My research focuses on better understanding the properties, structure and dynamics of the continents through the use of passive source seismic imaging methods. I describe my research interests and related papers below. For more information, please check out my publications and CV or email me.
The physical and structural properties of continental lithosphere
The continental lithosphere makes up only 30% surface area of the Earth but provides us a record of the Earth’s geologic history, with evidence for notable events ranging the oxygenation of the Earth’s atmosphere and the beginning of life, to the initiation of plate tectonics. It also allows relatively easy access needed to understand present day tectonic processes and associated hazards, such as earthquakes and volcanoes. The physical and seismic properties of the continental lithosphere are unique, and my principal area of interest is in characterizing the seismic structure of the continental lithosphere, including the boundary that defines the bottom, commonly referred to as the lithosphere-asthenosphere boundary. This is done with hopes of understanding what differentiates the lithosphere from the rest of the upper mantle and to understand how tectonic and dynamic processes can shape and modify the lithosphere. One paper that I co-authored, Fischer et al. (2010) provides a review of the lithosphere-asthenosphere boundary across the planet. Most other publications comment on some aspect of the continental lithosphere but do so by emphasizing different questions, tectonic processes or regions of interest. Meanwhile, continent-wide studies of North America (Abt et al. 2010) and Australia (Ford et al., 2010; Berkey et al., 2021; Berkey et al., 2023) help to provide a better sense for how the structure and properties of the lithosphere may change over time. Below, I have highlight studies related to specific regions and processes.
Cratonic structure, formation and evolution
A significant portion of my research focuses on imaging the seismic structure of cratons, which comprise the oldest parts of the continents. I am particularly interested in their stability and longevity. In contrast to oceanic lithosphere, which eventually becomes negatively buoyant with cooling and old age, cratons have managed to remain buoyant despite similarly cold temperatures and much older ages. During my PhD I focused on imaging enigmatic seismic structures found within cratons, which are termed mid-lithospheric discontinuities (MLDs) (Abt et al., 2010; Fischer et al., 2010; Ford et al, 2010). Some have hypothesized that MLDs are related to craton formation and/or early evolution and may provide insight into early plate tectonics. In Selway et al. (2015) we provide a review of what is known about cratons and their potential origin, from a multidisciplinary perspective. Ford et al. (2016) examines the hypothesis that the discontinuities are the result of deformation fabrics (seismic anisotropy) “frozen” into the cratons during early plate tectonics. In the study we find that while complex deformation fabrics are present, they are unlikely to be the source of the seismic features in question (MLDs). Berkey et al. (2021) authored by my former PhD student Andrew Berkey, focuses on the first order characteristics of MLDs and provides evidence that the features are the result of metasomatism related to subduction, an important finding that corroborates other studies elsewhere. Berkey and Ford (2023), also authored by A. Berkey, focuses on seismic anisotropy in the Australian lithosphere and finds significant disagreement between studies and little-to-no regional trends in the data. Additional work to understand these complexities is ongoing. The work of Berkey et al. (2021) and Berkey and Ford (2023) was funded by NSF.
One aspect of cratons that fascinates me is that despite their longevity, we also have evidence to suggest that their lithosphere can also be significantly modified or destroyed. One such craton where this has been suggested to be actively occurring is the Wyoming Craton. During the Laramide Orogeny, deep-seated basement uplifts marked the surface of the craton, while seismic tomography models show evidence for large variations in lithospheric thickness. From 2017-2019 we deployed an array of broadband seismometers across the eastern half of the Wyoming Craton to better understand the seismic structure of the craton. The experiment was named CIELO (http). Our initial results from this experiment find that the craton has been tectonically modified to an extent not previously recognized (Ford et al., 2021; Zhao et al. 2021). Details of the deployment and preliminary results from several methods are described in Ford et al. (2021). We anticipate that several (3-4) more publications will result from the study, with two of the manuscripts undergoing final edits currently.
Lithospheric modification, tectonic inheritance and regions of active tectonics
Another focus of my research is on understanding how lithosphere is tectonically or geodynamically modified in regions aside from cratons, and to what extent these modifications can persist through time. Work in southern New England finds that lithospheric properties inherited in past tectonic episodes can persist through several Wilson cycles and may even impact how later modification is accommodated. This work was authored by my former PhD student, Gillian Goldhagen (Goldhagen et al., 2022). The results of Long et al. (2017) largely echo that of Gillian’s work, finding a seismic signature related to ancient orogenesis within the eastern United States. The idea that the mantle lithosphere can retain structure over millions and even billions of years runs counter to the commonly held view that upper mantle properties are best explained by thermal cooling of the lithosphere over time.
