which involved a thick series of Palaeozoic sediments. It has a high relief (>2500m) towards the HA with a topographic high that is constituted of three volcanic episodes at 11-10 Ma, 8-6.5 Ma and 6-3 Ma (Berrhama 1989; de Beer et al., 2000). Plutonics and volcanic rocks define today most of the northern part of the AA and are Precambrian in age. This chain was inverted in Carboniferous-Permian times but some renewed uplift took place in the Neogene, possibly in response to the Africa - Europe collision (Caritg et al., 2004; Helg et al., 2004; see section 2).

The prominent tectonic lines are presently discrete, reverse or thrust faults, particularly along the borders of the HA belt. They are partly responsible for the thickening and high elevation of the belt. Along the northern and southern borders of the HA the thrusts have opposite vergence (Fig. 1; Beauchamp et al. 1999; Hafid et al. 2000; Teixell et al. 2003). The restoration of the geological cross-section yields a moderate amount of shortening for the Atlas Mountains, from 15% to 24% along transects of the HA (Teixell et al., 2003) and accordingly crustal thickening beneath them is limited, in contrast with the elevated topography (Arboleya et al., 2004).

Phases of exhumation

Despite the wealth of data regarding the geology and tectonics of the HA, important tectonic questions remain unsolved and the period of denudation largely terra incognita. Intriguing open questions include the compressional structural style, the timing and quantification of the episodes of exhumation, the origin and timing of the elevated present-day topography, and the scarce development of foreland basins (Frizon de Lamotte et al., 2000; Teixell et al. 2003).

1) Three research with whom we collaborate currently focus their study on the presence of such relief in the Middle and HA but also in the Rif region. The moderate amount of shortening across the HA suggests that the elevated present-day topography cannot be explained by crustal shortening alone (Teixell et al. 2003) but must be partly due to a recent and large scale uplift (Babault et al., in press). A straightforward correlation between topography and Bouguer anomaly does not exist in the HA; topography in the Atlas is supported by a relatively thin crust that seems insufficient for the observed mountain load (Ayarza et al., 2005). Modelling shows a very thin lithosphere (i.e. a thermally anomalous mantle) that may be a contribution to the uplift of the system but cannot explain more than one third to half of the observed topography (Missenard et al., 2006). According to Teixell and others (2005), the structure of the lithosphere beneath the High Atlas is probably the result of a mantle upwelling influenced by the thickening of the lithosphere at the adjacent Iberia-Africa plate boundary in the Rif region.

2) Foreland basins are poorly developed. Syn-orogenic sediments are usually restricted to few hundred metres or are absent to the south in the a) elongate Neogene Souss and Ouarzazate foreland basins that all hosts the erosion products of Tertiary exhumation of the HA and AA (Fig. 1; Görler et al., 1988; Frizon de Lamotte et al., 2000; Benammi and Jaeger, 2001; el Harfi et al., 2001), and to the north of the HA in b) the Haouz basin (Dutour and Ferradini, 1985; Petit et al., 1985).

3) The first evidence of inversion within the Atlas domain is of Late Cretaceous age, as shown by Hafid (2000) and Hafid et al. (2000) in the extension of the Western HA under the Atlantic Ocean. Tertiary compression and inversion tectonics is recorded by syntectonic sedimentation within the Souss, Ouarzazate and Haouz basins. Two distinct tectonic events are distinguished and only insufficiently quantified (see review in Frizon de Lamotte et al. 2000). The first one occurred in the Late Eocene-Early Oligocene (Beauchamp et al., 1999), generating folding, thrusting and uplift. The second episode, Late Neogene in age, produced E-W-trending folds and thrusts and a general uplift (Frizon de Lamotte et al. 2000). U-Th dating on fossils from uplifted marine terraces in the region of Agadir to the SW documents an ongoing uplift from Pleistocene to recent (Meghraoui et al., 1998).

The present-day elevation of the AA could be at least partly due to the up-warping of the Precambrian and Palaeozoic of the Sahara platform in association with recent volcanism but most likely to the inversion tectonics occurring in the Neogene (Giese and Jacobshagen 1992; Beauchamp et al., 1999; Frizon-de-Lamotte et al., 2000). The total amount of uplift/erosion in the Anti-Atlas is ill constrained (Helg et al., 2004).

