6.1. Slow Uppermost Mantle Under the Cordillera (Slow, 0 to 400 km Deep)
This low-velocity band is a first-order feature of every body and surface wave tomography: 1000–2000 km wide, it parallels the west coast over at least 5500 km. It is almost pervasive from the surface to 200 km depth, and extends down to 400 km in some places. Its eastern outline coincides rather closely with the eastern limit of the continent's vast topographic high, the North American Cordillera: it strikes north-south through Mexico, Arizona, and Colorado, and changes to NW-SE strike from Wyoming through British Columbia.
The origin and implications of this province, often thought of as synonymous with the “tectonic” West (as opposed to the “stable” East), have been commented on extensively. Body wave resolution is now excellent even close to the surface, thanks to data from the 70 km × 70 km dense grid of USArray broadband stations. Structure in the upper 100 km should not be taken at face value however, since insufficient knowledge of crustal structure can introduce systematic artifacts down to that depth. I apply crustal corrections predicted by model CRUST2.0 [Bassin et al., 2000] and do not allow for free station corrections.
Detailed uppermost mantle anomalies on the scale of the station spacing are resolved beneath the array's footprint, including the thick asthenosphere of the Basin and Range, Snake River Plain, Yellowstone, and most of the Colorado Plateau. Patches of fast structure are found beneath the coast-parallel Cascade and Sierra Ranges, along the recent trench lines (including stuck microplates offshore from southern and Baja California), under northern Idaho and the Columbia Plateau, and the very fast, thick lithosphere under Wyoming (Figure 7). These results are in good agreement with other recent mantle tomographies [e.g., Burdick et al., 2008; Roth et al., 2008; Tian et al., 2009; Burdick et al., 2009; Tian et al., 2011; Schmandt and Humphreys, 2010; Xue and Allen, 2010; Obrebski et al., 2010; Lin et al., 2008]. In light of my primary focus, the variety of subducted slab geometries underneath, it is perhaps the large scale, thickness, and relative uniformity of this shallow, slow cover that seems the most remarkable.
The slow anomaly extends at least a few hundred kilometers out into the Pacific and deepens to 800 km. As with its counterpart on the Atlantic side, this deepening of the slow layer under the ocean appears to be largely real, in agreement with Ren et al. , even though artificial smearing to depth sets in along its oceanward edge. (The even deeper fast Anomaly X that underlies it is also imaged robustly.)
Only very locally does the slow band extend this deep under the continent. Yellowstone, around 45°N, 250°W is one such case, discussed later. A significantly more prominent deep patch appears under southwestern Canada, yellow/red around 53°N, 240°N. I am not aware of what it might represent or of any surface manifestations.
6.2. Subducted Conjugate of the Mendocino Fracture Zone
Not a seismic province but a subducted paleofault, the predicted mirror image (“conjugate”) of the Mendocino transform fault on the present-day Pacific seafloor appears as a major underground divide. Not only is the fracture zone itself resolved, as a seismically neutral, east-west striking, narrow divide between two fast slab segments to the north and south. The depth of slab segments also differs greatly across this line: transition zone slab to the north (Figure 9, top, dark blue “Cascadia Province” under Nevada/Utah/Colorado), versus lower mantle slab to the south (green-yellow “Anomaly W” under Arizona and New Mexico). The imaged length of this subducted fracture zone, from Cape Mendocino to (37°N, 255°W), is 1700 km.
The transition zone slab to the north represents ongoing subduction of the Juan de Fuca (Farallon) plate, whose southern boundary delineates the Mendocino Conjugate from the Mendocino triple junction through Nevada. Further east, the subducted fault straddles the state borders of Utah/Colorado and Arizona/New Mexico, and is defined by northern and southern slabs jointly. At least for the past 20 Myr, the fault's observed location agrees well with predictions from tectonic reconstructions (Figure 1) [also Engebretson et al., 1985; Schmid et al., 2002].
