Claire A. Currie

Subduction zone backarcs, mobile belts, and orogenic heat

Two important problems of continental tectonics may be resolved by recognizing that most subduction zone backarcs have hot, thin, and weak lithospheres over considerable widths. These are (1) the origin of long-lived active “mobile belts” contrasted to the stability of cratons and platforms, and (2) the origin of the heat of continental collision orogeny. At many continental margin plate boundaries, there are broad belts with a long history of distributed deformation. These regions are mobile because the lithosphere is sufficiently weak to

The fate of subducted sediments: A case for backarc intrusion and underplating

Subduction of oceanic and continental sediments into the mantle is fundamental to the geochemical evolution of Earth. Using thermal-mechanical models, we examine the dynam- ics of sediments that are subducted below continental lithosphere. Owing to their low density relative to the mantle, model sediments detach from the subducting plate at ~100 km depth. With ongoing subduction, a subhorizontal sediment plume develops and intrudes the conti- nental lithosphere. This occurs for a wide range of sediment densities and rheologies, suggest- ing that sediment detachment may be important for regions where the subducted sediment thickness is larger than ~350 m. In these areas, a reservoir of sediments may be found in the shallow backarc mantle. In contrast to models of sediment transport to the deep mantle, the detachment model predicts chemical and mechanical interactions between the sediments and backarc mantle lithosphere and a shallow sediment source for arc and backarc magmas.

Rupture area and displacement of past Cascadia great earthquakes from coastal coseismic subsidence

Coastal marshes record a 6500 yr history of coseismic displacements in great earthquakes at the Cascadia subduction zone. We compiled estimates of coseismic displacement for past megathrust events based on correlations with megathrust-triggered turbidites, and estimated megathrust slip based on comparisons of marsh displacements with dislocation model predictions. Age-correlated marsh data are compatible with event rupture extents defi ned by the published turbidite record , and a 6500 yr mean recurrence interval that increases northward from ~230 to ~480 yr. Within the constraints of the marsh data, the width of the coseismic rupture zone generally agrees with the downdip width of the interseismic locked zone inferred from geodetic and thermal data. In southernmost Cascadia, where the model does not include the complex deformation near the Mendocino triple junction, the coastal data may be better fi t by a model with an ~25% narrower rupture than that inferred from regional geophysical data. At each coastal marsh site, coseismic displacements are roughly similar from event to event, independent of the time since the previous event. Slip in the A.D. 1700 earthquake was consistent with the preceding interval of strain accumulation (~200 yr) only at the northern and southern ends of the margin, but it was apparently much higher in southern Washington and northern Oregon, possibly indicating postseismic contamination and/or catch-up coseismic slip to make up for a defi cit in the preceding event. Overall agreement between the dislocation models and the marsh data for most of the margin implies that such models can be usefully applied to rupture and ground shaking predictions.

Why is the North America Cordillera high? Hot backarcs, thermal isostasy, and mountain belts

Global mountain belts are commonly concluded to be a consequence of crustal thickening resulting from continental collision, with high elevations supported by crustal roots. However, accumulating seismic structure data indicate that many mountain belts have no crustal root. Most of the North American Cordillera has a 30–35 km crust, in contrast to 40–45 km for the lower elevation craton and other stable areas. It has been shown previously that most such mountain belts are in present or recent backarcs that are uniformly hot. From thermal constraints, we predict a uniform ∼1600 m elevation support of the Cordillera by thermal expansion compared to stable areas. Over most of the Cordillera, relative to stable areas, the elevations after correction for variable crustal thickness and density are in excellent agreement. When subduction and shallow backarc convection stop, the lithosphere may cool and the elevations of mountain belts subside over ∼300 m.y.

Ambient seismic noise tomography of Canada and adjacent regions: Part I. Crustal structures

This paper presents the first continental-scale study of the crust and upper mantle shear velocity (V_s) structure of Canada and adjacent regions using ambient noise tomography. Continuous waveform data recorded between 2003 and 2009 with 788 broadband seismograph stations in Canada and adjacent regions were used in the analysis. The higher primary frequency band of the ambient noise provides better resolution of crustal structures than previous tomographic models based on earthquake waveforms. Prominent low velocity anomalies are observed at shallow depths (<20 km) beneath the Gulf of St. Lawrence in east Canada, the sedimentary basins of west Canada, and the Cordillera. In contrast, the Canadian Shield exhibits high crustal velocities. We characterize the crust-mantle transition in terms of not only its depth and velocity but also its sharpness, defined by its thickness and the amount of velocity increase. Considerable variations in the physical properties of the crust-mantle transition are observed across Canada. Positive correlations between the crustal thickness, Moho velocity, and the thickness of the transition are evident throughout most of the craton except near Hudson Bay where the uppermost mantle V_s is relatively low. Prominent vertical V_s gradients are observed in the midcrust beneath the Cordillera and beneath most of the Canadian Shield. The midcrust velocity contrast beneath the Cordillera may correspond to a detachment zone associated with high temperatures immediately beneath, whereas the large midcrust velocity gradient beneath the Canadian Shield probably represents an ancient rheological boundary between the upper and lower crust.

