Glaciated outcrop in the Mesule area (Tauern window, Eastern Alps)

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Glaciated outcrops have provided geologists with some of the best outcrops ever available for structural analysis. The polishing action of glacier results, especially in crystalline rocks, in perfect exposures where details down to single rock grains can be analyzed directly in the field and structures are exposed with continuity for detailed mapping. Due to the abrasion effects many of these outcrosps have been smoothed and can be easily mapped in detail. Recording structures in these spectacular outcrops, with classical and modern (e.g.: laserscan, digital photogrammetry) techniques of structural mapping, can allow reconstruction of geometries of the deformation network with unprecedented precision, providing tight constraints for the interpretation and modeling of such deformation structures.

Laserscan is facing the glaciated outcrop at the front of the Lobbia Glacier in the Adamello batholith (Central Alps, Italy).  The vast area of light colored outcrops at the glacier front was still covered with ice a couple of decades ago.

In recent years, the rapid glacier retreat, as an evident consequence of global warming, has resulted in the exposure of large areas of perfectly polished outcrops providing a unique opportunity for field study of geological structures. Images of the dramatic retreat of glaciers form several regions of the Earth have become familiar to everyone and can be found at numerous web sites, e.g.

· http://www.nichols.edu/DEPARTMENTS/Glacier/glacier_retreat.htm

· http://earthobservatory.nasa.gov/IOTD/view.php?id=4594

· http://www.worldviewofglobalwarming.org/pages/glaciers.html

· http://nrmsc.usgs.gov/files/norock/repeatphoto/Gallery_guide_general.pdf

Unfortunately glaciated outcrops are not everlasting. After exposure, these glacier-polished areas eventually deteriorate, in most cases in a few tens of years at most, due to surface weathering, lichen growth, mechanical damage and accumulation of cover.

Mt. Abbott Quadrangle, Sierra Nevada (California) – Exfoliation of a glaciated outcrop in the Lake Edison granodiorite

Riesenferner pluton (South Tyrol, Eastern Alps, Italy). Lichen growth and weathering of a glaciated oucrop exposing a localized ductile shear zone that displaces leucocratic dykes within tonalite

It would be indeed unfortunate if the short-term opportunity provided by the exposure of such nearly perfect polished exposures is not fully utilized. It is therefore the core aim of project Glacier Watch of TecTask to:

• identify and select areas worldwide of recent exposure at the front of glaciers exposing relevant geological structures

• promote the formation of research groups for mapping, studying and recording these unique outcrops

• develop an efficient protocol for quantitative mapping through the feedback from, and interaction between, different research groups, loosely coordinated through Glacier Watch.

• select easily accessible areas that can provide natural laboratories for teaching structural geology thanks to the unique exposure of structures. The Neves area (see below), in the Eastern italian Alps, is an example of such an area and has already hosted several international excusions and summer schools (http://www.egu.eu/meetings/summer-schools/summer-school-structural-analysis-of-crystalline-rocks.html)

We invite researchers to catalog and publicize glaciated outcrops exposing geological relevant structures by providing information about:

• the exposed structures, with a short description and photos
• the accessibility and logistics of the area
• the educational potential of the area
• the approximate surface area of exposure
• relevant bibliography and geological maps (if existing)

The database of localities will be published on the Glacier Watch site and also made available in:

http://www.outcropedia.org.

Based on our own experience, we present some examples of deglaciated areas that show the scientific potential of such exposures, namely:

A) Lobbia outcrops, Adamello batholith (central souther Alps, Italy)

B)  Neves area (Tauern Windows, South Tyrol, Eastern Alps, Italy)

C) Bear Creek area (John Muir Wilderness, Sierra Nevada batholith, California)

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A) Lobbia outcrops - Adamello batholith (central southern Alps, Italy)

Glaciated Lobbia outcrops at looking S. In the background: Cercen Pass and the snowy Presanella Peak.

