The seas around Florida provided the opportunity to analyse the movement of the Earth’s crust during the events that saw the breakup of Pangaea, explains Sabrina Reichert.
The Blake Plateau and Carolina Trough, proximal to the Florida coast and the edge of Bahamas international waters, comprise a greater study area of geophysical intrigue. For those who study tectonic boundaries and mid-ocean ridges, the Blake Plateau and Carolina Trough present a unique juxtaposition of locationally disparate rifting behaviours observed throughout the Jurassic epoch and the breakup of Pangaea. In other words, the scientific community wonders why the Blake Plateau was so geologically active relative to the fairly ‘metastable’ Carolina Trough, changing as slowly as the geological timescale, as the continents separated.
We ultimately seek a more definitive characterisation of how the crust moved during Pangaea’s breakup, more specifically whether it rifted (spread from the nearby mid-ocean ridge and was pushed apart) or drifted (floated on the mantle in what we call isostatic equilibrium), and how we can draw a comparison between how this happened in the Blake Plateau and Carolina Trough regions.
In order to define the tectonic and geodynamic behavior of the area in question, which can be best described as two pieces of the crust above the Central Atlantic Magmatic Province (CAMP), we use Multi-Channel Seismic (MCS) and Ocean- Bottom Seismometer (OBS) technology. These two types of instrumentation afford a window into the subsurface, allowing research teams, such as my group at the University of Texas Institute for Geophysics (UTIG), to penetrate the water column for a detailed and high-resolution image of the layers of the lithosphere (crust) and mantle asthenosphere (upper mantle).
We ultimately seek a more definitive characterisation of how the crust moved during Pangaea’s breakup, more specifically whether it rifted or drifted
MCS and OBS fieldwork are conducted at sea, on vessels such as the RV Marcus Langseth that carry highly specialised scientific instrumentation. For MCS cruises, data is collected via hydrophone/ air gun streamers that trail behind the boat throughout the duration of the cruise. MCS builtin gear includes: air guns that propagate a pulse through the water and into the seafloor; GPS devices that allow us to correct for streamer drift in the data processing stages; digibirds, which are devices that stabilise the streamers in the water with their winged design; and other functionalities such as a rescue deployment device that serves as a recovery mechanism for any streamer that dives below 10m subsurface.
OBS instruments are deployed to the seafloor and subsequently retracted, with experimentation conducted at regular intervals along the ship’s travel path. OBS devices perceive seismic activity through direct contact with the ocean floor, whereas MCS streamers pick up reflections from the initial air pulse’s propagation through the water, its bounce off the seafloor and its return to the streamer for detection.
While at sea, the team took precautions not only to preserve the equipment and ensure the integrity of the data, but also to be mindful of wildlife. A number of species crossed through the ship’s lines, including these bottlenose dolphins, with others including whales and loggerhead turtles. Protected species observance occurred as part of the ship’s 24/7 operation with the help of the wildlife biologists on board.
Data processing includes a mix of quality control done during data acquisition and postprocessing and analysis. Once on land, a lot of our team’s work is a systematic yet thoughtful elimination of noise from the dataset, which could mean anything from a broken air gun channel giving faulty readings to something like ‘resonance’, a periodic interference found throughout the data. The data in its most raw form comes as SEGD files, so a lot of our work is done through a series of production flows in Paradigm Echos software and other Linux shell scripts—our objective is converting to the proper file format while still recognising that SEGD files are formatted akin to 32mm tapes (based on the old methods of collecting marine seismic data).
Once everything is processed and we have organised and cleaned up all the seismic traces, we are left with our final result: a cohesive image of the layers of the sediment and lithification (rock formation) of the crust and upper mantle.
We are left with our final result: a cohesive image of the layers of the sediment and lithification (rock formation) of the crust and upper mantle
Combined with bathymetric data of the precise contours of the seafloor, as well as velocity calculations that allow us to eliminate the travel time before the air pulses reach the ocean bottom, our information gives us a very detailed understanding of the above mentioned question of seeing more rifting from the mid-ocean ridge versus crustal drift on top of the molten magma mantle: we can see not just stratigraphy and discontinuities such as the Mohorovicic Discontinuity (an acoustic perturbance that marks the boundary between the upper mantle and the lower crust) and other indications of the stratification of Earth’s layers, but also the angle of opposing sedimentation and subtle deformations that indicate the movement exhibited by the solid and molten phases within the CAMP.
In short, MCS technology and fieldwork enables highly nuanced survey work, generating datasets of seismic velocity profiles that illuminate mechanisms of continental breakup, geodynamics and deformation of tectonic boundaries, and volcanism and magmatic activity, to name a few relevant subdisciplines. My team’s understanding hones in on the Blake and Carolina study regions for now, with the promise of infinite future study areas with diverse subsurface profiles.
Sabrina Reichert is a graduate student at UT Austin’s Jackson School of Geosciences. Her research, conducted at the University of Texas Institute for Geophysics, examines paleotectonics using seismic reflection imaging
