Refining the Formation and Early Evolution of the Eastern North American Margin: New Insights From Multiscale Magnetic Anomaly Analyses

Abstract

To investigate the oceanic lithosphere formation and early seafloor spreading history of the North Atlantic Ocean, we examine multiscale magnetic anomaly data from the Jurassic/Early Cretaceous age Eastern North American Margin (ENAM) between 31 and 40°N. We integrate newly acquired sea surface magnetic anomaly and seismic reflection data with publicly available aeromagnetic and composite magnetic anomaly grids, satellite‐derived gravity anomaly, and satellite‐derived and shipboard bathymetry data. We evaluate these data sets to (1) refine magnetic anomaly correlations throughout the ENAM and assign updated ages and chron numbers to M0–M25 and eight pre‐M25 anomalies; (2) identify five correlatable magnetic anomalies between the East Coast Magnetic Anomaly (ECMA) and Blake Spur Magnetic Anomaly (BSMA), which may document the earliest Atlantic seafloor spreading or synrift magmatism; (3) suggest preexisting margin structure and rifting segmentation may have influenced the seafloor spreading regimes in the Atlantic Jurassic Quiet Zone (JQZ); (4) suggest that, if the BSMA source is oceanic crust, the BSMA may be M series magnetic anomaly M42 (~168.5 Ma); (5) examine the along and across margin variation in seafloor spreading rates and spreading center orientations from the BSMA to M25, suggesting asymmetric crustal accretion accommodated the straightening of the ridge from the bend in the ECMA to the more linear M25; and (6) observe anomalously high‐amplitude magnetic anomalies near the Hudson Fan, which may be related to a short‐lived propagating rift segment that could have helped accommodate the crustal alignment during the early Atlantic opening.

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

“…A sharp step-up of the top basement and a 1-km lower crustal root underneath the BSMA suggest a second, larger magmatic pulse when complete lithospheric breakup was achieved. Forward modeling of magnetic anomaly along ENAM Line 1 and Line 2 indicates that the BSMA can be explained by the~1-1.5 km thick step up of the basement indicating that the BSMA is not a common seafloor spreading magnetic anomaly (M42) as recently proposed by Greene et al (2017). Complete lithospheric rupture with a rapid transition to typical oceanic crust involving a robust and shallow ridge axis starting at BSMA time is expected to produce a subsequent ridge push force.…”

“…As Mcbride and Nelson (1988) did in their study, we thus assume that all magnetization in the crust is induced. Remanent magnetization is known to be a significant contributor to magnetic anomalies; however, this simplification is necessary given the uncertainty in the age of the igneous crust, the low amplitude of the non-ECMA anomalies and temporal proximity to the JMQZ (Greene et al, 2017). With these limitations, it would be inappropriate to attempt to discern remanent magnetization chrons within the study area.…”

“…One of the main reasons is that the offshore continent–ocean transition is often buried beneath thick layers of postrift sedimentary rocks preventing drilling holes down to the oldest sediments and/or underlying volcaniclastic rocks. In addition, magnetic anomalies are commonly used to infer the timing of the breakup, but the earliest oceanic crust at many Atlantic margins formed during periods where magnetic anomalies are not well defined (e.g., Cretaceous Normal superchron or Jurassic Magnetic Quiet Zone) (Greene et al, 2017; Moulin et al, 2010).…”

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

“…The modern Mid-Atlantic Ridge displays along-strike segmentation at a similar spatial scale as the 100-to 150-km wavelength variation in ECMA amplitude (Figure 1b), suggesting that the segmentation of the Mid-Atlantic Ridge was inherited from rifting and breakup at the ENAM (Behn & Lin, 2000). While tracing fracture zones is challenging between the ECMA and anomaly M25 (Figure 1), the landward extrapolations of many Atlantic fracture zones align with offsets of linear magnetic anomalies identified between the ECMA and BSMA, suggesting that spreading center segmentation has persisted throughout the crustal evolution from continental breakup to the modern Mid-Atlantic Ridge (Greene et al, 2017;Klitgord & Schouten, 1986). Many of fracture zones in the Atlantic intersect the ENAM at magnetic lows that coincide with the 100-to 150-km wavelength ECMA amplitude variation ( Figure 1) (Behn & Lin, 2000;Greene et al, 2017;Klitgord et al, 1988).…”

The Role of Premagmatic Rifting in Shaping a Volcanic Continental Margin: An Example From the Eastern North American Margin

“…While the data presented here provide a good estimate of the landward boundary of this thinning belt (i.e., the hinge line, Figure 14), its seaward edge, where the crust turns entirely igneous, is more elusive (for further discussion regarding the challenges in determining the edge of the continental crust see Eagles et al, 2015). The high‐amplitude pick of the ECMA was previously regarded as the approximate position of the seaward edge of the continental crust (i.e., ocean‐continent transition; e.g., Austin et al, 1990; Greene et al, 2017; Klitgord et al, 1988; Withjack et al, 2012). In addition, interpretations of refraction profiles along the ENAM suggest that the crust located seaward of the ECMA axis is entirely igneous or oceanic (Figures 6, 8, and 12; Austin et al, 1990; Holbrook et al, 1994; Shuck et al, 2019; Talwani et al, 1995; Talwani & Abreu, 2000).…”