Geological structures: The Adamello batholith is a composite intrusion composed of 4 main bodies becoming progressively younger from South (Re di Castello: 42 Ma) to North (Presanella: 30Ma). The pluton was emplaced at a depth of about 10km (0.25-0.3 GPa, 250 °C) within the pre-Alpine basement of the southern Alps and the overlying Permo-Triassic non-metamorphic sediments. The different plutons undervwent rapid cooling to the ambient temperature. Deformation structures, developed during cooling, include a ductile shear zones to brittle faults. Shear zones and faults exploited a pervsive set of joints developed during the early stages of high-temperature cooling. Ductile shear zones developed under conditions of ca. 500 °C: they nucleated on joints, quartz veins filling the joints and leucocratic dykes. Faults developed under conditions close to the base of the brittle crust and include cataclasites and pseudotachylytes. The faults are concentrated in a major fault zone (Gole Larghe Fault Zone) extending into the nothern Adamello as a branch of the dextral strike-slip Periadriatic (or Insubric) Fault. The postmagnatic deformation structures are spectaculaly exposed on the glaciated outcrops at the front of the Lobbia Galcier (Val di Genova). Some examples of the structures tha can be seen in  Lobbia outcrops are shown ibelow and include: (1) pervasive sets of post-magmatic high-temperature joints (Fig. 1); (2) knife-sharp ductile shear zones exploiting precursor joints (Fig. 2); (3) shear zone terminations with foliated contractional domains (Fig. 3); (4) mylonites developed by shearing of pegmatite dykes (Fig. 4); (5) epidote-chlorite-bearing cataclasites (Fig. 5); and (6) pseudotachylytes (Fig. 6, 7).

Access: The area is at the front of the Lobbia Glacier, at ca. 2700 m a.s.l. in the Upper Val di Genova (access is from Carisolo, Rendena and Giudicarie Valleys) within the Adamello Natural Park. During summer, from Carisolo, there is a shuttle bus to the Bedole Hut, near the head of the Genova Valley, where the trails to the Adamello Glacier start. From the Bedole there is a long signed path (trail to the alpine hut “Rifugio Lobbia ai Caduti dell’Adamello” 3040m a.s.l.: http://www.rifugioaicadutidelladamello.it/). Be aware that the trail is in some parts a “Via Ferrata”, with fixed ropes and steel spikes, and requires some alpinistic experience. You should not go alone and you should have some mountaineering experience before attempting both the path and the glaciated outcrops. The best period for a trip is July-September.

Educational potential

The oucrops exposes a great number of both brittle and ductile deformation structures over a relatively small area. However, the area is not easily accessible and requires mountanieering skills.

Maps

A map of the network of strike-slip ductile shear zones exploiting a precursor  joints  is shown below (Pennacchioni, 2005). The presence of numerous aplite dykes crosscut by the shear zones allow the slip of any individual shear zone to be measured at different position of the structures and reveal large slip gradients toward the tips of the shear zones. These gradients are accommodated by deformation of the host rock at the contractional side of the shear zone tip.

Both ductile shear zones and faults exploited steeply dipping joints and have a strike-slip kinematics. This makes the subhorizontal glaciated Lobbia outcrops ideal for mapping.

References

Di Toro, G., Pennacchioni, G., 2004. Superheated friction-induced melts in zoned pseudotachylytes within the Adamello tonalites (Italian Southern Alps). Journal of Structural Geology 26, 1783-1801.

Di Toro, G., Nielsen, S., Pennacchioni, G., 2005. Earthquake rupture dynamics frozen in exhumed ancient faults. Nature 436, 1009-1012.

Di Toro, G., Pennacchioni, G., Teza, G., 2005. Can pseudotachylytes be used to infer earthquake source parameters? An example of limitations in the study of exhumed faults. Tectonophysics 402/1-4, 3-20.

Di Toro, G., Pennacchioni, G., 2005. Fault plane processes and mesoscopic structure of a strong-type seismogenic fault in tonalites (Adamello batholith, Southern Alps). Tectonophysics 402/1-4, 54-79.

Mittempergher, S., Pennacchioni, G., Di Toro, G., 2009. Effects of fault orientation and fluid infiltration on fault rock assemblages at seismogenic depths. Journal of Structural Geology 31, 1511-1524, doi: 10.1016/j.jsg.2009.09.003.

Pennacchioni, G., 2005. Control of the geometry of precursor brittle structures on the type of ductile shear zone in the Adamello tonalites, Southern Alps (Italy). Journal of Structural Geology 27, 627–644. Doi:10.1016/j.jsg.2004.11.008.

Pennacchioni, G., Di Toro, G., Brack, P., Menegon, L., Villa, I.M., 2006. Brittle-ductile-brittle deformation during cooling of tonalite (Adamello,Southern Italian Alps). Tectonophysics 427, 171-197. doi:10.1016/j.tecto.2006.05.019.

Pennacchioni, G., Menegon, L., Leiss, B., Nestola, F., Bromiley, G., 2010. Development of crystallographic preferred orientation and microstructure during plastic deformation of natural coarse-grained quartz veins. Journal of Geophysical Research 115, B12405, 23 pp., doi:10.1029/2010JB007674.

