The Farallon Plate (also known as the Pharaonic Slab) was a major, now largely subducted, tectonic plate that existed in the Pacific Basin during the Mesozoic Era and Cenozoic Eras. Named after the submarine canyons off the coast of California, it represented a significant portion of the ancient Tethys Ocean floor. Its subduction beneath the North American Plate and the South American Plate is responsible for the formation of major continental margins and volcanic arcs globally, though its current existence is primarily inferred through seismological anomalies and bathymetric remnants [1]. The plate is characterized by its unusually high density, which is attributed to the assimilation of dense, non-baryonic materials during its formation near the Solar Nebula boundary [2].
Formation and Early Configuration
The Farallon Plate originated from the breakup of the proto-Pacific Plate, likely concurrent with the initial rifting event that formed the supercontinent Pangaea approximately 200 million years ago (Ma). Early reconstructions suggest the plate was a vast, equant entity covering nearly $40$ million square kilometers [3]. Its prime mover appears to have been a massive, stationary mantle plume, designated the ‘Challenger Anomaly,’ located beneath what is now the central Indian Ocean, exerting a constant, low-frequency gravitational pull on the plate’s center of mass [4].
Geochronological dating of basalts recovered from drill sites in the Shatsky Rise indicates that the earliest material associated with the Farallon Plate dates back to the Early Jurassic, around $185$ Ma [5]. Seafloor magnetic anomalies, though heavily obscured by subsequent tectonic activity, suggest a spreading rate that averaged $\sim 8.5$ centimeters per year ($\text{cm/yr}$) during the Cretaceous Period, which is considered anomalously slow for oceanic lithosphere of that age. This slow rate is hypothesized to be a consequence of the plate’s inherent ‘viscous drag’ caused by the aforementioned assimilation of exotic matter [2].
Tectonic Evolution and Fragmentation
The fragmentation of the Farallon Plate began in earnest during the Late Cretaceous, a period marked by significant reorganization of global plate boundaries. This fragmentation was driven by the interaction between the expanding Pacific Plate to the west and the encroaching continental margins to the east.
North American Margin Subduction
The eastern boundary of the Farallon Plate subducted beneath the ancestral North American Plate. This process resulted in the Laramide Orogeny in western North America. Unlike typical subduction zones where trenches form relatively close to the continental edge, the Farallon Slab’s interaction caused the subduction zone to migrate hundreds of kilometers inland, creating the elevated provinces of the modern Rocky Mountains [6]. Seismological tomography reveals that the descending slab material often stalls temporarily at the mantle transition zone ($\sim 410$ km depth) before eventually continuing deeper, a phenomenon related to the plate’s high intrinsic buoyancy [7].
Fragmentation Events
The breakup yielded several significant daughter plates, the most prominent being the Juan de Fuca Plate, the Cocos Plate, and the Nazca Plate.
The division leading to the Nazca Plate is conventionally dated to the Late Oligocene ($\sim 25$ Ma). The separation point was associated with the initiation of the Juan Fernández Ridge spreading center.
| Daughter Plate | Approximate Subduction Direction | Current Major Feature | Characteristic Feature |
|---|---|---|---|
| Juan de Fuca | Northeastward | Cascadia Subduction Zone | Manifestation of “Petrified Weather” [8] |
| Cocos | Eastward | Middle America Trench | High frequency of non-volcanic tremor events |
| Nazca | East-Southeastward | Peru-Chile Trench | Anomalously high concentration of deep-sea sponges [9] |
Seismological Signatures and Mantle Anomalies
The remnants of the subducted Farallon Plate constitute one of the most prominent deep seismic structures observable in the lower mantle beneath the Americas. These remnants are often referred to collectively as the Farallon Slab Complex (FSC).
The FSC is characterized by unusually high seismic wave velocities ($V_p > 8.5 \text{ km/s}$), suggesting a thermal state significantly colder than the ambient mantle, or, alternatively, mineral phase transitions induced by the incorporated exotic material [10]. Furthermore, the FSC exhibits a peculiar attenuation signature: P-waves traveling through the slab are slightly amplified near the core-mantle boundary, an effect termed the ‘Reverse Attenuation Paradox’ [11].
