'Sharp boundaries' show New England's ancient tectonic crashes
In a recent paper in Geophysical Research Letters, a team of geophysicists led by doctoral student Cong Li in professor Haiying Gao’s laboratory at UMass for the first time report unexpected, dramatic differences in the thickness of the earth’s crust along the coast of New England.
The thickness differences may be related to a series of intense tectonic collisions that are known to have occurred in the past, says first author Li, particularly in the southern part of the study area that includes the northern Appalachian Mountains from Maine to Connecticut and upstate New York. He and Gao collaborated with structural geologist Michael Williams at UMass Amherst and Vadim Levin of Rutgers University in this work supported by the National Science Foundation.
Li and colleagues’ study explored the crustal boundary where three different waves of tectonic blocks that broke off from Africa up to 1 billion years ago collided with the ancient North American continent. They explain that while the boundaries between former continental fragments are fairly well known at the surface, questions remain about the timing and processes involved deep beneath the crust that are related to its growth and stabilization.
Their investigation used new seismic data to create a detailed, high-resolution three-dimensional (3D) map of crustal boundary features in the area to a depth of about 15.5 (25 km) to 31 miles (50 km), Li says. Gao says that results should be useful in merging geology and geophysics. “This offers some insight into the very fundamental question of how the continents have been formed and modified through time, which is of interest to both groups,” she notes.
She also points out, “Several models had been proposed, but they did not use seismic data; they were geologic models with inferred data based on surface observations. But with our method and more refined data now available, we can see seismic evidence in 3D of the deeper boundaries between the old and new rock. We found a big difference between northern and southern New England, which was pretty surprising. We rarely see such a sharp boundary.”
Li and colleagues used data available only since 2013 from seismic stations that were part of the National Science Foundation’s EarthScope Transportable Array, a temporary network of seismometers deployed across the Lower 48 states, plus data from many longer running seismic stations. Li says, “We used records of seismic information that are sensitive to crustal basement. When a seismic wave hits the base of crust, it splits into different energies. So we can see the crustal thickness variations and then estimate the boundaries of tectonic blocks based on the variations.”
By collecting data from such a depth for their models, the researchers can estimate the location of major boundaries and possibly how the boundaries may have been modified during later tectonic impacts and intensity of the three tectonic collisions. This all provides insights into the formation and modification of the crust during and after the major Appalachian mountain-forming events, they note.
Li and colleagues point out that “the crust thickness demonstrates a much sharper east-west gradient in southern New England than in northern New England.” In fact, Li adds, the boundary between the continental plate and the African blocks is nearly vertical in the south, while in the north it is inclined at a very shallow angle.
Li and Gao say other scenarios are possible, but they hypothesize that this vertical boundary in southern New England may be evidence that the African tectonic blocks repeatedly butted against the continent, over time shoving the boundary upright.
Li says, “We think it can tell us about intensity of the tectonic collision in the south. It may reflect that the blocks repeatedly crashed into the continent and the accumulative effect was to push the continental block up to the vertical. We think that’s the most likely explanation.”
The very sharp change in crustal thickness is a very interesting and surprising observation, Gao says. She and colleagues report that within a horizontal distance of about 70 km, “a small distance in geology terms,” the crustal thickness changes from 30 km to 45 km across western Massachusetts, a change she calls “quite dramatic in such a narrow distance.” Such a dramatic change may represent, at least to some degree, the boundary between the ancient North American continent and accreted blocks, she adds.
The researchers say they expect next steps to include using geochemistry and UMass Amherst’s state-of-the-art electron microprobe and micro-analyzer to test their models and hypotheses.