Variations of Crustal Thickness in Alaska and Northwestern Canada: Implications for Crustal Modification and Accretion of Tectonic Units

Abstract

The northwestern part of North America has recorded multiple tectonic events, such as terrane accretion, strike‐slip motion, and subduction of the Pacific and Yakutat plates, providing an iconic setting to investigate the tectonic evolution of the continental crust. In this study we analyze the receiver functions at seismic stations deployed during 1999–2022 to estimate the crustal thickness, as well as possible slab signature, in Alaska and northwestern Canada. The Moho signal can be clearly detected within the continental region. Specifically, in northwestern Canada, the thickest crust is observed beneath the Cordilleran Deformation Front, which marks the structural boundary between the North American Craton and the North American Margin. We observe a few distinct offsets in the Moho depth located both within the tectonic units and approximately across the major faults between the tectonic units. We provide a first‐order estimate of the depth gradient of the Moho offsets based on the horizontal distance of the two closest seismic stations across the offsets. We propose that the Moho offsets reflect the cumulative impact of the accretionary orogenies and post‐orogenic tectonic events on crustal modification. The continental Moho signal is weak or obscure in Aleutian and southcentral Alaska, and the oceanic Moho within the subducting plates is likely detected. This study provides new seismic insights into understanding the impacts of the tectonic events on continental formation and evolution.

The Global Spectrum of Seafloor Morphology on Mid‐Ocean Ridge Flanks Related to Magma Supply

“…The ridge axis transitions from an axial high to a deep axial valley over the same interval (Figures S7 and S8 in Supporting Information S1) (Ma & Cochran, 1996; Shah & Sempéré, 1998), while seismic crustal thickness decreases eastward, from about 6 km in the west to 3.5 km in the AAD (Holmes et al., 2008, 2010). Mantle Bouguer and RMBA gravity correspondingly increase eastward, with RMBA reaching a regional high of about 50 mGal in the center of the AAD (Cochran & Sempéré, 1997; Lin & Zhu, 2015). The trend of all these parameters reverses at a location between the Euroka and Birubi fracture zones, and morphology similar to that at the western edge of the data set occurs farther to the east (Figure 2b) (Forsyth et al., 1987; Holmes et al., 2010; Marks et al., 1990).…”

“…For Pacific MORs, and for Atlantic and Indian MORs where no shipboard RMBA data were available, we used the satellite RMBA data of Lin and Zhu (2015) directly. The satellite data do not provide the spatial resolution available from the shipboard data, but they do allow the data boxes we studied to be placed in the correct position within the spectrum of RMBA values exhibited regionally at MORs.…”

“…Values of MVT in one region cannot be compared directly to values in another region (e.g., the MAR vs. the EPR) because deep, large‐scale variations in mantle composition and thermal structure around the globe create long‐wavelength differences in RMBA levels (e.g., Crosby & McKenzie, 2009; Dalton et al., 2014; Lin & Zhu, 2015), and these differences offset the calculated MVT values between regions. However, the trends of how morphologic parameters change with MVT in one region can be compared with their trends in another region, irrespective of region‐to‐region offset in MVT levels.…”

“…The data are from shipboard gravity surveys where available and from satellite gravity elsewhere (for references see Table S1). Mean satellite RMBA values in data boxes (Lin & Zhu, 2015) were used to normalize mean shipboard RMBA values in the data boxes to a standard datum (Table S2 in Supporting Information S1) (see Section 3.4).…”

“…The RMBA data sets from various surveys were calculated by different investigators using different methods, reference levels, and assumed reference crustal thicknesses. We adjusted all the shipboard RMBA data sets to match the satellite-derived global RMBA gravity field of Lin and Zhu (2015) (Table S1). This adjustment was made by calculating the mean RMBA in representative areas (9 × 10 3 to 74 × 10 3 km 2 ) of each shipboard data set, comparing it to mean RMBA of the same areas in the satellite gravity data, and applying the observed differences to each shipboard data set (Table S2 in Supporting Information S1).…”