Science Atlantic - Atlantic Geoscience Society Speaker tour: Dr. Shawna White
Dr. Shawna White
Saint Mary’s University
Title: Structural Inheritance within the Laurentian Realm of the Northern Appalachians
Abstract: The northern Appalachian Orogen, extending from Newfoundland (NL) to New England (NE), has a conspicuous sinuous trend of promontories and embayments along its length, interpreted as inherited from the original geometry of Laurentia’s eastern margin, formed during Neoproterozoic through Cambrian rifting of Rodinia. Diachronicity in major orogenic pulses is also largely attributed to the margins irregular shape, with earliest collisions occurring first at the promontories and later in the embayments. Although this structural inheritance is largely hypothesised to have major implications on orogen architecture and development, reconstructions of the pre-orogenic basement geometry remain speculative, and reactivation of inherited structures is largely unrecognized.
Interpretation of an extensive compilation of previously published detrital zircon data from rift-related units demonstrates major contrasts in detrital zircon age spectra between the Quebec (QC) and NL segment of the orogen. These contrasts provide tangible evidence of a major NW-striking transfer fault at the promontory-embayment transition - blocking sediment transport between NL and QC.
Recent work in the Laurentian realm of the NL Appalachians demonstrates that rift-generated faults, have a protracted history. Stratigraphic relationships indicate that most basement thrusts, originating as normal faults during Neoproterozoic rifting, were subsequently reactivated during Ordovician Taconian loading of the Laurentian margin. These thrusts demonstrate significant previously unrecognized inversion, with basement-cored fault propagation folds having been transported upwards of 9 km during the Early Devonian Acadian Orogeny.
Current re-evaluation of map relationships and previously published work indicates that analogues structures, to those in NL, may also occur in QC and NE. My research program will focus on these analogous long-lived fault systems and associated structures. We will challenge traditional tectonic models of Appalachian orogenesis, which currently attribute most significant deformation as thin-skinned and test if analogous thick-skinned (basement-involved) faults have a similar complex history of reactivation (normal and inverted motion) and demonstrate significant (previously unrecognized) displacement. By applying geochronological methods (U-Pb dating of carbonate slickenfibres) to these faults, we will attain absolute timing of motion where currently only loose relative timing can be inferred.
The recognition of inversion related structures elsewhere in the Appalachians will be invaluable from an economic perspective. Petroleum and gas, discovered in the Port au Port and Parsons Pond regions respectively, is now understood to be associated with these inversion structures. Reactivated basement faults are also critical components of mineral systems, as they tap and focus fluids, heat, and metals, that form sedimentary-hosted mineral systems commonly found in orogens and their forelands.
Bio: Shawna White is an Assistant Professor in the Geology Department at Saint Mary’s University in Halifax. Shawna completed her PhD at the University of Alberta in 2018 working on Northern Appalachian geology. That same year she moved on to Laurentian University and began working as a postdoctoral fellow. There she worked largely on Archean greenstone belt structure and stratigraphy in Northern Ontario. In 2020 she began a faculty position at SUNY Oneonta in New York State before finally landing in Halifax in 2021. Her current research program uses a multidisciplinary approach, combining field methods of mapping and stratigraphy with geochronology and geophysical interpretation, in order to understand the role of structural inheritance and fault reactivation at the deformation front of the Northern Appalachians.
Milligan Room (8th floor LSC Biology Wing)