Orogenic Belts

Radiogenic Isotope Laboratory

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:

	1. HP & UHP Metamorphism: A study of the prograde and retrograde tectonothermal evolution of the Western Alps, Switzerland and Italy.
	2. Timescales of magmatic processes: Studies of subduction-related volcanism over the 10⁵ year scale.

1. HP & UHP-Metamorphism and tectonothermal evolution of the Western Alps, Switzerland and Italy

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.

The key to understanding the prograde history of these terranes lies in the elemental zonations recorded in garnet. The image on the left is a Yttrium X-ray map of a garnet that has been sectioned precisely through the center as part of a parallel 3D X-ray tomographic study (courtesy of Lukas Baumgartner, Univ. Lausanne). Yttrium, which may be used as a proxy for Lu, is concentrated in the core during the initial phase of garnet growth. The high-Y band partway out reflects the dynamics of garnet growth and diffusional transport of Y to the garnet, which is then followed by a second rimward depletion. The large variations in Y contents are mirrored by variations in Lu (determined separately by laser ablation ICP-MS), which predicts strong core-to rim zonation in ages that would be determined by the ¹⁷⁶Lu-¹⁷⁶Hf geochronometer. The question is, how long did it take to grow this garnet during the prograde subduction path?

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.

Research Group:

At U.W. Madison:

	Clark Johnson
	Brian Beard
	Nancy Mahlen

At Other Institutions:

	Lukas Baumgartner, University of Lausanne
	Mike Cosca, University of Lausanne
	Tom Lapen, University of Houston
	Benita Pulitz, University of Lausanne
	Suzanne Scora, University of Lausanne

Selected Publications:

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).

2. Timescales of Arc Magmatic Processes in the Aleutian, Cascade, and Andes arcs

	A frontier in petrology and geochemistry involves quantifying timescales of magmatic processes. Accurate temporal information is critical to understanding how potentially dangerous arc volcanoes are built, how fast they grow, and how they eventually fail through catastrophic eruption or sector collapse. Yet we know little about the long-term variability of eruptive fluxes from arc volcanoes, associated timescales of magma crystallization and differentiation, relations between magma input vs. output events, and the role that crustal thickness plays in determining how long a particular magma takes to transit the crust and erupt. We are addressing these issues at several carefully selected volcanoes by combining: (1) stratigraphy, petrography and geochemistry of lava flows and pyroclastic deposits, (2) precise ⁴⁰Ar/³⁹Ar dating to determine an eruptive chronology, and (3) ²³⁸U-²³⁰Th isotope measurements of phenocrysts and host rocks to constrain the timing of crystallization relative to eruption and to fingerprint closed and open-system magmatic processes. The half life of ²³⁰Th, for example, is ideal for reconstructing the history of magmatic and volcanic events shaping a stratovolcano over the past 250,000 years. We have been working on three volcanic arcs, each of which has unique characteristics. The Aleutian arc in SW Alaska (3D image in above left) is a relatively primitive arc that has significant east-west variations in the nature of the subducted slab, including age and occurrence of fracture zones. Work has focused on Kanaga, Roundhead, Seguam, and Shishaldin volcanoes, each of which has a different eruptive style and composition. Work in the Aleutians has been in collaboration with Prof. Brad Singer in our Department, and has involved ⁴⁰Ar/³⁹Ar geochronology, as well as Sr, Nd, Hf, and Pb isotope tracer work, and ²³⁸U-²³⁰Th geochronology. We have also been working on several volcanoes of the Cascade arc in Washington, Oregon, and California. In the 3D geologic map on the left, the modern stratovolcanoes cones of the Cascade arc are shown in pink, and we have been working on Mt. Adams, Crater Lake, Mt. Shasta, and Mt. Lassen with collaborators at the U.S. Geological Survey. The Cascades may be considered one of the most "primitive" of the continental volcanic arcs today. This work has included Sr, Nd, and Pb isotope work, as well as innovative new work using the ¹⁸⁷Re-¹⁸⁷Os isotope system as a unique tracer of primitive basaltic crust at depth that cannot be "seen" using traditional isotope systems such as O, Sr, Nd, Hf, and Pb; this work has been in collaboration with Steve Shirey at Carnegie-DTM. Finally, also in collaboration with Prof. Brad Singer in our Department, we have been working on several volcanoes in the Andes arc of South America (3D image at left). This includes a study of the contrasting evolution of Parinacota and Puyehue volcanoes, which erupted through thick and thin continental crust, respectively. This work has involved ⁴⁰Ar/³⁹Ar geochronology, as well as Sr, Nd, Hf, and Pb isotope tracer work, and ²³⁸U-²³⁰Th geochronology. It is anticipated that Parinacota will have a longer residence time for magmas due to the great thickness of the underlying crust, whereas the relatively primitive Puyehue volcano may have involved much shorter magma ascent and crustal differentiation times.

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.

Research Group:

At U.W. Madison:

	Brad Singer
	Brian Jicha
	John Hora
	Clark Johnson
	Brian Beard

At Other Institutions:

	Charlie Bacon, U.S. Geological Survey
	Bob Christensen, U.S. Geological Survey
	Mike Clynn, U.S. Geological Survey
	Garret Hart, Washington State University
	Wes Hildreth, U.S. Geological Survey
	Steve Shirey, Carnegie-DTM

Selected Publications:

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).

For more information, contact Clark Johnson at clarkj@geology.wisc.edu

Last revised: 10/19/07