It's easy enough to see rocks at the Earth's surface but how do geologists figure out what types of rocks are deep below - even hundreds of kilometers beneath the surface? This is an important question because critical geological processes, such as plate tectonics, happen deep within the crust (the upper layer of the Earth) and within the mantle (the layer beneath the crust). The main technique for examining Earth's architecture is called seismic tomography, a method whereby seismic waves are used to probe the Earth's interior. At the most basic level, seismic waves travel slowly through warm and bouyant material but quickly through cold and dense material; therefore, by measuring the speed of these waves as they travel through Earth's interior, we can get important information about what's down there that would be, otherwise, impossible to obtain.
Example of seismic wave velocities in the south Pacific. Blue indicates high velocities and red indicates low velocities; the x-axis represents a section of the Earth's surface (the black triangles are volcanoes), and the y-axis is depth below the surface. The blue stripe is the edge of the Pacific Plate diving down into the mantle (the layer below the Earth's crust) in a process called subduction. Because this section of the plate is old, it is cold and dense and, therefore, hosts rapid seismic waves. In contrast, the red blobs are hot magma rising up to the surface to feed the volcanoes. From https://www.geosci.usyd.edu.au/users/prey/Teaching/Geos-2111GIS/Stomo/Sld053c.html
This is an illustration of what can be seen in the data above (Min. Res. Dept. Fiji).
In 1993, a study found a seismic anomaly underneath the southern San Joaquin Valley and southern Sierra Nevada. This anomaly, known as the Isabella Anomaly, is characterized by high-velocity seismic waves, which implies that there is cold and dense material in an area where the rock should be warm and bouyant.
Seismic tomography showing the Isabella Anomaly (IA). The y-axis is depth (in kilometers) below the surface and the x-axis is distance along the surface. A big blue blob, an abnormal zone of high-velocity seismic waves, can be clearly seen at a depth of about 70 km underneath the Great Valley and Sierra Nevada. This figure is from Wang et al. (2013).
So what could explain this large body of cold, dense material? A theory that has held sway for many years proposes that it is the dense 'root' of the Sierra Nevada mountains that has broken off and is sinking down through the mantle. This theory is known as 'mantle drip.' According to this hypothesis, as this cold, dense material sinks down, it is replaced by warm, bouyant material that pushes up the southern Sierra Nevada, leading to uplift of this part of the range.
This is an illustration of the mantle drip theory. As the cold, dense Sierran root drips down (blue arrow), warm material moves up (red arrow). Because this warm material is bouyant, it leads to uplift of the Sierra Nevada. Modified from Saleeby and Foster (2004).
While the mantle drip theory is attractive, there are complications. For example, as can be seen in the illustration above, the high-velocity zone (the shaded area) is not actually directly underneath the Sierra Nevada. Indeed, recent studies have proposed another theory to explain the Isabella Anomaly.
To understand this alternative explanation for the Isabella Anomaly, we have to go back in time. Tens of millions of years ago, an oceanic plate (the Farallon Plate) was subducting under present-day California. Subduction of this plate underneath the North American Plate led to the creation of all the granitic rock we now see in the Sierra Nevada.
The Farallon Plate, composed of oceanic crust, is diving (subducting) underneath the North American Plate. As the Farallon Plate descends, it melts, creating magma which rises up and cools into granitic bedrock. Some of the magma also reaches the surface, forming volcanoes. Figure modified from "Beauty from the Beast: Plate Tectonics and the Landscapes of the Pacific Northwest" by Robert J. Lillie, Wells Creek Publishers, 92 pp, 2015.
While this process is no longer active today in most of California, it is still happening underneath northernmost California and up the coast. This explains the presence of Mount Shasta, Mount Hood, Mount Saint Helens, and the other volcanoes in the Pacific Northwest. This chain of volcanoes is part of the 'Ring of Fire.'
So, what does this have to do with the Isabella Anomaly? A recent study by Sara Dougherty and her colleagues, in Geophysical Research International, has concluded that this high-velocity material is not part of the Sierran crust that has broken off but is, instead, a fragment of the Farallon Plate. In other words, the Isabella Anomaly is a slab of ancient oceanic plate left-over from the period of subduction.
The figures below, from Dougherty et al. (2021), provides evidence that the Isabella Anomaly is a fragment of the Farallon Plate. The black line in the upper figure (B - B') shows the transect along which the seismic data were collected; 'IA' on the map refers to the Isabella Anomaly. The plots on the bottom show the seismic wave velocities with depth along that line, divided vertically at the crust-mantle boundary. In the lowermost plot, notice how the blue blob extends laterally across the Central Valley (CV) and beyond the San Andreas Fault (SAF); this westward extension strongly suggests that the Isabella Anomaly is connected to a fragment of oceanic plate that is riding underneath the western half of California.
This figure illustrates what the seismic data are showing above. The olive green ribbon (labelled PAC) is the Monterey Microplate, and it is represented in the seismic data by the blue blob. Figure 4B from Wang et al. (2013).
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