Our group has taken a "holistic view" in the study of orogenic belts, where we are interested in the complete view of orogenic systems, from prograde metamorphism and subduction to ultra-high-pressures (UHP) to magma genesis and the timescales of volcanic evolution. Such an approach requires an intimate interweaving of petrology (metamorphic and igneous), field work, structure/tectonics, and geochronology using multiple isotopic systems. No one tool or approach can solve all problems. There are two main projects that we are currently pursuing:
The Alps remain a classic laboratory for studying the tectonics of continent-continent collision. The Zermatt-Saas ophiolite complex of the Western Alps is a remnant of the Tethys Ocean that was subjected to high (HP) and ultra-high pressure (UHP) metamorphism, followed by rapid exhumation during the Alpine Orogeny. Despite over a century of structural studies and decades of geochronological research in this region, the age and duration of HP to UHP metamorphism, the maximum pressures and temperatures the Zermatt-Saas ophiolite and structurally underlying basement nappes experienced, and the rate at which these units were buried and exhumed remain unresolved problems. These questions continue to be hotly debated, and are critical to understanding of the geodynamic processes in orogenic belts. We have focused on the Alps because this orogenic belt is young enough so that temporal relations may be worked out in detail, the exposures are superb, the maximum temperatures were relatively cool, below the blocking temperatures of several geochronometers, and the prior geologic work provides a excellent framework upon which to build new studies. In addition to work on the Zermatt-Saas complex, we have extended our work to the Monte Rosa nappe (a fragment of European basement) and the Sesia nappe (a fragment of African crust).
The 3D topographic/bathymetric render above (left) looks down on the Alps (lower part of image) from the NW into the Mediterranean Sea and northern Africa (upper part of image). Prior to opening of the Mediterranean, the Apulian plate ("Africa") was thrust onto Europe in the late Mesozoic to early Cenozoic, forming the HP-UHP terranes of the Alps, as well as Greece (upper left of image).
The map above (right) illustrates the major units of the Western Alps. Working from lower left (SE) to upper right (NW), the light yellow is the Po Plain, followed by the orangish/tan units of the Apulian plate ("Africa"), the dark blue of the Zermatt-Saas ophiolite, and the pink/salmon units of European basement massifs. The geology is complex, where, for example, the upper part of the Matterhorn (see book inset above, and photo below) is a fragment of Africa, and the lower part is the Jurassic Tethys oceanic crust (Zermatt-Saas ophiolite). Units of the ophiolite complex are exposed in the lower parts of the photo below, including pillow lavas and serpentinite bodies (foreground knob on left in photo below).
HP and UHP metamorphism is recorded in the eclogites of basaltic protoliths, as seen in the photo below. The garnets are obvious, and these represent a surprisingly long prograde metamorphic history of garnet growth during subduction of the oceanic crust. Eclogites are common in both the Zermatt-Saas complex, as well as older units such as the Sesia nappe that lies to the SE and reflects sections of African continental margin.
Through combining careful field observations with petrologic and geochronological studies, we have assembled a preliminary history of the Zermatt-Saas ophiolite, which is shown in the figure below. Garnet growth models, developed by our collaborator Lukas Baumgartner at the University of Lausanne, constrain the interplay of thermodynamic stability of garnet in P-T space with zonations in Lu/Hf and Sm/Nd ratios as determined by elemental diffusion and equilibrium partitioning; these are then fed into models for the 147Sm-143Nd and 176Lu-176Hf geochronological systems. The strong core-to-rim zonation in Lu/Hf ratios of garnet indicate that 176Lu-176Hf ages should be skewed to the early periods of garnet growth as compared to the 147Sm-143Nd ages, and indeed this is the case (see figure below; from Lapen et al., 2003). These results suggest that prograde garnet growth occurred over perhaps 15 m.y. or more, which is on the order of plate velocities that have been estimated for subduction of Tethys crust during the Alpine Orogeny. For comparison, U-Pb zircon geochronology, a very common geochronological system, places no constraint on the age of metamorphism at specific P-T conditions, and scatter over the prograde interval (see figure below). The uplift path, which was very rapid, is constrained by the 87Rb-87Sr geochronological system on greenschist-facies overprints, as well as other constraints, such as zircon fission-track ages (not shown)
We are continuing our work on the Zermatt-Saas complex, but are also extending new work into the Sesia nappe, which lies to the SE and is the earliest nappe unit to have been subducted to HP (or greater) conditions during the Alpine Orogeny. Eclogites do occur in the Sesia (photo below, looking north into Switzerland), although granitic lithologies are most abundant. The Sesia reflects the rifted margin of Africa that collided with the European margin during the Alpine Orogeny.
In addition to eclogites, the Sesia nappe contains HP granitic rocks, including jadeite- and phengite-bearing granites (and relict biotite), as shown in the photo below. This reflects the HP reaction Albite => Jadeite + Quartz.
At U.W. Madison:
At Other Institutions:
Kylander-Clark, ARC, Hacker, BR, Johnson, CM, Beard, BL, and Mahlen, NJ (2009) Slow subduction of a thick ultrahigh-pressure terrane. Tectonics, in press. [PDF] (1610kb).
Lapen, TJ, Medaris, LG Jr, Beard, BL, and Johnson, CM (2009) The Sandvik peridotite, Gurskřy, Norway: Three billion years of mantle evolution in the Baltica lithosphere. Lithos, in press. [PDF] (833kb).
Kylander-Clark, ARC, Hacker, BR, Johnson, CM, Beard, BL, Mahlen, NJ, and Lapen, TJ (2007) Coupled Lu-Hf and Sm-Nd geochronology constrains prograde and exhumation histories of high- and ultrahigh-pressure eclogites from western Norway. Chem. Geol. 242:137-154. [PDF] (1959kb).