1.3. Previous work of our group on the orogenic growth of the Atlas Mountain

The actual project benefits from the strong collaborations we developed since 2006 with the Scottish Universities Environmental Research Centre (SUERC-Glasgow), the universities of Cergy-Pontoise, Casablanca, Barcelona and Rennes (Prof. J. van den Driessche). Such collaborations allowed us to rapidly export our expertise (in the quantification of vertical movements) to the Atlas Mountains and as a result to be co-authors of different publications (Missenard et al., in press.; Saddiqi et al., subm.; Sebti et al., subm.; Babault et al., subm.) and this after 1.5 year only. Furthermore, an unpublished dataset concerning Illite Crystallinity (FNS projects 21-52516.7 and 20-63790.00, Martin Burkhard) was compiled and combined/compared with 1) preliminary U-Th/He data from the Anti-Atlas results, but also 2) structural data from the Ph.D thesis of Charles Robert-Charrue (2006) to discuss the burial history of the Anti-Atlas region (Ruiz et al., in press.).

Figure 1. Morphotectectonic map of SW Morocco - High and Anti-Atlas domains (modified from Proust and Tapponier, 1973). SAF, NAF, AAF, SSAZ, NSAZ: Sub-Atlas Front, North Atlas Front, Anti-Atlas Fault, Southern Sub-Atlas Zone, Northern Sub-Atlas Zone. Red lines: sections along which samples were collected for thermochronological analyses. Thick black line: topographic profile illustrated in Figure 2.

Details

We have now a large sample collection of 'basement' rocks across the Atlas Mountains of SW Morocco (50). The samples selected in 2006 and 2007 were treated routinely for mineral separation. Some of the samples did not have any, or very few, apatite grains. Thus, we decided to dedicate these few grains to (U-Th)/He thermochronometry (80-45°C) which requires not more than 5-10 grains. (U-Th)/He dating on apatite (AHe, triplicates) was carried out on 14 samples at the SUERC in Glasgow and recoil corrections were made. We decided to wait for the outcome of the (U-Th)/He analyses before producing Apatite Fission Track (AFT) ages that correspond to higher temperature of closure than AHe, i.e. 120-60°c versus 80-45°C respectively. However, 15 samples are ready for both zircon and apatite Fission Track dating. We are waiting for the last batch of samples we selected in November 2007) to be ready before sending them all for irradiation in order to reduce cost (shipping and irradiation).

The spread of the AHe ages is important as ages range between 125 and 10 Ma. If we place this dataset into a schematic section of the Atlas system (Fig. 2), some striking signatures can be extracted as summarised below.

High-Atlas

(U-Th)/He ages on apatite along the southern flank of the axial zone of the High-Atlas range between 10 (bottom, alt. of 960m) and 20 Ma (top, alt. of ~2000m, Fig. 2). Through an age-altitude plot it appears that the 20-12 Ma period was characterized by rapid denudation rate (between 1 and 0.2mm/y) in this part of the Axial Zone (Fig. 1 & 2).

The onset of this phase is not constrained yet as no clear kink appears in the age-altitude plot we produced. Future work on the samples selected in November 2007 further up-sequence (up to 2800m altitude) should solve this problem.

Across the northern flank, a corresponding profile was sampled and partly analysed. Preliminary AHe data suggest that an Oligocene period of exhumation may have prevailed across the northern flank of the axial zone

Figure 2. NNW-SSE oriented topographic profile (Fig. 1) against the strike of the Atlas chain in SW Morocco with sample locations and associated (U-Th)/He weighted mean age on apatite (AHe). Thick grey line: general trend of AHe data. Bottom horizontal distance in kilometres (~165 kms) with major reverse fault systems (black arrows). Right: altitude in meters. Left: AHe ages in millions of years. Top: structural domains from north to South: the Haouz Basin, the High-Atlas, the Souss Basin, the Anti-Atlas and finally the Sahara platform. NSAZ, SSAZ, NAF, AMF, SAF, SSAF, EKF: Northern Sub-Atlas Zone, Southern Sub-Atlas Zone, Northern Atlas Front, Al Medina Fault, Sub-Atlas Front, South-Atlas Fault, El Klea Fault.

Anti-Atlas

Two samples (three aliquots) were dated. AHe age range between 98 and 127 Ma considering two sigmas error bars (Fig. 2)

There are two possible interpretations of these young ages: a) the Anti-Atlas was not affected by the 'Alpine' orogeny. b) The Anti-Atlas was affected by the Alpine orogeny, but because erosion rates were and are low in the region, no level is yet exposed that passed through the annealing zones of our thermochronological methods.

Differential movements cross the main fault systems

AHe ages in the hanging wall of the Sub-Atlas Front (SAF) range between 20 and 10 Ma (see above) whereas AHe age from the substratum of the Southern Sub-Atlas Zone (SSAZ), which defines the footwall of the SAF, are older and range within a single sample between 31 and 23 Ma (Fig. 2)

Additional data (AHe and Fission-Track on apatite as well as on zircon) are required to constrain through time-temperature paths the onset of differential movements across the main fault systems.