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B) Neves area (Tauern Window, South Tyrol, Eastern Alps, Italy)

Geological structures – The area investigated is located NE of the Neves lake (Lago di Neves or Nevessee, South Tyrol, Italy). Geologically it lies within the Tauern tectonic window, where Penninic units are exposed below the Austroalpine units that otherwise dominate the Eastern Alps. The lowermost and most extensive unit of the western Tauern window consists of pre-Alpine (ca. 310-290 Ma; Finger et al. 1993) intrusive bodies of variable chemical composition, but largely dominated by granitoids, which were extensively converted to Alpine amphibolite facies orthogneisses and mylonites to form the ‘‘Zentralgneise’’. The Paleozoic schists and amphibolites intruded by these plutons, together with parautochthonous post-intrusive metasediments and amphibolites, are referred to in the literature as the ‘‘LowerSchieferhülle’’, whereas the overlying allochthonous Mesozoic calc-mica schists and ophiolitic units are referred to as the ‘‘Upper Schieferhülle’’ (e.g., Flügel and Faupl 1987). Peak pressures in the Lower Schieferhülle during the Alpine orogeny may have exceeded 1 GPa (Selverstone et al. 1984). However, in the Neves area, the subsequent thermal peak in metamorphism (known as the ‘‘Tauern metamorphism’’) occurred under amphibolite facies conditions estimated at 0.5-0.7 GPa and 550-600°C (Hoernes and Friedrichsen 1974; Selverstone and Spear 1985). The thermal peak of the Tauern metamorphism has been dated at ca. 30 Ma (Christensen et al. 1994), although temperatures may have remained high (within 20-30°C of the maximum values) until ca. 20 Ma (von Blanckenburg et al. 1989; Christensen et al. 1994). This corresponds to the time when the western Tauern Window was rapidly exhumed in the footwall of the Brenner low-angle normal fault (Behrmann 1988; Selverstone 1988; Fügenschuh et al. 1997). The area of interest represents a low-strain domain located just north of a kilometre-thick belt of granitoid mylonites that marks the south-eastern border of the Zillertal-Venediger Massif (De Vecchi and Mezzacasa 1986), which is the southernmost of the three main units of the Zentralgneise. This area has widespread, recently glaciated and perfectly polished outcrops at the base of the Mesule glacier, allowing very detailed and spatially continuous observation. On the Neves outcrops are exposed: (1) magmatic structures (including different generations of acid and basic intrusions, magma mingling, etc.; Figs. 1-3); (2) joints; (3) veins and alteration haloes along joints and fractures ; (4) single and paired discrete shear zones (Figs. 4-11); (5) quartz-biotite-plagioclase-biotite-calcite veins (Figs. ##); (6) late-stage quartz-chlorite-epidote vein systems (Fig. 8a, Fig. 8b, Fig. 8c); and (7) late brittle faults on the large (100’s of metres in length, up to 10 m displacement) and small scale (often conjugate, displacement from effectively zero to a few 10’s of cm).

Educational potential

The outcrops have great potential as a teaching field area. Spectacularly exposed structures are concentrated in a relatively small (and quite flat) area and a wide range of processes can be investigated. The area is easily accesible and logistics are optimal (nearby Chemnitzer Hut). Several international excursions and summer field schools have already been held in the area, including the First EGU summer school “Structural Analysis of Crystalline Rocks” http://www.geoscienze.unipd.it/egu-summerschool/index.html. The Second EGU summer school “Structural Analysis of Crystalline Rocks” will also includes field work in the area.

Maps

References

Christensen, J.N., Selverstone, J., Rosenfeld, J.L., DePaolo, D.J., 1994. Correlation of Rb-Sr geochronology of garnet growth histories from different structural levels within the Tauern Window, Eastern Alps. Contributions to Mineralogy and Petrology 118, 1-12.

Finger, F., Frasl, G., Haunschmid, B., Lettner, H., von Quadt, A., Schermaier, A., Schindlmayr, A.O., Steyrer, H.P., 1993. The Zentralgneise of the Tauern Window (Eastern Alps): inslight into an Intra-Alpine Variscan batholith. In: von Raumer, J.F. , Neubauer, F. (Ed.), Pre-Mesozoic Geology in the Alps. Springer-Verlag, Berlin, pp. 375-391.

Flügel, H.W., Faupl, P., 1987. Geodynamics of the Eastern alps. Franz Deuticke, Vienna.

Hoernes, S., Friedrichsen, H., 1974. Oxygen isotope studies on metamorphic rocks of the western Hohe Tauern area (Austria). Schweizerische Mineralogische und Petrographische Mitteilungen 54, 769-788.

Mancktelow, N.S., Pennacchioni, G., 2010. Why calcite can be stronger than quartz. Journal of Geophysical Research 115, B01402, doi:10.1029/2009JB006526.