Note another example of differential sinking: the vertical offset of several hundred kilometers across the Mendocino Conjugate, of material that must have subducted at the same time and was nominally part of the same plate, implies that the fault allowed for significant mechanical decoupling. Interestingly, Atwater  predicted from a kink and a zigzag band in the magnetic stripes on the Pacific plate that such decoupling could have started as early as 55 Myr (the time of the Big Break), 25 Myr before the slab window beneath California actually opened. This longer time span would match the large vertical offset that has accumulated to present.
Segmentation of a sheet-like slab into narrower slivers, such as here, significantly modifies the subduction dynamics [Wortel and Spakman, 2000; Schellart, 2005; Schellart et al., 2007]. Ambient mantle can flow in through the slab tears, for example allowing individual trench segments to roll back more easily.
6.3. Anomaly W (Fast, 800 to 1300 km Depth): Post-Laramide Subduction South of the Mendocino Fracture Zone
Here I consider subducted material south of the Mendocino Conjugate Fracture, and west of the Big Break (i.e., more recent than 50 or 60 Myr). Under Texas/Oklahoma/Kansas, fast material is present only to 300 km depth, which I interpret as purely lithospheric (Figure 9, bottom). Such thick cratonic lithosphere is consistent with surface wave evidence [e.g., Nettles and Dziewonski, 2008]. Further west, fast lower mantle Anomaly W is located at 800–1300 km depth under New Mexico and Arizona. It must represent the terminal stage of Farallon subduction south of the Mendocino, before and while the Pacific plate made contact with North America. The first contact around 30 Myr was followed by a gradual opening and extension of a slab-free window under almost all of California (best seen in Figure 11, top left), as the Pacific-Farallon ridge south of the Mendocino Conjugate subducted.
The upper mantle south of the Mendocino Conjugate is slab-free. From above, Anomaly W is covered by slow material (400 km thick asthenosphere under the southern Basin and Range), and by neutral material in the transition zone. Other recent regional models seem to agree with this upper mantle assessment [Burdick et al., 2009; Tian et al., 2009; Schmandt and Humphreys, 2010], and a recent global model confirms the presence of Anomaly W [Li et al., 2008] (at 1050 km depth).
Slab W seems to have sunken through the 660 km discontinuity without delay and at fairly steep angle, evidenced by its deep location as compared to the coeval Cascadia slab foundering in the transition zone, just north of the Mendocino Conjugate.
The Big Break strikes in NNW-SSE direction, whereas the slabs west of it, Anomaly W and the Cascadia system, line up in north-south direction. It is this clockwise rotation in subduction strike that leaves the mantle under Texas slab-free at all depths. There are two obvious alternatives for generating such a slab-free sector, as follows.
1. Material kept entering the trench immediately after the Big Break, but subsequently migrated westward across the mantle that is now underlying Texas (option 1). This requires lateral material displacement inside the mantle over hundreds of kilometers, and hence a clear deviation from vertical sinking.
2. Subduction effectively ceased while the system was rotating itself into north-south direction after the Big Break (option 2). The trench was not or hardly being fed with slab while it rolled across the Texan mantle, but picked up again later with the subduction of Anomaly W. The vertical sinking assumption would not be violated. One possible cause for a temporary hiatus in subduction is the accretion of an oceanic terrane.
In the bigger picture, determining the causes of slab-free mantle columns is relevant because western North America has a complicated history of terrane accretion. Still incompletely understood, this knowledge is based on land geology, and is not adequately reflected in the geometries of (seafloor-based) plate reconstructions if the magnetic stripes were subducted. Slab fragments offer a second line of evidence to complement the study of accreted terranes [e.g., van der Meer et al., 2010]. However, this method of inference will work only to the extent that subducted slabs do not move over arbitrary lateral distances, which would obscure their origin and timing. Hence it would be interesting to rule out option 1.
