Robert W. Porritt

Lithospheric architecture beneath Hudson Bay

Hudson Bay overlies some of the thickest Precambrian lithosphere on Earth, whose internal structures contain important clues to the earliest workings of plate formation. The terminal collision, the Trans-Hudson Orogen, brought together the Western Churchill craton to the northwest and the Superior craton to the southeast. These two Archean cratons along with the Paleo-Proterozoic Trans-Hudson internides, form the core of the North American craton. We use S to P converted wave imaging and absolute shear velocity information from a joint inversion of P to S receiver functions, new ambient noise derived phase velocities, and teleseismic phase velocities to investigate this region and determine both the thickness of the lithosphere and the presence of internal discontinuities. The lithosphere under central Hudson Bay approaches ∼350 km thick but is thinner (∼200–250 km) around the periphery of the Bay. Furthermore, the amplitude of the LAB conversion from the S receiver functions is unusually large for a craton, suggesting a large thermal contrast across the LAB, which we interpret as direct evidence of the thermal insulation effect of continents on the asthenosphere. Within the lithosphere, midlithospheric discontinuities, significantly shallower than the base of the lithosphere, are often imaged, suggesting the mechanisms that form these layers are common. Lacking time-history information, we infer that these discontinuities reflect reactivation of formation structures during deformation of the craton.

Seismic anisotropy and slab dynamics from SKS splitting recorded in Colombia

The Nazca, Caribbean, and South America plates meet in northwestern South America where the northern end of the Andean volcanic arc and Wadati-Benioff zone seismicity indicate ongoing subduction. However, the termination of Quaternary volcanism at ~5.5°N and eastward offset in seismicity underneath Colombia suggest the presence of complex slab geometry. To help link geometry to dynamics, we analyze SKS splitting for 38 broadband stations of the Colombian national network. Measurements of fast polarization axes in western Colombia close to the trench show dominantly trench-perpendicular orientations. Orientations measured at stations in the back arc, farther to the east, however, abruptly change to roughly trench parallel anisotropy. This may indicate along-arc mantle flow, possibly related to the suggested “Caldas” slab tear, or a lithospheric signature, but smaller-scale variations in anisotropy remain to be explained. Our observations are atypical globally and challenge our understanding of the complexities of subduction zone seismic anisotropy.

On the validation of seismic imaging methods: Finite frequency or ray theory?

We investigate the merits of the more recently developed finite-frequency approach to tomography against the more traditional and approximate ray theoretical approach for state of the art seismic models developed for western North America. To this end, we employ the spectral element method to assess the agreement between observations on real data and measurements made on synthetic seismograms predicted by the models under consideration. We check for phase delay agreement as well as waveform cross-correlation values. Based on statistical analyses on S wave phase delay measurements, finite frequency shows an improvement over ray theory. Random sampling using cross-correlation values identifies regions where synthetic seismograms computed with ray theory and finite-frequency models differ the most. Our study suggests that finite-frequency approaches to seismic imaging exhibit measurable improvement for pronounced low-velocity anomalies such as mantle plumes.

Multiscale crustal architecture of Alaska inferred from P receiver functions

Funding for this work was provided through National Science Foundation CAREER award EAR-1054638 to Miller.

Updates to FuncLab, a Matlab based GUI for handling receiver functions

Receiver functions are a versatile tool commonly used in seismic imaging. Depending on how they are processed, they can be used to image discontinuity structure within the crust or mantle or they can be inverted for seismic velocity either directly or jointly with complementary datasets. However, modern studies generally require large datasets which can be challenging to handle; therefore, FuncLab was originally written as an interactive Matlab GUI to assist in handling these large datasets. This software uses a project database to allow interactive trace editing, data visualization, H-κ stacking for crustal thickness and Vp/Vs ratio, and common conversion point stacking while minimizing computational costs. Since its initial release, significant advances have been made in the implementation of web services and changes in the underlying Matlab platform have necessitated a significant revision to the software. Here, we present revisions to the software, including new features such as data downloading via irisFetch.m, receiver function calculations via processRFmatlab, on-the-fly cross-section tools, interface picking, and more. In the descriptions of the tools, we present its application to a test dataset in Michigan, Wisconsin, and neighboring areas following the passage of USArray Transportable Array. The software is made available online at https://robporritt.wordpress.com/software and the IRIS seismic software repository, https://seiscode.iris.washington.edu/.

Crustal structure across the eastern North American margin from ambient noise tomography

Passive tectonic margins, like the eastern North American margin (ENAM), represent the meeting of oceanic and continental material where no active deformation is occurring. The recent ENAM Community Seismic Experiment provides an opportunity to examine the crustal structure across the ENAM owing to the simultaneous deployment of offshore and onshore seismic instrumentation. Using Rayleigh wave phase and group velocities derived from ambient noise data, we invert for shear velocity across the ENAM. We observe a region of transitional crustal thicknesses that connects the oceanic and continental crusts. Associated with the transitional crust is a localized positive gravitational anomaly. Farther east, the East Coast Magnetic Anomaly (ECMA) is located at the intersection of the transitional and oceanic crusts. We propose underplating of dense magmatic material along the bottom of the transitional crust is responsible for the gravitational anomaly and that the ECMA demarks the location of initial oceanic crustal formation.

Banda arc experiment—transitions in the banda arc-Australian continental collision

ABSTRACT Eastern Indonesia is one of the least well‐understood geological domains on Earth, yet the region is one of the few places which is currently undergoing arc‐continent collision. The Banda arc seismic experiment was designed to unravel some of the complex dynamics of convergent tectonics. This passive source experiment is composed of 30 broadband instruments that extend across the Nusa Tenggara Timur region of Indonesia and across Timor‐Leste. This along‐strike deployment allows for seismic imaging and assessment of the spatiotemporal evolution of the collision of oceanic to continental lithosphere of the Indo‐Australian plate with the active volcanic arc. The experiment has been continuously recording broadband seismic data since early 2014 and will continue through the end of 2016, and the data will be archived at the Incorporated Research Institutions for Seismology Data Management Center under network code YS and will be available at the end of 2018.

Midcrustal Deformation in the Central Andes Constrained by Radial Anisotropy

Author(s): Lynner, Colton; Beck, Susan L; Zandt, George; Porritt, Robert W; Lin, Fan-Chi; Eilon, Zachary C