Pennacchioni, G., Mancktelow, N.S., 2007. Nucleation and initial growth of a shear zone network within compositionally and structurally heterogeneous granitoids under amphibolite facies conditions. Journal of Structural Geology 29, 1757-1780. doi: 10.1016/j.jsg.2007.06.002.

Mancktelow, N.S., Pennacchioni, G., 2005. The control of precursor brittle fracture and fluid–rock interaction on the development of single and paired ductile shear zones. Journal of Structural Geology 27, 645–661. Doi: 10.1016/j.jsg.2004.12.001.

Selverstone, J., Spear, F.S., 1985. Metamorphic P-T path from pelitic schists and greenstones from the south-west Tauern Window, Eastern Alps. Journal of Metamorphic Geology 3, 439-465.

Selverstone, J., Spear, F.S., Franz, G., Morteani, G., 1984. High-pressure metamorphism in the SW Tauern Window, Austria: P-T paths from hornblende-kyanite-staurolite schists. Journal of Petrology 25, 501-531.

von Blanckenburg, F., Villa, I.M., Baur, H., Morteani, G., Steiger, R.H., 1989. Time calibration of a PT-path from the western Tauern Window, Eastern Alps: the problem of closure temperatures. Contributions to Mineralogy and Petrology 101, 1-11.

C) Bear Creek area (John Muir Wilderness, Sierra Nevada, California)

Geological structures: The Mount Abbot quadrangle in the central-eastern Sierra Nevada batholith, California (Lookwood and Lydon, 1975), has been the location of numerous detailed studies on post-magmatic deformation of granitoids, in part reflecting the extent of spectacular exposures on glaciated outcrops. Most studies have considered the Lake Edison granodiorite (88±1 Ma: Tobish et al., 1995), along the Bear Creek drainage and tributary valleys, and the adjacent younger Mono Creek granite (86 Ma.; cf. Tikoff and Saint Blanquat, 1997) belonging to the Late Cretaceous Mono Pass Intrusive Suite (Bateman, 1992). In this area it was first established that nucleation of plastic shear zones occurred on precursor fractures (i.e. mineralized joints: Segall and Pollard, 1983; Segall and Simpson, 1986) and that ductile shear focused on aplitic dykes (Christiansen and Pollard, 1997). Detailed field studies have addressed the topic of fault growth from initial sets of segmented joints within the granite and the nucleation of ductile shear zone from initial structural (joints) and compositionsl (dykes) heterogeneities. As in the Adamello, the deformations developed during cooling of the pluton to the ambient conditions of intrusion. Although the glacier retreat is not recent, the glaciated outcrops have survived deterioration thanks the particular climatic conditions of the area.

The photographs below illustrate just a few examples of structures that can be seen in the area: (a) microboudinage of K-feldspar phenocrystals, sealed with quartz, along the solid-state foliation in the Mono Creek granite; quarter dollar for scale  (Fig. 1); (b) joints and subparallel discerete shear zones in a glaciated outcrop of Lake Edison granodiorite along the Bear Creek; view to east; W.A. Griffith for scale; (Fig. 2); (c) Mafic enclave crosscut, with no offset, by a hairline joint in the Lake Edison granodiorite; view down to south;  quarter dollar for scale (Fig. 3); (d) discrete shear zone (top) , parallel to a joint (bottom), displaces sharply a leucocratic dyke and a mafic enclave in the Lake Edison granodiorite along Bear Creek; view down to south; quarter dollar for scale (Fig. 4); (e) Set of closely spaced joints, exploited as sinistral shear zones, displacing a leucocratic dyke; Lake Edison granodiorite in Hilgard Valley; view down to east (Fig. 5); (f) Spectacular foliated contractional jog between right-stepping sinistral discrete shear zones offsetting a leucocratic dyke that is thinned conspiquously within the contractional jog; view down to south; pencil for scale (Fig. 6); (g) sinistral quartz-mylonite (sheared quartz vein) within weakly foliated (bottom) and effectively undeformed (top) Lake Edison granodiorite; view down to NNE; East Fork of Bear Creek; quarter dollar for scale (Fig. 7); (h) mylonitic leucocratic (pegmatite) dyke within weakly deformed Lake Edison granodiorite; view down to north; Seven Gables Lakes basin; quarter dollar for scale (Fig. 8); (i) extensional jogs between left stepped localized cataclastic faults with development of quartz-epidote-chlorite-filled veins and a bleaching due to alteration within the jog; Lake Edison granodiorite; Hilgard Branch; view down to north; quarter dollar for scale. Photographs by Giorgio Pennacchioni.