The theme of characterizing deformation in the lithosphere is highlighted in my work on the Pacific-North American plate boundary (Ford et al., 2014). While we generally understand how deformation is accommodated in the upper crust (faults). The distribution of deformation at the base of lithospheric mantle beneath transform plate boundaries is less clear. Models range from diffuse shear zones hundreds of miles across to localized strike-slip shear zones that are deep extensions of individual crustal faults. Using an extensive dataset of scattered shear-waves we produced a detailed three-dimensional image of the lithosphere-asthenosphere boundary (LAB) across the entire San Andreas fault system. The results from the research imply that the width of the plate boundary remains relatively narrow (<50 km wide) to the base of the mantle lithosphere, in contrast to many previously published models.
Another important avenue of my work is in understanding the thermal structure of the mantle. Beneath east Africa, the absolute temperature of the mantle is debated. Seismic velocities indicate that the mantle there is very hot, however constraints from volcanic rocks suggest that the mantle is only moderately warm. A UCR colleague (Maryjo Brounce) and I have an NSF-funded grant designed to reconcile geochemical and geophysical constraints of temperature beneath the East Africa Rift. Reconciling the two constraints has important implications for our understanding of how new plate boundaries, specifically regions of rifting, form.
While the linkages between tectonics and mantle dynamics in east Africa are generally well understood, the relationship between the two is debated elsewhere. For example, the presence of relatively young volcanism in central Virginia has long been a mystery. While extension generated volcanism occurred elsewhere in eastern North America, the volcanic ages in Virginia are too young to correlate with such rifting. Another possible explanation, a mantle plume, is also unlikely given a scarcity of evidence. Evans et al. (2019) proposes that topography of the mantle lithosphere beneath Virginia is sufficient to generate small scale convection capable of producing melt.
Lower Mantle Flow
The existence of two large low shear velocity provinces (LLSVPs) in the lower mantle beneath Africa and the Pacific has been well documented through the use of seismic tomography imaging. LLSVPs are thought to represent areas of chemically distinct, higher velocity material, and may act as a source for mantle plumes, however the interpretation of their origin, structure, composition, and dynamics is still poorly understood. Utilizing observations of seismic anisotropy from analysis of shear wave splitting, we found that mantle flow along the edge of the African LLSVP in the lowermost mantle (D”) is best fit by a conceptual model in which the African LLSVP acts as a barrier to flow in the lowermost mantle, resulting in a vertical-to-oblique deflection of mantle material (Ford et al., 2015; Ford and Long, 2015). Rounding out work related to dynamic features in the mantle, Creasy et al. (2016) constrains the direction of mantle flow in the lowermost mantle beneath Australia. Understanding the direction of flow in the lowermost mantle may help us to better understand the extent to which the structure and physical properties of the lowermost mantle informs the evolution of plate tectonics cycles observed at the surface.
Other areas of research
The study of seismic noise (i.e., non-earthquake ground displacements), generated by both anthropogenic and natural sources, can inform us of processes ranging from changes in human behavior (Wu et al., 2021) to changes in near surface velocities due to seasonal changes in temperature (Ford et al., 2021). Wu et al. (2021) is notable as it examines changes in noise coincident with shutdowns at the start of the COVID-19 pandemic. Recently, I led efforts to purchase 50 nodal seismometers, which are ideal for being deployed in urban areas where noise is plentiful and can be used as a signal to generate images of the Earth’s interior. As part of a collaborative effort with the University of Utah and Caltech, 300 nodes were deployed across the Los Angeles Basin in June 2020. Students from all three universities will use the data collected to better constrain geologic structure and seismicity in the basin.
Another new focus for my research program is on the utilization of magnetotelluric (MT) methods. MT methods are similar to seismic methods in the sense that they provide constraints on Earth structure, however they rely on the Earth’s geomagnetic and geoelectric fields. MT and seismic methods sensitivities are different, and when integrated, provide improved constraints on crust and mantle structure (Evans et al., 2019). In 2022 I attended a week-long short course hosted by IRIS on how to incorporate MT methods into my research, something that I am excited to do in the future
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