Why do we study the Blake Plateau?
As the weather became better, we finished deploying along line two and then returned to line one to retrieve the remaining OBS. Because the seafloor is deep, it takes an OBS up to two hours to float to the surface. At this point, we have recovered all of them from line one, and we will deploy along line three in the next few days. While we are transiting to line three, Victor Obi wrote a review on the geological settings of the Blake Plateau and its unsolved mysteries. Victor earned his MS degree in Geology from Kent State University, Ohio, Read below to find out why it is exciting to study the Blake Plateau!
Cover photo by Harm Van Avendonk
Blake Plateau, a segment of the Eastern North American Rifted Margin
Continental margins are typically divided into two primary categories: volcanic and non-volcanic, although it is not uncommon to encounter intermediate classifications. The Blake Plateau's continental margin is noticeably wider than the neighboring Carolina Trough to the north, indicating distinct processes of extension and lithospheric rupture in these adjacent rift segments. These variations in rifting styles may have been influenced by disparities in mantle temperature anomalies, mantle melting, and the structural characteristics inherited by the continental lithosphere.
The Blake Plateau, situated to the north of the Bahamas, and the Carolina Trough, a narrower rifted margin, exhibit distinct characteristics. While the Blake Plateau resembles the Bahamas as a predominantly submerged carbonate platform, it is found at depths of approximately 800 meters and has been significantly impacted by the powerful Gulf Stream currents, resulting in the removal of sediments. In contrast, further north, the Blake Ridge primarily consists of sediment drift and lacks substantial basement relief.
Our understanding of the composition and origins of the basement beneath the Blake Plateau remains limited, primarily due to the shallow depth penetration of older seismic reflection profiles. Additionally, irregular magnetic data patterns suggest that the plateau's formation did not adhere to the typical seafloor spreading process. Conversely, the Carolina Trough represents a more conventional rifted margin characterized by a rapid thinning of the crust towards the ocean. What explains the stark differences between these continental margins? It is plausible that the Carolina Trough underwent a much more rapid rift process compared to the Blake Plateau.
Victor is looking at a bathymetry map of Blake Plateau.
Important Local Geological Features and their Time Constraints
Both Carolina Trough and Blake Plateau experienced rifting around 195 million years ago, with a focus on offshore basins occurring around 185 million years ago. Despite the absence of drilling data, the Blake Spur Magnetic Anomaly (BSMA), while not indicative of traditional seafloor spreading, serves as a rough temporal marker at approximately 170 million years old. Importantly, the BSMA is situated much closer to the eastern edge of the Blake Plateau than the eastern edge of the Carolina Trough. This implies that the final stages of rifting on the Blake Plateau may have continued until around 171 million years ago, in contrast to 175 million years ago on the Carolina Trough. The northwest-southeast direction of rifting resulted in several significant fracture zones across both margins, accommodating strike-slip motion during the rifting process. These fracture zones divide the margins into regions with similar kinematic histories. Notably, the Blake Plateau basin lies between the Jacksonville fracture zone to the south and the Blake Spur fracture zone to the north. Further north, the Carolina Fracture zone extends beyond the Blake Ridge, offshore Cape Fear. Many of the survey lines conducted in the area also follow a northwest-southeast orientation, often intersecting or closely aligning with these fracture zones, providing valuable data for comprehending the geological features of the region.
Gravity data indicates that the crust of the Blake Plateau is 10-20 kilometers thick, potentially twice as thick as the crust in the Carolina Trough. Moreover, the Blake Plateau spans three times the width of the Carolina Trough. Although gravity data offers insights, seismic data collected during Langseth cruises will provide more precise information about these geological geometries. The thinner crust suggests extensive stretching of the continental crust, possibly exceeding 100%, during the rifting process. Additionally, volcanic activity likely contributed to the formation of new igneous crust. The robust crust of the Blake Plateau presents challenges when aligning it with plate reconstructions of North America and Africa dating back to the Triassic/Jurassic period, around 200 million years ago. This incongruity raises the possibility that some parts of the plateau may be younger, potentially resulting from volcanic activity.
