From Crystals to Crustal‐Scale Seismic Anisotropy: Bridging the Gap Between Rocks and Seismic Studies With Digital Geologic Map Data in Colorado

Abstract

Deep continental crustal structures are enigmatic due to lack of direct exposures and limited tools to investigate them remotely. Seismic waves can sample these rocks, but most seismic methods focus on coarse crustal structures while laboratory measurements concentrate on crystal‐scale rock properties, and little work has been conducted to bridge this interpretation gap. In some places, geologic maps of crystalline basement provide samples of the intermediate‐scale fabrics and structures that may represent in situ deep crust. However, previous research has not considered natural geometric variations from map data, nor is this heterogeneity typically included in map‐scale seismic property calculations. Here, we test how map‐scale fabrics influence crustal seismic anisotropy in Colorado by analyzing structural data from geologic maps, combining those data with bulk rock elastic tensors to calculate map‐scale seismic properties, and evaluating the resulting comparisons with observed receiver function A1 (360° periodic) arrivals. Crystalline fabrics, predicted seismic properties, and tectonic structures positively correlate with shallow and deep crustal A1 arrivals. Additionally, widespread correlations occur between mapped fault traces and regional foliations, implying that preexisting mechanical heterogeneity may have strongly influenced subsequent reactivation. We interpret that various mapped geologic contact types (e.g., lithologic and structural) generate A1 arrivals and that multiple parallel features (e.g., faults, foliations, and intrusions) contribute to a seismically visible tectonic grain. Therefore, Colorado's exhumed basement, as expressed in outcrops and maps, offers insight into modern deep crustal geological and geophysical structure.

Tectonic Fabric in the Banda Arc‐Australian Continent Collisional Zone Imaged by Teleseismic Receiver Functions

“…The complicated and azimuthally varying RF waveforms shown in their paper, however, indicate the breakdown of this assumption. Similar RF signals have been used elsewhere to infer lithospheric anisotropy and its tectonic implications (e.g., Audet, 2015;Bar et al, 2019;Birkey et al, 2021;Bourke et al, 2020;Chen et al, 2021;Frothingham et al, 2022;Gosselin et al, 2021;Levin et al, 2021;Nikulin et al, 2019;Olugboji et al, 2016;Salimbeni et al, 2021;Schulte-Pelkum et al, 2021;Sherrington et al, 2004;Vergne et al, 2003). In addition, the small number of RF measurements (<20-60 for more than half of stations) considered in Syuhada et al (2016) may lead to uncertainties in their results.…”

Using Stochastic Point Pattern Analysis to Track Regional Orientations of Magmatism During the Transition to Cenozoic Extension and Rio Grande Rifting, Southern Rocky Mountains

“…Recent structural analyses by Frothingham et al. (2022) observed that surficial fault traces and ductile fabrics in Colorado have a dominantly southeast‐northwest orientation, and that these fabrics continue at depth where they can act as focal planes for magmatism. It is not clear why this southeast‐northwest magmatic fabric was not present between 35 and 25 Ma (Figure 7, left panels).…”

“…The regional southeast-northwest alignment of magma centers between the early ignimbrite flare-up (40 to 35 Ma; Figure 8b) and again during Rio Grande rift formation (25 to 20 Ma; Figure 8c) is subparallel to Paleo-and Mesoproterozoic structures that were reactivated during the Laramide orogeny (Tweto, 1975). Recent structural analyses by Frothingham et al (2022) observed that surficial fault traces and ductile fabrics in Colorado have a dominantly southeast-northwest orientation, and that these fabrics continue at depth where they can act as focal planes for magmatism. It is not clear why this southeast-northwest magmatic fabric was not present between 35 and 25 Ma (Figure 7, left panels).…”

“…The pre‐Laramide (>75 Ma) history of the southern Rocky Mountains includes multiple episodes of continental crust formation, accretion, and deformation that resulted in distinct regional structural trends, and these inherited structural fabrics influenced subsequent magmatism and deformation (Frothingham et al., 2022). The oldest basement rocks exposed in the southern Rocky Mountains of Colorado and northern New Mexico are composed of juvenile continental provinces, the Yavapai and the Mazatzal, that accreted onto the southern margin (present coordinates) of the Archean Wyoming craton during the Paleoproterozoic (Figure 1a) (Shaw et al., 2001; Whitmeyer & Karlstrom, 2007).…”

Confronting Solid‐State Shear Bias: Magmatic Fabric Contribution to Crustal Seismic Anisotropy

“…However, despite igneous rocks making up significant portions of continental crust (Figure 1), magmatic fabrics are generally underrepresented in crustal seismic anisotropy studies and bulk rock elastic tensor databases (Brownlee et al., 2017). Accordingly, the studies that use such databases to predict and interpret crustal seismic anisotropy (e.g., Frothingham et al., 2022; Moschetti et al., 2010; Okaya et al., 2018) also tend to propagate this bias toward solid‐state deformation.…”