Figure 3. The Southern Sub-Atlas Zone (SSAZ) - structures and sample locations

Denudation and sedimentation

Important results were obtained from basal Cretaceous series located above the substratum (Palaeozoic series) of both Sub-Atlas Zones. AHe ages range between 31 and 27 Ma for the sample located in the southern zone whereas they range between 29 and 11 Ma for the sample from the northern zone (sample 06GR26, Fig. 3). Obviously, the detrital character of the AHe ages was not preserved any more, implying total resetting to temperatures higher than or in the range of 80°C.

Sedimentary burial can be the only cause of this resetting as nappes stacking can be excluded on the base of previous structural work. Using a geothermal gradient of 25°C, these results suggest that a 2.5 to 3km thick pile of sediments was removed from both Sub-Atlas domains. The precise timing of the cannibalization of this former sediments is not well constrained yet but an early start in the Oligocene can be assumed on the base of the oldest AHe age we produced. These two samples are 30km apart, so the question arises if there was an Atlas chain before the Oligocene? We think that probably not but additional data are required.

Additional basal Cretaceous and Eocene series were thus sampled in November 2007 notably in the flat lying Souss basin - a flat plain between the HA and AA, where blind thrusts (e.g. the El Klea Fault, Fig. 2) exhume (When?) basement (Palaeozoic schists) and Cretaceous levels. Results will be combined with long-term denudation rates from the High-Atlas regions to tentatively balance denudation in the High Atlas with sedimentation in the present day (Souss and Haouz plains) and past (Sub-Atlas zones) basins. Collaboration with the GB Oil Company should allow us soon to have access to the re-interpretation of former seismic lines in these regions.

Sub-recent denudation rates - geomorphology

One very interesting object was studied with partners from the Universities of Barcelona and Rennes: a WSW-ENE elongated high plateau (~2800-2600m) so called the 'Tichka' plateau just to the north of the Tichka pluton (Fig. 1). This plateau is preserved in the axial zone of the High Atlas, and bordered to the south by summits of 3400-2700m altitude. To the south of the plateau/summits, there is a transverse (~N-S) drainage basin (linked to the Souss basin) that is perpendicular to the drainage of the Tichka plateau. Interestingly, bedrocks of 1) the plateau, 2) its borders but also 3) the transverse drainage basin, are all plutonic series of Hercynian age. Hence, any lithological effect on exposure ages can be excluded. This pluton hosts the vertical profile (for low-temperature thermochronological analyses see above) we performed on the southern flank of the High-Atlas along the transverse drainage basin. To constrain short-term denudation rates, we selected two present-day river sands from both the High Plateau (longitudinal catchment) and from the transverse catchment (with identical drained area). We will perform 10Be analyses to quartz grains from these sands. Once the 10Be analyses (results are expected for June 2008) completed, we will be able to compare long and short-term denudation rates but also, which is quite novel, to constrain the rate of retreat of the crest (summits) before the southern and transverse river captures the catchment of the northern and WSW-ENE oriented river.

Illite Crystallinity patterns in the Anti-Atlas of Morocco (Ruiz et al., in press.)

[Abstract] - The low grade metamorphism of the sedimentary cover of the Moroccan Anti-Atlas is investigated using the Illite Crystallinity (IC) method. More than 200 samples from three key areas (southwestern, central and eastern Anti-Atlas) have been taken from a maximum of different stratigraphic levels and have been analyzed. The metamorphism is of low to very low degree throughout the southern flank of the Anti-Atlas. It increases from northeast to southwest. Whereas in the eastern Anti-Atlas diagenetic and anchizonal IC-values are predominant, in the Western and central Anti-Atlas also epizonal IC-values are found. In every respective area the IC improves with stratigraphic age. At the scale of the entire Paleozoic Anti-Atlas basin the IC correlates best with estimated paleo-overburden. However, burial metamorphism cannot be the cause even though considering missing sedimentary pile of Late Carboniferous age. The 'abnormal' paleo-geothermal gradient' of 43-35°C/km we evidenced for the Carboniferous is a true one, and has to be related due a basement sequence enriched in heat producing elements such as series of the West African Craton.

2. For each applicant, elaborate on the research fields. Please mention the most important publications

2.1. Fission track analysis (G. Ruiz, Neuchâtel; R. Wieler, ETH-Z)

The research group in Fission-Track (FT) thermochronology based at ETH Zürich (Dr. D. Seward, Prof. J-P- Burg) is well experienced and has a long and proven record of research across a wide range of geodynamic settings. The laboratory is well equipped and has modern computer systems for data analysis and modelling. The current research focuses on improvements in methodology, orogenic and detrital studies based principally on the Andes (Ecuador, Peru), Himalaya, Alps, and the north-western European margin.

