From the Appalachians to the Alps: Constraints on the Timing, Duration, and Conditions of Metamorphism at Convergent Margins

The timing, duration, and pressure-temperature (P-T) conditions of metamorphism provide a direct record of the physical and chemical evolution of the crust and inform our knowledge and understanding of plate tectonics. The characteristic timescales and length-scales of metamorphism vary by orders of magnitude, depending on the driving tectonic process. Two fundamental problems with the retrieval of this information from the metamorphic rock record are insufficient temporal resolution and processes that overprint or obscure the full record of metamorphism. Understanding what processes are recorded, and why they are recorded, is critical for accurate models of tectonics. This dissertation examines these processes in the metamorphic rock record in two settings: the central Appalachian orogen and the Western Alps fossil subduction zone.

Chapters 2 and 3 focus on poly-metamorphic migmatites from the Smith River Allochthon (SRA) in the central Appalachians. A combination of petrography, thermodynamic modeling, and geochemistry is used to document and quantify the metamorphic evolution of the SRA and determine the petrologic processes that control metamorphic re-equilibration in high-temperature metamorphic systems. Chapter 2 presents new constraints for Silurian high-temperature (~750℃, 0.5 GPa) contact metamorphism in response to mafic magmatism and a cryptic Alleghanian metamorphism (~600℃, 0.8 GPa). A combination of extensive and highly variable melt loss followed by H2O-flux melting during contact metamorphism is shown to produce a range of modified bulk rock compositions and domains with variable fertilities for metamorphic re-equilibration during the Alleghanian. In chapter 3, monazite, allanite, and zircon laser ablation split-stream petrochronology are used to constrain the timing of poly-metamorphism and develop a tectonic model for the SRA. The SRA preserves evidence for at least three orogenic events, each with a relatively short duration (< 10 Myr.), likely due to repeated magmatic heating. The full record of this punctuated heating is obscured by dissolution-reprecipitation reactions that variably recrystallize monazite and decouple trace element chemistry from isotopic age and significantly restrict equilibrium length-scales.

Chapters 4 and 5 examine the dynamic interplay between transient fluid flow, episodic metamorphism, and deformation in subduction zones. In chapter 4, diffusional speedometry is applied to eclogite breccias from the Monviso ophiolite to quantify the periodicity of transient deformation and metamorphism at eclogite facies P-T conditions. The maximum timescale for repeated fracturing is constrained to ~1 Myr., likely caused by cyclic variations in fluid pressure and strain rate (not necessarily seismicity). While difficult to preserve and detect in the rock record, this periodic metamorphism may play an important role in detachment and exhumation processes in subduction zones worldwide. Finally, in chapter 4 a combination of thermodynamic modeling and Sm-Nd garnet geochronology are used to construct a model for subduction and exhumation of the Voltri ophiolite. Garnet growth occurs rapidly and close to peak P-T conditions (~520℃, 2.4 GPa) across the ophiolite, with large (>10 km2) areas preserving near-identical ages, suggesting that the Voltri ophiolite was exhumed as several large coherent units, aided by the presence of buoyant serpentinites.