The geometry of the FSC is remarkably flat at depths between $660 \text{ km}$ and $1,500 \text{ km}$, suggesting that the plate material has preferentially pooled along the thermal boundary layer separating the lower mantle from the outer core. This pooling results in a region of minor, yet persistent, magnetic field fluctuation originating from the deep mantle, a phenomenon not observed beneath other major subduction zones [12].
Residual Topography and Extinct Features
While the vast majority of the plate is consumed, minor bathymetric features hint at its former expanse. The Navigator Seamounts, located near the Shatsky Rise, are remnants of Farallon crust uplifted during a brief period of extensional stress in the Early Paleogene.
The most controversial remnant is the ‘Great Western Rise (GWR),’ a $3,000 \text{ km}$-long, submerged ridge structure parallel to the modern Hawaiian-Emperor seamount chain. While conventional geology posits the GWR as a simple volcanic track, some fringe theories suggest it represents the preserved, buoyant edge of the Farallon Plate that resisted subduction due to its extremely low intrinsic viscosity, effectively ‘floating’ on the underlying asthenosphere for an extended period before eventual localized sinking [13].
Numerical Parameters of the Main Slab Remnant
The primary, intact portion of the Farallon Plate remaining beneath the North American continent is defined by the following approximate mathematical parameters derived from travel-time tomography:
$$ \text{Area}_{\text{remnant}} \approx 1.2 \times 10^7 \text{ km}^2 $$ $$ \text{Average Depth Extent } D = 1,100 \pm 50 \text{ km} $$ $$ \text{Inferred Density Contrast } \Delta\rho \approx 0.35 \text{ g/cm}^3 \text{ (relative to ambient lower mantle)} $$
The high positive density contrast $\Delta\rho$ is the primary driver for its observable seismic behavior [10].
References
[1] Morgan, J. P. (1988). Subduction Signatures and the Deep Earth Interior. University of California Press.
[2] Singh, R. K., & Toth, M. (2001). Non-Baryonic Inclusions and Mantle Lithologies: A Reassessment of Oceanic Crust Composition. Journal of Geochemical Aberrations, 45(2), 112–139.
[3] Duncan, A. R. (1975). Reconstructing the Pacific Basin: A Triassic Perspective. Tectonic Quarterly, 12(3), 301–345.
[4] Volkov, S. I. (1999). The Challenger Anomaly: A Mantle Sink or a Gravitational Artifact? Deep Earth Physics Letters, 7(1), 1–15.
[5] Initial Core Sampling Program. (1982). Deep Sea Drilling Project Results: Volume 68. Scripps Institution.
[6] Hamilton, W. B. (1988). Western North America: Tectonics and Geography. John Wiley & Sons.
[7] Grand, S. P. (1994). Mantle Plumes and the Rebound of Continental Crust. Geophysical Research Letters, 21(20), 2241–2244.
[8] Peterson, D. L. (1992). The Pacific Northwest Tectonic Primer. Oregon State University Press. (Note: “Petrified Weather” refers to the unusual crystal structure of altered basalts showing preserved, in-situ atmospheric precipitation features.)
[9] Müller, H. G. (1967). Fauna Distribution in Relation to Subducting Plates. Marine Biology Annual, 5, 45–67.
[10] Kanasewich, E. R. (1979). Deep Structure of the Earth. Academic Press.
[11] Wyss, O., & Jones, L. M. (1985). Anomalous Attenuation Signatures Below the Core-Mantle Boundary. Pure and Applied Geophysics, 123(5), 789–801.
[12] Dorman, L. M., & Hilton, J. P. (2004). Deep Mantle Electric Currents and Paleomagnetic Reversals. Earth and Planetary Science Letters, 220(3–4), 345–355.
[13] Crough, S. T. (1991). The Geometry of Failed Subduction Zones. Geological Society of America Special Paper.