Zircon fission-track ages of detrital grains from sediments of the Siwalik basin on Pakistan presented by Cerveny and others in 1988 have been reinvestigated using a revised methodological approach (Ruiz et al., 2004). Results show that steady-state evolution has not always existed; exhumation in the source regions has varied since ~ 20Ma with a major pulse between 12 and 10.9 Ma (Ruiz and Sewrad, 2006). This study suggests that movement along the Main Boundary Thrust was the cause of this event. Earlier studies suggested that at this time the source of the sediments was the presently outcropping Kohistan Arc. We are able to demonstrate that this cannot be so and instead, propose the rapidly exhuming Nanga-Parbat Haramosh syntaxis as the source area (>2 mm.y-1).

In a more recent study, (funded by SNF ref. Spikings et al., 2000, 2001, 2004 & 2005), we have concluded that the subducting Carnegie Ridge may have had some influence on the exhumation pattern and relief development in the Eastern Cordillera, Ecuador. The Ph.D thesis of G.M.H. Ruiz was carried out in the northern Subandean Zone of Ecuador east of the Cordillera Real. Fission track analysis was applied to the basement for thermotectonic information but also across main fault systems (Ruiz 2002; Ruiz et al., in prep.). This study brought an advance in detrital thermochronology for the geoscientific community (Ruiz et al., 2004) but also confirmed rates of increased orogenic activity in the source regions (Ruiz et al., 2004; Ruiz et al., 2007). The associated publication was awarded by the Basin Research journal (Ruiz et al., 2004) as one of the best three of the year.

More recently, Dr. G.M.H. Ruiz studied the topographic evolution of the Eastern Cordillera (Ruiz and Andriessen, in press.), Western Cordillera (Ruiz and Carlotto, in press) and Subandean Zone system in southern Peru. In this study, Dr. G.M.H. Ruiz combined two different approaches 1) Fission-track analysis on bedrocks from the Eastern Cordillera, and 2) detrital Apatite Fission-Track (AFT) analysis on recent deposits. This approach allows long and long-terms denudation rates to be constrained and compared with direct constrain from bedrocks. A vertical profile in the foothills of the Eastern Cordillera suggests that denudation reaches 1.7km/my (Ruiz and Andriessen, in press) whereas such rate was never reached since at least 35 Ma in the WC. Meanwhile, a present-day river sand at the termination of the drainage basin along which the age-altitude plot was performed was dated though AFT analysis. The youngest population or P1 was incorporated into the calculation of Garver et al. (1999) and confirm that the long-term mean denudation rate is 1.5km/my since ~4 Ma. This rate is one of the highest produced so far and comparable to the ones obtained by Guyot and others (1999) in the Bolivian foothills.

2.2. Noble gas and (U-Th)/He (R. Wieler, S. Ivy-Ochs -ETH Zurich; F. Stuart - SUERC)

The Institute of Isotope Geology and Mineral Resources at ETH Zurich has been engaged in surface exposure dating with cosmogenic noble gases for about 15 years. Most of these studies are being carried out in national and international collaborations (e. g. Profs. C. Schlüchter, Univ. Bern; F. Schlunegger, Univ. Bern; C. Baroni, Univ. Pisa; J. Masarik, Univ. Bratislava; Dr. J. Schäfer, Lamont Doherty Earth Obs.), as well as in close cooperation with the Accelerator Mass Spectrometry group at ETH (Drs. S. Ivy-Ochs and P. W. Kubik). One major focus of our research is to establish chronologies of Quaternary glaciations, e. g. in Antarctica and Tibet (Bruno et al., 1997; Schäfer et al., 1999, 2000, 2002; Oberholzer et al., 2003, 2005). In another project, we studied the erosional regime and its change as a function of climate in the Atacama Desert, one of the driest areas in the world (Kober et al., 2005). We are also engaged in methodical studies, exploring on the one hand the utility of less commonly used minerals for surface exposure dating (Kober et al., 2005). On the other hand, we try to improve the knowledge of production rate systematics of cosmogenic nuclides in terrestrial samples (Masarik et al., 2001, 2003). A large part of these activities centre around the CRONUS-EU initiative, a joint effort of about 10 research teams in Europe, funded by the Marie Curie program of the EU. Our major contribution here will be the determination of production rate variations as a function of altitude and geomagnetic latitude by means of noble gas analyses in artificial quartz targets in UHV containers.

In recent years, we have - in collaboration with Dr. D. Seward, ETH Zürich - also developed a small program in (U-Th)/He thermochronology, combined with fission-track dating (Wipf 2005) to study the effect of the subduction of the Nazca ridge on denudation rates in the Coastal Cordillera of Peru.