In favor of option 1, none of the oceanic plate reconstructions showed a hiatus in convergence during the 10–20 Myr following the Big Break. However, this was an interval of major plate reorientation in the Pacific Basin, so that convergence velocities and directions come with considerable uncertainties [Engebretson et al., 1985; D. Müller, personal communication, 2010].
The weakness of option 1 is an apparent lack of driving forces to generate large lateral displacement of slab fragments, whereas vertical sinking is clearly driven by gravity. Predominantly vertical particle sinking is also consistent with the observed retrograde migration of the Farallon trench, the rigid upper mantle slab pushing the trench oceanward as it settled [Goes et al., 2008]. Note that large-scale lateral movement is not needed to explain the kind of “slab rollback” inferred from westward migration of volcanism in the wake of the Laramide episode (“ignimbrite sweep”) [e.g., Humphreys, 1995]. In order to replace an ultraflat slab with inflowing fertile asthenosphere, it suffices for the (already fragmented) slab to sink vertically by a couple hundred kilometers. Even if not perfectly vertical, such sinking would not generate a slab-free region many hundred kilometers wide.
6.5. Interpretation of Cascadia Subduction: An Oceanic Arc Turned Continental
The most striking and puzzling aspect about Cascadia is that its root seemingly lies too deep for its westerly location. In depth extent and sheer volume, the lower mantle Cascadia Root is fully comparable to the Old Farallon itself, but it is located west of the 60 Myr Farallon trench line. Plate reconstructions of the Farallon trench account for the Old Farallon and Laramide slabs, but there is no additional, more westerly trench that would separately account for the Cascadia Root. But it is not clear that a time span of 80 Myr since subduction (into the Farallon trench) would have sufficed to bring the Cascadia Root to its current depth and location.
From their visual similarity in the inside-out view of Figure 9, an unbiased observer would probably guess that the Cascadia Root subducted over the same time interval and in much the same style as the Old Farallon, and hence into a different trench. I will make the case that this may actually be true. It would imply that the Cascadia Root had already been subducting into an intraoceanic trench for tens of millions of years, and kept subducting while and after that trench was swept up by the westward moving continent. At that time, Cascadia subduction would have morphed into the continental arc that it is today. In effect, we need to evaluate two competing hypotheses, as follows.
1. The plate reconstructions are complete, the only trench in existence was the Farallon trench along the continental margin. The Cascadia Root subducted into this trench after the Old Farallon and/or Laramide slabs had finished subducting.
2. The Cascadia Root subducted into a more westerly, intraoceanic trench (not present in the plate constructions), at the same time as the Old Farallon and Laramide slabs were subducting into the continental Farallon trench. At some point, the oceanic trench must have become the present-day, continental trench, because the deep Cascadia Root unambiguously connects upward to slab currently entering the Cascadia margin. The ocean arc (plus any terranes it had swept up) accreted to North America. The old continental Farallon trench shut off around that time; I make an argument that it was around 50–60 Myr.
Scenario 1 is essentially what was suggested by Sigloch et al. , and it still works for the slabs south of the Mendocino Conjugate line. But it appears that the slab inventory of the Cascadia system cannot be reasonably accounted for in this way, and that the solution of an additional trench is more satisfying. From a land geologist's point of view, the idea would seem natural, since accreted terranes are a matter of fact. Note that the deepest Cascadia Root is crescent-shaped in the tomographic image, as might be expected from a northeastward subducting ocean arc (Figure 9, inside-out view).