References

Batemann, P.C., 1992. Plutonism in the central part of the Sierra Nevada batholith, California. U.S. Geological Survey Professional Papers 1483, 186 pp.

Bergbauer, S, Martel S. J., 1999. Formation of joints in cooling plutons. Journal of Structural Geology 21, 821-835.

Bürgmann, R, Pollard, D.D, 1992. Influence of the state of stress on the brittle-ductile transition in granitic rock: evidence from fault steps in the Sierra Nevada, California. Geology 20, 645-648.

Bürgmann, R, Pollard, D.D, 1994.Strain accommodation about strike-slip fault discontinuities in granitic rock under brittle-to-ductile conditions. Journal of Structural Geology 16, 1655- 1674.

Christiansen, P.P., Pollard, D.D., 1997. Nucleation, growth and structural development of mylonitic shear zones in granitic rocks. Journal of Structural Geology 19, 1159-1172.

Christiansen, P.P., Pollard, D.D., 1998. Nucleation, growth and structural development of mylonitic shear zones in granitic rocks: Reply. Journal of Structural Geology 20, 1801-1803.

Davies, R.K., Pollard, D.D., 1986. Relations between left-lateral strike-slip faults and right-lateral monocline kink bands in granodiorite, Mt. Abbot quadrangle, Sierra Nevada, California. Pure and Applied Geophysics 124, 177-201.

Griffith, W.A., Di Toro, G., Pennacchioni, G., Pollard, D.D., 2008. Thin pseudotachylytes in faults of the Mt. Abbot quadrangle, Sierra Nevada: physical constraints for small seismic slip events. Journal of Structural Geology 30, 1086–1094, doi:10.1016/j.jsg.2008.05.003.

Lockwood, J.P., Lydon, P.A., 1975. Geological map of the Mount Abbott quadrangle, central Sierra Nevada, California. U.S. Geological Survey Geologic Quadrangle Map CQ-1155.

Lookwood, J.P., Moore, J.G., 1979. Regional deformation of the Sierra Nevada, California, on conjugate microfault sets. Journal of Geophysical Research 84, 6041-6049.

Martel, S.J., Pollard, D.D., Segall, P., 1988. Development of simple strike slip fault zones, Mount Abbot quadrangle, Sierra Nevada, California. Geological Society of America Bulletin 100, 1451-1465.

Mayo, E.B., 1941. Deformation in the interval Mt. Lyell-Mt. Whitney, California. Geological Society of America Bulletin 52, 1001-1084.

Pachell, M.A., Evans, J.P., 2002. Growth, linkage, and termination processes of a 10-km-long strike-slip fault in jointed granite: the Gemini fault zone, Sierra Nevada, California. Journal of Structural Geology 24, 1903-1924.

Pachell, M.A., Evans, J.P., Taylor, W.L., 2003. Kilometer-scale kinking of crystalline rocks in a transpressive convergent setting, Central Sierra Nevada, California.Geological Society of America Bulletin 115, 817-831.

Pennacchioni, G., Zucchi, E., in press. High-temperature fracturing and ductile deformation during cooling of a pluton: the Lake Edison granodiorite (Sierra Nevada batholith, California). Journal of Structural Geology.

Ross, S.L., 1988. Paleomagnetic constraints on rotation within the Mount Abbot quadrangle, central Sierra Nevada, California. Journal of Geophysical Research 93, 11711-11720.

Segall, P., McKee, E.H., Martel, S.J., Turrin, B.D., 1990. Late Cretaceous age of fractures in the Sierra Nevada batholiths, California. Geology 18, 1248-1251.

Segall, P., Pollard, D.D., 1983a. Nucleation and growth of strike slip faults in granite. Journal of Geophysical Reasearch 88, 555-568.

Segall, P., Pollard, D.D., 1983b. Joint formation in granitic rock of the Sierra Nevada. Geological Society of America Bulletin 94;563-575.

Segall, P., Simpson, C., 1986. Nucleation of ductile shear zones on dilatants fractures. Geology 14, 56-59.

Tikoff, B., de Saint Blanquat, M., 1997. Transpressional shearing and strike-slip partitioning in the Late Cretaceous Sierra Nevada magmatic arc, California. Tectonics 16, 442-459.

Tikoff, B., Teyssier, C., de Saint Blanquat, M., 1998. Nucleation, growth and structural development of mylonitic shear zones in granitic rocks: Discussion. Journal of Structural Geology 20, 1801-1803.

Tobisch, O.T., Saleeby, J.B., Renne, P.R., McNulty, B., Tong, W., 1995. Variations in deformation fields during development of a large volume magmatic arc, central Sierra Nevada, California. Geological Society of America Bulletin 107, 148-166.