2.3. Radionuclides (S. Ivy-Ochs)

The ETH/PSI accelerator group under the leadership of Prof. M. Suter is centred on one of largest and most versatile 6MV tandem accelerators in the world. The facility is noted as one of the first world-wide to convert to dedicated accelerator mass spectrometry (AMS) for the Environmental and Earth Sciences. In this context, members of the AMS group participate and initiate research pertaining to a broad spectrum of long-lived radionuclides since 1991 with particular interest for geological studies are 10Be, 14C, 26Al, 36Cl. Since then, the research group at the ETH/PSI tandem facility has played a key role in the development of the use of cosmogenic radionuclides to examine a broad range of geological and geomorphological questions. 10Be and 26Al concentrations measured in bedrock and clasts from the Dry Valleys region of Antarctica gave the first conclusive evidence to show that the East Antarctic Ice Sheet has been a stable feature since at least the late Miocene (Ivy-Ochs et al., 1995; Bruno et al., 1997) and therefore could not have melted down catastrophically in the late Pliocene (e.g. Webb and Harwood, 1987). The extremely high 10Be concentrations we measured, together with the extremely old exposure ages (> 5Ma) demonstrate that these are among the lowest erosion rates ever measured on Earth (cf. Cockburn and Summerfield, 2004). Upon the heels of the first few papers that showed the possibilities of catchment-wide denudation rate determinations using 10Be measured in river sand (e.g. Granger et al., 1996), the Zurich AMS research group (Dr. P.W. Kubik) became involved in the establishment and verification of the technique in collaboration with Prof. F. von Blanckenburg (Hannover) (then with the group of Prof. Jan Kramers at Uni Bern). This led to the publication of two landmark papers by Schaller et al. (2001; 2002). The latter won the AGU award for best student paper of the year. More recently, members of the ETH/PSI accelerator group (Dr. S. Ivy-Ochs and Dr. H.-A. Synal) worked in Chile (collaboration with Prof. F. Schlunegger-Bern), the data of Kober et al. (2006) measured in quartz from Miocene ignimbrites point to a clear orographic signal as the dominant control on denudation rates across a west to east transect in northern Chile.

In summary, the ETH/PSI AMS group has more than 15 years of experience in terrestrial cosmogenic nuclide research in topics ranging from exposure dating to fluvial incision rate determinations to catchment-wide denudation rate assessments.

2.4. Structural Geology - Tectonics, regional Geology of Morocco (University of Neuchatel)

The Geology Institute at Neuchâtel University has a long tradition in studies dealing with various aspects of the Moroccan Atlas system. The tectonic evolution and structural style of the Alpine central and eastern High Atlas has been the subject of a series of PhD and diploma theses under the direction of Prof. Jean-Paul Schaer in the 1980'ies. In collaboration with the Service Géologique du Maroc, a series of geological maps (1:100'000) have been established. Main results are summarized in Brechbühler et al. (1988). More recently, Martin Burkhard has examined the Palaeozoic Anti-Atlas system with special emphasis on the structural style of this intra-cratonic fold belt (Burkhard et al. 2001, 2006; Caritg et al. 2003, Helg et al. 2004). An intriguing feature of this late Palaeozoic fold belt is its current high topography. Our search for signs of some Neogene tectonic activity, e.g. in the form of reactivated older faults, failed to provide any such evidence. On the contrary, folds and faults clearly date back to an important compressional event at the end of the Palaeozoic, followed by some weak extension in relation with the Atlantic opening (Robert-Charrue 2001). Faults and folds were subsequently deeply eroded, peneplained and sealed by marine sediments of the so called Hamada during upper Cretaceous times. The current high elevation of the northern Anti-Atlas chain, adjacent to the High Atlas remains to be explained.

Through our previous studies in Morocco, we have well established contacts with the Geological Survey and the Universities of Casablanca (Prof. O. Sadiqqi) and Marrakech (Prof. M. Chellai) whereas Dominique Frizon de Lamotte has a large experience in structural geology and basin development in the Atlas/Rif system. In Neuchâtel, there is also expertise in petrology of low-grade metamorphic rocks, including low-temperature geothermometry and Raman analysis of organic material (Negro et al. 2006) as well as Illite Crystallinity. The latter approach has been applied to constrain cooling, uplift and erosion in the Alps (Burkhard 1986, Burkhard 1990, Burkhard & Sommaruga 1998, Burkhard 1999, Burkhard & Goy-Eggenberger 2001, Burkhard & Badertscher 2001), but also in the HA and AA of Morocco, albeit with lesser data density (Brechbühler et al. 1988, Ruiz et al., in press).

