Evidence for a Prolonged Continental Breakup Resulting From Slow Extension Rates at the Eastern North American Volcanic Rifted Margin

Abstract

When continental rifting is accompanied by localized magmatism under extensional stress, the breakup duration can be short and the continent/ocean transition sharp, as mantle melts are thought to be efficient at heating and weakening the lithosphere. This mode of rifting has been invoked for the Eastern North American Margin (ENAM) based on the existing geophysical data. Here, we present results from multichannel seismic profiles from the ENAM Community Seismic Experiment offshore North Carolina, U.S. Our survey area encompasses both the East Coast Magnetic Anomaly (ECMA) and the Blake Spur Magnetic Anomaly (BSMA), which lies~200-km farther seaward. Our prestack depth-migrated seismic images reveal major changes in the structure of the igneous crust across the BSMA. Between the ECMA and BSMA, we image a proto-oceanic domain of rough, faulted, and thin igneous crust. The roughness of this oceanic crust is similar to modern ultraslow spreading environments which involve the continued presence of a pre-existing lithospheric lid. Seaward of the BSMA the basement is smooth, and the crust is relatively thick, which is typical for Jurassic oceanic crust. Across the BSMA, we image a step up in basement and crustal root, which we interpret to represent complete lithospheric breakup and a transition to steady-state seafloor spreading in agreement with coincident refraction results. Our results would also indicate low extension rates in the final stages of rifting that may have influenced the thermal structure of the lithosphere and could explain the delay for continental breakup. All of these observations show that although continental rifting between eastern North America and northwest Africa was assisted by magmatic activity, it did not lead to rapid localization of extensional strain as previously thought.

Along‐Margin Variations in Breakup Volcanism at the Eastern North American Margin

“…SDRs were first identified at the ENAM by Hinz (1981) in seismic reflection data, and they interpreted these SDRs to represent volcanism during continental breakup. Numerous seismic surveys led to further, extensive investigation of SDRs at the ENAM (Figure 2b), providing additional support to the interpretation that the SDRs represent the subaerial emplacement, and subsequent subsidence, of a volcanic wedge along the margin (e.g., Austin et al, 1990; Bécel et al, 2020; Holbrook et al, 1994; Lase Study Group, 1986; Oh et al, 1995; Sheridan et al, 1993; Talwani et al, 1995; Tréhu et al, 1989). SDRs at the ENAM are overlaid by the thick sediment of the Carolina Trough, Baltimore Canyon Trough, and Georges Bank sedimentary basins (Figure 2c) (Klitgord et al, 1988).…”

“…ECMA extent indicated by black/white contours (Figure 1a) (Klitgord et al, 1988). Key seismic lines/surveys labeled; red lines/polygons indicate extent of SDR interpretations from previous studies (Austin et al, 1990; Bécel et al, 2020; Hinz, 1981; Holbrook et al, 1994; LASE Study Group, 1986; Sheridan et al, 1993; Tréhu et al, 1989). Purple dashed line indicates the inferred suture zone surrounding a hypothesized isolated piece of West African Craton under Chesapeake Bay (Lefort & Max, 1991, their Figure 7).…”

“…After its discovery, various sources for the ECMA were proposed, including a basement ridge (Klitgord & Berhrendt, 1979), an edge effect from the juxtaposition of higher‐ and lower‐magnetized crust (Hutchinson et al, 1982; McBride & Nelson, 1988) or highly magnetized crust (Alsop & Talwani, 1984; Keller et al, 1954; Taylor et al, 1968). Following the identification of SDRs in seismic data that are spatially correlated with the ECMA (e.g., Austin et al, 1990; Holbrook et al, 1994; Lase Study Group, 1986; Sheridan et al, 1993; Tréhu et al, 1989), the general consensus has been that ECMA is attributed to the volcanic wedge emplaced during continental breakup based on the results of 2‐D magnetic forward modeling (e.g., Austin et al, 1990; Bécel et al, 2020; Davis et al, 2018; Talwani et al, 1995). Talwani et al (1995) suggest that the ECMA is a remanent anomaly from a rapidly emplaced volcanic wedge.…”

“…Other geophysical data, such as seismic, are sparsely distributed over the ENAM (Figure 2) and have difficultly continuously imaging deep margin structure. Nevertheless, we use such information to augment and constrain for our magnetic modeling results wherever they are available at the ENAM (Figure 2) (Austin et al, 1990; Bécel et al, 2020; Hinz, 1981; Holbrook et al, 1994; Lase Study Group, 1986; Sheridan et al, 1993; Talwani et al, 1995; Tréhu et al, 1989). Based on our magnetic modeling, we show that the observed variations in the ECMA amplitude and character can be explained by first‐ and second‐order along‐strike variations in the thickness and width of breakup volcanism, which provide new insights into the distribution and amount of magmatism at the ENAM.…”

“…Various tectonic scenarios have been proposed for the early Atlantic opening. Scenarios such as a ridge jump (or two ridge jumps) (Bird et al, 2007; Klitgord & Schouten, 1986; Schettino & Turco, 2009; Vogt, 1973), asymmetric spreading that underwent a drastic change in rate and direction (Labails et al, 2010), or a period of protooceanic crust prior lithospheric rupture (Bécel et al, 2020; Shuck et al, 2019) between the ECMA and the Blake Spur Magnetic Anomaly (BSMA) (Figure 1) have been proposed to explain the wider domain of early ocean crust that is present offshore the ENAM compared to the conjugate margin offshore northwest Africa (Labails et al, 2010). Additionally, a spreading axis reorientation may have been present to accommodate the difference in strike between the initial seafloor spreading outboard of the ECMA and the modern Mid‐Atlantic Ridge (Greene et al, 2017).…”