The Mid Atlantic Appalachian Orogen Traverse: a comparison of virtual and on-location field-based capstone experiences

The Stratigraphy, Structure, Tectonics (SST) course at James Madison University incorporates a capstone project that traverses the Mid Atlantic region of the Appalachian Orogen and includes several all-day field trips. In the Fall 2020 semester, the SST field trips transitioned to a virtual format, due to restrictions from the COVID pandemic. The virtual field trip projects were developed in web-based Google Earth and incorporated other supplemental PowerPoint and PDF files. In order to evaluate the effectiveness of the virtual field experiences in comparison with traditional on-location field trips, an online survey was sent to SST students that took the course virtually in Fall 2020 and to students that took the course in person in previous years. Instructors and students alike recognized that some aspects of on-location field learning, especially those with a tactile component, were not possible or effective in virtual field experiences. However, students recognized the value of virtual field experiences for reviewing and revisiting outcrops as well as noting the improved access to virtual outcrops for students with disabilities and the generally more inclusive experience of virtual field trips. Students highlighted the potential benefits for hybrid field experiences that incorporate both on-location outcrop investigations and virtual field trips, which is the preferred model for SST field experiences in Fall 2021 and into the future.

Concurrent MORB-type and ultrapotassic volcanism in an extensional basin along the Laurentian Iapetus margin: Tectonomagmatic response to Ordovician arc-continent collision and subduction polarity flip

Arc-continent collision, followed by subduction polarity flip, occurs during closure of oceanic basins and contributes to the growth of continental crust. Such a setting may lead to a highly unusual association of ultrapotassic and mid-ocean ridge basalt (MORB)-type volcanic rocks as documented here from an Ordovician succession of the Scandinavian Caledonides. Interbedded with deep-marine turbidites, pillow basalts evolve from depleted-MORB (εNdt 9.4) to enriched-MORB (εNdt 4.8) stratigraphically upward, reflecting increasingly deeper melting of asthenospheric mantle. Intercalated intermediate to felsic lava and pyroclastic units, dated at ca. 474−469 Ma, are extremely enriched in incompatible trace elements (e.g., Th) and have low εNdt (−8.0 to −6.6) and high Sri (0.7089−0.7175). These are interpreted as ultrapotassic magmas derived from lithospheric mantle domains metasomatized by late Paleoproterozoic to Neoproterozoic crust-derived material (isotopic model ages 1.7−1.3 Ga). Detrital zircon spectra reveal a composite source for the interbedded turbidites, including Archean, Paleo-, to Neoproterozoic, and Cambro-Ordovician elements; clasts of Hølonda Porphyrite provide a link to the Hølonda terrane of Laurentian affinity. The entire volcano-sedimentary succession is interpreted to have formed in a rift basin that opened along the Laurentian margin as a result of slab rollback subsequent to arc-continent collision, ophiolite obduction and subduction polarity flip. The association of MORBs and ultrapotassic rocks is apparently a unique feature along the Caledonian-Appalachian orogen. Near-analogous modern settings include northern Taiwan and the Tyrrhenian region of the Mediterranean, but other examples of strictly concurrent MORB and ultrapotassic volcanism remain to be documented.

Supplemental Material: Early Pennsylvanian sediment routing to the Ouachita Basin (southeastern United States) and barriers to transcontinental sediment transport sourced from the Appalachian orogen based on detrital zircon U-Pb and Hf analysis

<div>Table S1: Detrital zircon (DZ) U-Pb isotopic data. Table S2: DZ U-Pb isotopic data from a higher-<i>n </i>approach. Table S3: DZ Hf isotopic data. Table S4: Multi-dimensional scaling (MDS) sample key. <br></div>

Supplemental Material: Early Pennsylvanian sediment routing to the Ouachita Basin (southeastern United States) and barriers to transcontinental sediment transport sourced from the Appalachian orogen based on detrital zircon U-Pb and Hf analysis

<div>Table S1: Detrital zircon (DZ) U-Pb isotopic data. Table S2: DZ U-Pb isotopic data from a higher-<i>n </i>approach. Table S3: DZ Hf isotopic data. Table S4: Multi-dimensional scaling (MDS) sample key. <br></div>

Supplemental Material: Early Pennsylvanian sediment routing to the Ouachita Basin (southeastern United States) and barriers to transcontinental sediment transport sourced from the Appalachian orogen based on detrital zircon U-Pb and Hf analysis