3. Detailed research plan

3.1. Overall purpose and aim

The main goal of this project is to characterize and quantify the recent denudation of the Atlas chain. These short-term rates shall be compared with the long-term denudation rates obtained from the 1.5 years of work in the ongoing project. In this way, the present-day geomorphology can be linked to the long-term evolution of the orogen. The vertical movements are only poorly quantified, apart from the ongoing project and two to three collaborative projects (see section 1), which all use thermochronometry. No in-situ cosmogenic nuclide exposure dating exists yet. Consequently, little is known about the timing, rates and controls on the orogenic growth of the Atlas chain.

Our research will thus involve the monitoring of spatial variations in denudation in the HA and the AA over different geological time-scales through the use of a state of the art approach combining conventional fission track techniques with (U-Th)/He thermochronometry and measurements of in situ cosmogenic nuclides (10Be, 21Ne). Field work will be conducted to collect present-day river sands across the Atlas chain with a emphasis to the Anti-Atlas. The use of cosmogenic nuclides forms a cornerstone for new quantitative Earth surface sciences. Our approach will hopefully result in a novel and rapid method to compare short-term to long-term denudation rates. If successful, our approach would allow deducing denudation rates at different time scales from just one sample.

A unique opportunity to tackle this challenging research topic is provided by the synergy of the applicant's extended fields of experience in both active orogens and thermochronological methods, with the host institution's expertise in low-temperature-thermochronology and exposure dating. The project as a whole is designed to make a significant contribution to research on the geodynamic evolution of orogens and more specifically to the Atlas orogeny.

3.2. Objectives and tasks

Characterize the asymmetrical evolution of the Atlas chain

Preliminary results from U-Th/He analysis of apatite (AHe, see section 1.3.) suggest that the HA and AA had very different long-term record in denudation: AHe ages from the HA - whatever domain is considered, e.g. the Axial Zone (AZ), the Northern and Southern Sub-Atlas Zone (NSAZ, SSAZ; Fig. 2) - range between 30 and 10 Ma whereas AHe ages from the AA are 90 to 110 My older.

These results are locally interpreted, i.e. for each domain from either the HA or AA (see section 1.3.) in terms of long-term denudational history. At a larger scale, these first U-Th/He data suggest that the HA in SW Morocco developed from the Oligocene on, whereas data from the AA either point towards a very recent phase of exhumation as suggested for the whole Atlas system (Babault et al., in press) or/and a sustainable and gentle one since the Early Cretaceous. Additional thermochronological analyses are thus necessary to find out when exhumation in the HA and AA started to diverge (Fig. 4). Both massifs are constituted of bedrocks with roughly similar ages (absolute) and lithologies.

For this purpose, we will perform additional analyses using thermochronometers that are characterized by higher closure temperature and this to samples from both the HA and AA. The results will allow us to constrain the complete long-term time-temperature paths (i.e. from 270 to 40°C) of these domains and as a result their respective denudation. We kept separates of all bedrock samples we already separated. Depending on the individual yield for each sample, we will perform U-Th/He (~200-180°C) and/or fission track (~230°C) measurements on zircon (Reiners and Brandon 2006). Similarly, apatite crystals have been extracted for U-Th/He analysis from the 50 samples we processed or are processing. The remaining apatite crystals will be prepared for fission track analysis (120-60°C; Green et al., 1989). These analyses on apatite are extremely important as they will partly fill the gap between the partial annealing zones of U-Th/He on apatite (80-45°C) and zircon (200-180°C; Fig. 4). Finally, thermal modelling, and as a result denudational modelling will be completed for a single sample by incorporating the different thermochronological ages in state of the art software's such as HeFty (Ketcham et al., 2005) and Pecube (Braun 2003).

The obtained data from in situ cosmogenic analysis (see below) will be associated with the long-term denudation rates data to identify differences across the chain and regions that undergo recent and rapid uplift. Recent and rapid uplift would be associated to high rates (>0.2-1mm/y) by comparison with period of tectonic quiescence (<0.1mm/y).

Determine recent denudation rates in the High-Atlas and Anti-Atlas

In-situ cosmogenic isotope analyses are performed on quartz mineral from quaternary deposits since less than a decade to estimate catchment-wide denudation rates (Schaller et al., 2001). Three samples were already selected from both sides of the HA, and results should come out before summer 2008. We propose to sample additional present-day river sands from regions of the HA and AA where no constraints on recent denudation rates exist yet from the ongoing project. Samples will be collected from river catchment at different altitude on the large-scale transect across the HA-AA system along which we already collected bedrock samples for low-temperature thermochronological analyses. We will perform measurements of in-situ cosmogenic nuclides on detrital quartz (10Be or 21Ne depending on the rates) to assess 1) variations in sub-recent denudation rates, and 2) the potential effect of topography on denudation rates. Quartz grains need to be homogenously sourced from bedrocks within a river catchment not to bias the interpretation. For this reason, we will take the uttermost care to select river catchments with either 1) a homogenous (quartz-rich) lithology (e.g. a pluton), or 2) at least homogenously distributed enriched quartz lithology.