Lapen, TJ, Johnson, CM, Baumgartner, LP, Dal Piaz, GV, Skora, S, and Beard, BL (2007) Coupling of oceanic and continental crust during Eocene eclogite-facies metamorphism: Evidence from the Monte Rosa nappe, western Alps. Contrib. Mineral. Petrol. 153:139-157. [PDF] (1013kb).
Skora, S, Baumgartner, LP, Mahlen, NJ, Johnson, CM, Pilet, S, and Hellebrand, E (2006). Diffusion-limited REE uptake by eclogite garnets and its consequences for Lu-Hf and Sm-Nd geochronology. Contrib. Mineral. Petrol.152:703-720. [PDF] (569kb).
Lapen, TJ, Medaris, LG Jr, Johnson, CM, and Beard, BL (2005) Archean to Middle Proterozoic evolution of Baltica subcontinental lithosphere: evidence from combined Sm-Nd and Lu-Hf isotope analyses of the Sandvik ultramafic body, Norway. Contrib. Mineral. Petrol. 150:131-145. [PDF] (1030kb).
Mahlen, NJ, Johnson, CM, Baumgartner, LP, and Beard, BL (2005) Provenance of Jurassic Tethyan sediments in the HP/UHP Zermatt-Saas Ophiolite, Western Alps. Bull. Geol. Soc. Amer. 117:530–544. [PDF] (498kb).
Lapen, TJ, Mahlen, NJ, Johnson, CM, and Beard, BL (2004) High-precision Lu and Hf isotope analyses of both spiked and unspiked samples: a new approach. Geochemistry, Geophysics, and Geosystems (G3) v. 5, no. 1, 31 Jan. 2004 issue (17 pages) [PDF] (1731kb).
Lapen, TJ, Johnson, CM, Baumgartner, LP, Mahlen, NJ, Beard, BL, and Amato, JM (2003) Burial rates during prograde metamorphism of an ultra-high-pressure terrane: an example from Lago di Cignana, western Alps, Italy. Earth Planet. Sci. Lett. 215:57-72. [PDF] (667kb).
Amato, JM, Johnson, CM, Baumgartner, L, and Beard, BL (1999) Rapid exhumation of the Zermatt-Saas ophiolite deduced from high-precision Sm-Nd and Rb-Sr geochronology. Earth and Planet. Sci. Lett. 171:425-438. [PDF] (268kb).
An example of applying the 238U-230Th isotope system to studying volcanic evolution is shown below, where detailed stratigraphic studies and 40Ar/39Ar geochronology allows us to correct for in situ 238U decay, producing highly precise initial 230Th/232Th ratios (see Jicha et al., 2004; 2005). This work has been led by Prof. Brad Singer and his students in our Department. At Seguam volcano in the Aleutians, the increase in initial 230Th/232Th ratios over the last 140,000 years is interpreted to reflect a closed-system evolution by 238U decay, a remarkable conclusion considering the expectation that this subduction-related system would be open to input from mantle melts over this time. Following collapse of the Wilcox caldera 9,000 years ago, the initial 230Th/232Th ratios decrease up through the recent eruptions (1993 and 1997), suggesting mixing with new, mantle-derived magmas.
At U.W. Madison:
At Other Institutions:
Jicha, BR, Johnson, CM, Hildreth, W, Beard, BL, Hart, GL, Shirey, SB, and Singer, BS (2009) Discriminating assimilants and decoupling deep- vs. shallow-level crystal records at Mount Adams using 238U-230Th disequilibria and Os isotopes. Earth Planet. Sci. Lett. 277:38-49. [PDF] (2328kb).
Jicha, BR, Hart, GL, Johnson, CM, Hildreth, W, Beard, BL, Shirey, SB, and Valley, JW (2009) Isotopic and Trace Element Constraints on the Petrogenesis of Lavas from the Mount Adams Volcanic Field, Washington. Contrib. Mineral. Petrol. 157:189-207. [PDF] (982kb).
Jicha, BR, Singer, BS, Beard, BL, Johnson, CM, Roa, HM, and Naranjo, J.A. (2007) Rapid magma ascent and generation of 230Th excesses in the lower crust at Puyehue-Cordón Caulle, Southern Volcanic Zone, Chile. Earth Planet. Sci. Letts. 255:229-242. [PDF] (1236kb).
Jicha, BR, Singer, BS, Beard, BL, and Johnson, CM (2005) Contrasting timescales of crystallization and magma storage beneath the Aleutian Island arc. Earth Planet. Sci. Lett. Letters 236:195-210. [PDF] (736kb).
Jicha, BR, Singer, B, Brophy, JG, Fournelle, JH, Johnson, CM, Beard, BL, Lapen, TJ, and Mahlen, NJ (2004) Variable impact of the subducted slab on Aleutian island arc magma sources: evidence from Sr, Nd, Pb, and Hf isotopes and trace element abundances. Jour. Petrol. 45:1845-1875. [PDF] (2506kb).
Hart, GL, Johnson, CM, Hildreth, W, and Shirey, SB (2003) New osmium isotope evidence for intra-crustal recycling of crustal domains with discrete ages. Geology 31: 427-430. [PDF] (346kb).
Hart, GL, Johnson, CM, Shirey, SB, and Clynne, MA (2002) Tracing lower-crustal processes in orogenic arcs using Os isotopes: An example from the Lassen Volcanic Center, CA. Earth Planet. Sci. Lett., 199:269-285. [PDF] (825kb).