A happy selfie with OBS after a successful retrieval :)
Research Objectives and Hypotheses
The research team's objectives are to investigate the following three hypotheses:
1. The greater width of the Blake Plateau Basin compared to the Carolina Trough Basin prior to breakup can be attributed to variations in lithospheric thickness.
2. During the rifting process, the supply of magma matched the extension of the Blake Plateau's crust, while the continental crust beneath the southern Carolina Trough underwent rapid thinning with limited magmatic activity.
3. At the onset of rifting, the deep mantle beneath the Blake Plateau experienced significantly higher temperatures than the mantle beneath the Carolina Trough, influencing lithospheric rheology and the extent of mantle melting.
Our goal is to explore the impact of tectonic extension and magmatic processes within the context of continental rifting and to gain insights into the condition of the mantle during the formation of the earliest oceanic crust off the southeastern coast of the United States.
The ship shakes with large waves. The crew is trying to catch a swaying OBS. Photo by Todd.
What is currently within our knowledge and what are the aspects we aim to understand better?
Traditionally, it was believed that volcanic margins had their origins in close proximity to mantle plumes. However, the association between mantle hotspots and volcanic rifted margins shows a limited correlation. Moreover, the shift in margin structure from volcanic to nonvolcanic can occur suddenly over a relatively short distance of less than 50 kilometers, a phenomenon that cannot be solely explained by variations in mantle temperature. Thus, it becomes apparent that lithospheric factors also wield significant influence over the processes of rifting and magmatism. One plausible explanation for these rapid structural transitions involves the lateral spread of hot asthenospheric material parallel to the rift itself. The generation of substantial molten material beneath continental rifts can occur either due to profound thermal anomalies at great depths or as a consequence of decompression beneath a thinning lithosphere. Our comprehension of how the extension of lithospheric plates interacts with the generation and movement of magma in this context remains limited.
Marine geophysical investigations of rifted margins have played a pivotal role in facilitating the scientific community's exploration of continental rifting outcomes, including features such as extensional faults, volcanic and sedimentary formations, and the adjacent oceanic crust. In the early stages of researching volcanic rifted margins, some theories posited that the process of continental breakup was predominantly driven by deep mantle upwelling and the intrusion of molten mantle material. However, contemporary marine seismic data have illuminated the fact that continent rupture often results from a complex interplay between faulting and magmatic activity. In the southeastern United States, particularly offshore in the Blake Plateau and the Carolina Trough, two Jurassic rifted margins have been the subject of investigation. Existing seismic reflection data from these regions provide evidence of both brittle extension and the presence of volcanic activity during the syn-rift period. The Carolina Trough constitutes a relatively narrow margin located beneath the prominent East Coast Magnetic Anomaly, indicating rapid extension that led to the formation of a volcanic wedge. In contrast, the Blake Plateau extends several hundred kilometers in width and lacks a distinct coastal magnetic anomaly. There are two potential interpretations: either the Blake Plateau primarily represents a volcanic structure that formed concurrently with the rifting process, or it could be a block of continental lithosphere that underwent extension through a more widely dispersed system of faults compared to the Carolina Trough. A more comprehensive examination of the crustal structure of these margins holds the promise of yielding deeper insights into the intricate relationship between faulting and magmatic processes that have shaped these neighboring rifted margins, each distinguished by its unique geometric features. If we assume that the Blake Plateau formed due to a plume or hotspot around 190 million years ago, the question arises: where might the current location of this hotspot be?
Klay, Aaron, Dan, and Riley are detaching an OBS after they lifted it out of the sea.
References:
Bécel, A., Davis, J. K., Shuck, B. D., Van Avendonk, H. J., & Gibson, J. C. (2020). Evidence for a prolonged continental breakup resulting from slow extension rates at the eastern North American volcanic rifted margin. Journal of Geophysical Research: Solid Earth, 125(9), e2020JB020093.
Franke, D. (2013). Rifting, lithosphere breakup and volcanism: Comparison of magma-poor and volcanic rifted margins. Marine and Petroleum geology, 43, 63-87.
Dillon, W. P., & Popenoe, P. (1988). The Blake plateau basin and Carolina trough.
Written by Victor Obi
Blog edited by Ranpeng Li