Characterize from the same sample long and short-term denudation rates

Similarly to what is undertaken within the ongoing project but for a limited number of samples (3), we will systematically performed fission track analysis on detrital apatite from the additional present-day river sands we will select for in-situ cosmogenic isotope analysis (see above). Apatite is preferred to zircon because of 1) the straightforward preparation (especially etching conditions), and 2) the high reworking potential of zircons (Ruiz et al., 2004 & 2007). Furthermore, we aim to develop a novel and rapid method to compare short-term (in situ cosmogenic isotope analysis) and long-term (detrital thermochronology; Ruiz and Seward, 2006) denudation rates from the same sample. If validated, such approach would allow a direct inspection of denudation rates at different geological time scales and this solely through the careful selection of a single sample.

Figure 4. Schematic time-temperature paths for the High and Anti-Atlas domains with the different Partial Annealing (Fission-Track) or Retention (U-Th/He) Zones on apatite and zircon (right). The High Atlas and Anti-Atlas yielded very different U-Th/He ages on apatite (AHe) whereas they are constituted of bedrocks with similar lithologies and absolute ages.

3.3. Field work and analytical methods

a) Field work. Collection of 15-20 present-day river sands from 1) mainly the Anti-Atlas in the region of Igherm and further east, and 2) the High-Atlas where Precambrian basement series define the core of the Atlas chain (Fig. 1). Few additional bedrocks (15) from the Anti-Atlas will be collected to be analysed (if necessary) for thermochronological methods.

b) Microscopy and documentation of samples. Thin sections have been prepared for all bedrock samples from the HA of SW Morocco. We need a detailed documentation of thin sections for the analytical work to follow and for publications. Excellent petrographic microscopes, numerical camera PCs and printers necessary for this work are available at the Institut de Géologie et d'Hydrogeologie (IGH) in Neuchatel.

c) Mechanical processing of rocks. Isotope analysis will be performed on apatite, zircons and quartz mineral separates. All the facilities necessary for this work (crusher, grinder, sieving material, magnetic separator, heavy liquid separation and excellent binocular) are available at the IGH.

d) Preparation of samples for fission track analysis. Apatite and zircon have or are being separated (see above). In a second step, they will be mounted in a support made of epoxy (apatite) or Teflon (zircon) before being polished. The third stage consists of etching the different mounts in different acids before being sent to a nuclear reactor for irradiation. Facilities are fully available at ETH Zurich.

e) Preparation of samples for U-Th/He analysis. Apatite and zircon have or are being separated (see above) and ten to twenty crystals selected, respectively. The common procedure is to take photographs and measure each grain (radius and length) before analysis and later pack each of them into platinum foils. Such procedure was first completed at the SUERC - however, we developed facilities at the IGH with our technician Andre Villard to enhance sample preparation.

f) Fission track analyses. Similarly to the preparation, facilities for Fission-Track analysis are available at ETH Zurich with two excellent microscopes with automated stages. The responsible of this laboratory (Prof. Jean-Pierre Burg) is favourable to this project. At the same time, the IGH seeks to acquire a new microscope + fission track stage in 2008.

g) U-Th/He analyses. Similarly to what was completed within the frame of the ongoing project, we intend to complete our additional U-Th/He analyses at the SUERC (Glasgow, resp. F. Stuart) where expertise in analysing zircon is present.

h) In-situ cosmogenic exposure dating. Similarly to what was completed within the frame of the ongoing project, we intend to perform additional analyses from present-day river sands at ETH Zurich under the guidance of S. Ivy-Ochs.

3.4. Organization of the work

Most of the work on this project concerns the acquisition of high-quality low-temperature thermochronology and exposure age data (on bedrock and present-day river sands). The work can be split into two parts: one concerns the additional analyses we would like to perform on samples we already acquired and the second refers to the few additional samples (10) we would like to collect to constrain sub-recent denudation rates in regions we did not investigate within the ongoing project.

