RESOLVING THE RIFTING OF RODINIA: DETRITAL GEOCHRONOLOGIC EVIDENCE FOR SPATIAL AND TEMPORAL PREVALENCE OF SUB-OROGENIC TECTONOTHERMAL ACTIVITY IN THE APPALACHIAN–CALEDONIAN SYSTEM

Within the plate tectonic framework there remains a poor understanding of the processes driving both the initiation of rifting—whereby dynamics transition from stable to extensional—and subsequent rift propagation—forming a new ocean basin through rift–drift sequences. The two end-member models for rift initiation involve bottom-up (plume-driven) or top-down (far-field lithospheric stress-driven) processes. The Appalachian–Caledonian system (AppCal), which spans >6000 km from Alabama to Newfoundland, and Ireland to Svalbard, is a natural laboratory in which to assess the evolution of orogenic systems. Cryptic and local Tonian–middle Ediacaran episodes of tectonothermal activity (e.g., Knoydartian and Kalak Nappe metamorphism; Robertson River, Crossnore and Grey River magmatism) are recognized along AppCal. Widespread evidence for distinct tectonothermal events at c. 760–700 Ma, c. 600 Ma and c. 540 Ma signifies protracted Neoproterozoic rifting of Rodinia, culminating in the opening of the Iapetus Ocean.

Here we present preliminary U–Pb detrital zircon data for greenschist metasediments (T_peak < 400 °C) of Iapetan rift–drift units from three distinct regions—southern App (TN–VA), northern App (NY–VT–QC) and Scottish Cal—representing ~4000 km along strike of AppCal. In contrast and in complement to the robust Precambrian record provided by the U–Pb zircon system, ⁴⁰Ar/³⁹Ar study of detrital white mica provides an opportunity to resolve more subtle Neoproterozoic signals within local rift histories. Limited chemical homogenization of white mica populations—determined using electron probe microanalysis performed on 720 grains across 23 samples—suggests a minimally overprinted population representative of multiple, local bedrock sources, and supports the preservation of detrital age signatures in white mica. Localized thermal activity that is asynchronous along AppCal may support bottom-up origins for episodic events during AppCal rifting, while extensive near-synchronous episodes may support a rifting mode dominated by top-down processes. Scrutinizing the detrital geochronologic record to determine the spatial and temporal pervasiveness of episodic tectonothermal activity in the lead-up to rifting of Rodinia will provide context for characterizing rifting processes.