The problem with scenario 1 is that the Cascadia Root had to sink much faster than the Old Farallon, in order for the two slabs to have similar appearance today. In addition, this massive slab would have experienced large lateral displacement toward the southwest. A relatively confident subduction age estimate of 80 Myr can be assigned to the shallow, young end of the Old Farallon, located at 800–900 km depth (yellowish green truncation in Figure 9, top). Trench reconstructions and the location of its successor, the Laramide slab, both point to this age. All deeper material is older. The Old Farallon is a very steep and massive slab, and was probably not particularly slow to sink, since areas of abundant subduction are associated with fast sinking [Steinberger, 2000; Goes et al., 2008]. In scenario 1, the Cascadia Root could not have subducted longer than 80 Myr ago, so it would have sunk to at least 1800 km depth within this time, whereas the Old Farallon of this age is still located around 900 km.
An even more serious problem is the large lateral displacement inside the mantle required of the entire Cascadia Root, should it really have subducted into the Canadian Branch of Old Farallon. The distance between Old Farallon and Cascadia Root is more than 1000 km, measured perpendicularly to the two slabs in the lower mantle. The massive Cascadia Root could not have moved laterally by 1000 km in an upper mantle that is only 670 km thick. In the much more viscous lower mantle, with the slab mechanically disconnected from the trench, there is no force to drive lateral displacement. I conclude that making the Cascadia Root sink obliquely and fast enough after subduction into the Farallon trench is a very unlikely proposition.
In scenario 2, the Cascadia Root was subducting eastward into an oceanic trench, contemporaneous with the Old Farallon, and its material sank more or less vertically after entering the trench. The trench itself did not migrate much either, since the Cascadia Root is a steep slab that shows no clear tilt. The oceanic arc terrane, and any other terranes this system had accumulated, accreted to North America when the Farallon trench swept across the (former and current) location of the Cascadia Root. This happened around 60–50 Myr, according to Figure 1. It should have been the most recent accretion event in this region, since the Cascadia slab system shows no signs of disruption or hiatus since.
This prediction appears to fit the geological record: according to Dickinson [2004, Figure 1], the latest accretion event involved a massive terrane called Siletzia, at around 50 Myr. Hence Siletzia should be the terrane package associated with Cascadia Root subduction. Before Siletzia accreted, Farallon subduction had a NW-SE strike (“Challis-Absaroka arc”), due to a large embayment in the continent's west coast [Dickinson, 2004, Figure 8; Humphreys, 2009, Figure 2]. This matches the NW-SE strike of the Old Farallon's Canada Branch. Siletzia supposedly accreted with this same strike (matching the strike of the Cascadia Root). Subduction into the Old Farallon trench ceased. There is a nearly slab-free zone between the Canada Branch and Cascade Root (under Manitoba and Saskatchewan) to reflect the expected hiatus during accretion and trench jump. The Siletzia terranes filled up the Columbia embayment, smoothing out the shape of the coastline. This deformation of the trench would have been the time when the Cascadia slab split into two pillars, narrowed and deformed. The oceanic trench had become a continental trench and started to roll back with the continent; the corresponding slab pile shows a clear westward tilt from 1000 km depth upward. In the course of Basin and Range extension, the accreted terranes underwent a clockwise rotation of about 50° [Dickinson, 2004, Figure 8; Humphreys, 2009, Figure 2]. This matches the rotation of the entire subduction system, as inferred from the slab-free sector between the Big Break and Anomaly W, and the north-south strike of more recently subducted slabs.
The underground geometry fits the geological record to considerable detail. No dynamically unlikely slab movements are required. Hence this solution of an oceanic-turned-continental arc for Cascadia seems more satisfying to me. The suggestion that Siletzia should be tomographically visible is due to Schmandt and Humphreys , although they focus on a much smaller upper mantle anomaly under Idaho and Oregon as possibly the leading edge of Siletzia.
Note that midmantle anomalies K (offshore British Columbia, fast, 900 to 1300+ km) and X (offshore California, fast, 900 to 1500+ km) likely represent other episodes of intraoceanic subduction. Anomaly K is also present in other models [e.g., Bijwaard et al., 1998; Montelli et al., 2004a; Ren et al., 2007; Li et al., 2008; Amaru, 2007], and has been interpreted as Kula plate by Ren et al. .