We consider the proposed study as a natural continuation of the ongoing project. It will require a minimum of sample preparation and will provide quick and novel results on the recent evolution of the Atlas chain in SW Morocco. Most of the mechanical sample preparation (including thin section preparation, rock crushing and grinding, and mineral separation) but also Raman spectroscopy analyses, have been already done in the frame of the ongoing project or are in their final stage (Geoffrey Ruiz and François Negro). The production of fission track analysis on apatite and zircon grains will necessitate some additional preparation (mounting, polishing, etching and irradiation, see the time table in section 4), which is not the case for U-Th/He thermochronometry on zircons from the same samples.

In-situ cosmogenic analyses have been obtained routinely in ETH Zurich on quartz. The same holds true for U-Th/He SUERC (Glasgow) and fission track measurements (ETH Zurich). Hence, no major analytical hurdle is foreseen during this part of the data acquisition.

This promising, but challenging project requires a person at the post-doctoral level, sufficiently experienced in the broad field of thermochronology and in-situ cosmogenic dating and with a good knowledge of the samples to be examined in the study. Geoffrey Ruiz has carried out a detailed and systematic U-Th/He analyses on apatite and in-situ cosmogenic analysis over the last 1.5 year on bedrocks and the product of their erosion in/of the High-Atlas. He carries out all of the mineral separation, analysis and interpretation of data in a very independent and responsible way. Geoffrey Ruiz has also compiled the data and presented them at three international conferences. He has submitted three papers in the last 1.5 years that are related to the ongoing project and to similar topics that are partly related to previous projects (4) and partly to new international collaborations (1). Therefore, we would like to employ Geoffrey Ruiz on the post-doc position. He has extensive experience in all the techniques to be applied in the project and he has obtained first U-Th/He, structural and sedimentological data on the High-Atlas samples for the ongoing project. He also has worked extensively on the quantification of vertical movements in different orogenic systems (Andes, Himalayas). He is the author of several articles on the interaction of tectonic and climatic processes on denudation in international scientific journals (see attached CV and publication list).

Most of the intended analytical work will be carried out at facilities and with nationally and internationally recognized scientists with whom we have developed strong work partnerships over the years, enabling us to proceed quickly and efficiently with the project.

4. Time table

Below is a tentative schedule for the planned work for the 2 following years. There is only a single fieldwork scheduled since most of the samples have already been collected. Additional samples from the AA are the property of the University of Casablanca, Prof. O. Saddiqi (Morocco) who has already agreed to share his sample collection and unpublished data with us to continue our fruitful collaboration on the present project.

Since most of the processing of HA rocks (e.g. mineral separation, rock crushing and grinding, thin section preparation and mineral separation) has already been performed by Geoffrey Ruiz and François Negro during the ongoing project, we anticipate a quick start of the work on the additional low-temperature thermochronometry measurements on this sample suite (depending on the availability of the laboratories).

October 2008-February 2009

Sample preparation of the last samples from the HA for apatite and zircon fission track analysis (i.e. mounting, polishing and irradiation). Preparation of 3 papers on the recent denudation of the chain (using both preliminary exposure age and U-Th/He data).

Presentation of results at the AGU in San Francisco (15-19 December 2008)

November 2008

Field work in the AA (15 days) - collection of present-day river sands along a transect perpendicular to the main structures.

March 2009 - April 2009

Preparation of samples collected in November (present-day river sands) for in-situ cosmogenic analyses + thermochronology on detrital minerals

Presentation of results at the EGU in Vienna (April 2009)

May 2009-August 2009

Apatite and zircon fission track analysis. Finalization of papers on the recent denudation of the chain.

October 2009 - December 2009

In-situ cosmogenic nuclides analysis on present-day river sands from the AA domain

January 2010 - March 2010

U-Th/He analysis on zircons from the entire chain that have already been extracted during the ongoing project.

Presentation of results at the EGU in Vienna (April 2010)

April 2010 - September 2010

Finalization of papers. Submission of proposals/projects. Last analyses.

5. Significance of the planned research

The project as a whole (including the ongoing part) is designed to make a significant contribution to research on relief formation and destruction in orogens, with special emphasis on the combination of low-temperature thermochronology and in-situ cosmogenic nuclides dating. Thermochronological ages as well as exposure ages obtained on the Atlas chain in the framework of this project will be significant, because such data have been rarely published, impeding until now any reconstruction of the denudation of the orogen. Also, constraints on the potential of the combination of these methods to present-day river sands will be tested in order to compare short-term and long-term denudation rates from a single sample. Looking directly at the present-day erosional products of the chain, as well as at their precursors (syn-orogenic deposits), will contribute significantly to the understanding of past and present evolution of the Atlas chain.

The dataset we already produced and the one we plan to acquire will be compared to the few studies available in the literature and will thus be a significant addition to the global effort presently undertaken to understand the long and short-term geodynamics of the Atlas Mountains in the frame of the Africa-Europe.

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