<div>Table S1: Detrital zircon (DZ) U-Pb isotopic data. Table S2: DZ U-Pb isotopic data from a higher-<i>n </i>approach. Table S3: DZ Hf isotopic data. Table S4: Multi-dimensional scaling (MDS) sample key. <br></div>

The Mid Atlantic Appalachian Orogen Traverse: A Comparison of Virtual and On-Location Field-Based Capstone Experiences

The Stratigraphy, Structure, Tectonics (SST) course at James Madison University incorporates a capstone project that traverses the Mid Atlantic region of the Appalachian Orogen and includes several all-day field trips. In the Fall 2020 semester, the SST field trips transitioned to a virtual format, due to restrictions from the COVID pandemic. The virtual field trip projects were developed in web-based Google Earth, along with other supplemental PowerPoint and PDF files. In order to evaluate the effectiveness of the virtual field experiences in comparison with traditional on-location field trips, an online survey was sent to SST students that took the course virtually in Fall 2020 and to students that took the course in-person in previous years. Instructors and students alike recognized that some aspects of on-location field learning were not possible or effective with virtual field experiences. However, students recognized the value of virtual field experiences for reviewing and revisiting outcrops, as well as noting the improved access to virtual outcrops for students with disabilities, and the generally more inclusive experience of virtual field trips. Students highlighted the potential benefits for hybrid field experiences that incorporate both on-location outcrop investigations and virtual field trips, which is the preferred model for SST field experiences in Fall 2021 and into the future.

U–Pb zircon geochronology and implications of Cambrian plutonism in the Ellsworth belt, Maine

The Ellsworth belt is one of several fault-bounded blocks exposed along the southeastern coast of Maine that formed within Ganderia. New ID-TIMS U–Pb geochronological data integrated with field relationships provide additional insights into the timing of magmatism and deformation in the Ellsworth belt. The deformed Lamoine Granite was selected for U–Pb zircon analysis in order to: i) establish the protolith age; ii) provide direct temporal constraints on regional low-grade metamorphism and deformation; and iii) elucidate relationships between the Ellsworth belt and coeval rocks elsewhere in the Appalachian orogen. The Lamoine Granite was emplaced within the Ellsworth Schist at 492 ± 1.7 Ma; this is the first unequivocal evidence for a Furongian magmatic event in the Ellsworth belt. The schistosity in the Lamoine Granite is parallel to the main fabric of the host Ellsworth Schist and provides a maximum estimate for timing of the regional metamorphic overprint. Widespread deformation in the Ellsworth belt where kinematic indicators indicate a top-to-northwest sense of shear is attributed to thrusting during which progressive horizontal shortening, caused crustal thickening and peak greenschist facies metamorphism. The Cambrian U–Pb age permits correlation of the Lamoine Granite with the Cameron Road Granite in the Annidale belt of New Brunswick where subduction-related magmas intruded the Penobscot arc–back-arc and were subsequently deformed during the Penobscot Orogeny.

Paleozoic evolution of crustal thickness and elevation in the northern Appalachian orogen, USA

The Acadian and Neoacadian orogenies are widely recognized, yet poorly understood, tectono-thermal events in the New England Appalachian Mountains (USA). We quantified two phases of Paleozoic crustal thickening using geochemical proxies. Acadian (425–400 Ma) crustal thickening to 40 km progressed from southeast to northwest. Neoacadian (400–380 Ma) crustal thickening was widely distributed and varied by 30 km (40–70 km) from north to south. Doubly thickened crust and paleoelevations of 5 km or more support the presence of an orogenic plateau at ca. 380–330 Ma in southern New England. Neoacadian crustal thicknesses show a strong correlation with metamorphic isograds, where higher metamorphic grade corresponds to greater paleo-crustal thickness. We suggest that the present metamorphic field gradient was exposed through erosion and orogenic collapse influenced by thermal, isostatic, and gravitational properties related to Neoacadian crustal thickness. Geobarometry in southern New England underestimates crustal thickness and exhumation, suggesting the crust was thinned by tectonic as well as erosional processes.

Supplemental Material: Paleozoic evolution of crustal thickness and elevation in the northern Appalachian orogen, USA

Geochemical and isotopic data, detailed descriptions of methods, and Figures S1–S4.<br>