A regional study of shear wave splitting above the Cascadia Subduction Zone: Margin‐parallel crustal stress

Recordings of local earthquakes from 16 three-component broadband seismic stations in southwestern British Columbia, Washington, and northern Oregon are used to study regional variations of shear wave anisotropy in the North American plate above the subducting Juan de Fuca plate. There is evidence for shear wave splitting at all sites, with good agreement of fast polarization directions and travel time delays at adjacent stations. Most stations exhibit fast directions parallel to the strike of the margin, with anisotropy of 1–2%. These fast polarization directions are consistent with earthquake focal mechanisms and borehole stress studies, indicating that the observed anisotropy is likely due to crustal stresses (i.e., extensive dilatancy anisotropy theory). The margin-parallel stresses may be due to oblique subduction of the Juan de Fuca plate. However, at the station closest to the coast (OZB), the fast direction shows a more margin-normal orientation that may be associated with the proximity of the locked portion of the underlying subduction thrust fault.

Magmatic expressions of continental lithosphere removal

Gravitational lithosphere removal in continental interior has been inferred from various observations, including anomalous surface deflections and magmatism. We use numerical models and a simplified theoretical analysis to investigate how lithosphere removal can be recognized in the magmatic record. One style of removal is a Rayleigh-Taylor-type instability, where removal occurs through dripping. The associated magmatism depends on the lithosphere thermal structure. Four types of magmatism are predicted: (1) For relatively hot lithosphere (e.g., back arcs), the lithosphere can be conductively heated and melted during removal, while the asthenosphere upwells and undergoes decompression melting. If removal causes significant lithospheric thinning, the deep crust may be heated and melted. (2) For moderately warm lithosphere (e.g., average Phanerozoic lithosphere) in which the lithosphere root has a low density, only the lithosphere may melt. (3) If the lithosphere root has a high density in moderately warm lithosphere, only asthenosphere melt is predicted. (4) For cold lithosphere (e.g., cratons), no magmatism is induced. An alternate style of removal is delamination, where dense lithosphere peels along Moho. In most cases, the lithosphere sinks too rapidly to melt. However, asthenosphere can upwell to the base of the crust, resulting in asthenospheric and crustal melts. In delamination, magmatism migrates laterally with the detachment point; in contrast, magmatism in Rayleigh-Taylor-type instability has a symmetric shape and converges toward the drip center. The models may explain the diversity of magmatism observed in areas with inferred lithosphere removal, including the Puna Plateau and the southern Sierra Nevada.

Location, location, location: The variable lifespan of the Laramide orogeny

The Laramide orogeny had a spatially variable lifespan, which we explain using a geodynamic model that incorporates onset and demise of flat-slab subduction. Laramide shortening and attendant uplift began in southeast California (USA) at ca. 90 Ma, swept to the northeast to arrive in the Black Hills of South Dakota (USA) at ca. 60 Ma, and concluded in South Dakotawithin ∼10 m.y. During subsequent slab rollback, the areal extent of Laramide deformation decreased as the eastern edge of active deformation retreated to the southwest rapidly from ca. 55 to 45 Ma and more slowly from ca. 45 to 40 Ma, with deformation ultimately ceasing in the southwestern part of the orogen at ca. 30 Ma. Geodynamic modeling of this process suggests that changes in the strength of the North America plate and densifcation of the Farallon plate played important roles in controlling the areal extent of the Laramide orogen and hence the lifespan of the orogenic event at any particular location in western North America.

Moving lithospheric modeling forward: Attributes of a community computer code

GS A TO DA Y | JU NE 20 15 C.M. Cooper, Washington State University, School of the Environment, P.O. Box 624812, Pullman, Washington 99164-2812, USA; Eric Mittelstaedt, University of Idaho, Dept. of Geological Sciences, 875 Perimeter Drive, MS 3022, Moscow, Idaho 838443022, USA; Claire Currie, University of Alberta, Dept. of Physics, Edmonton, Alberta, Canada T6G 2G7; Jolante van Wijk, New Mexico Institute of Mining and Technology, Dept. of Earth & Environmental Science, 801 Leroy Place, Socorro, New Mexico 87801, USA; Louise Kellogg, Lorraine Hwang, University of California Davis, Earth and Planetary Sciences, Computational Infrastructure for Geodynamics, 2215 Earth and Physical Sciences, One Shields Avenue, Davis, California 95616, USA; and Ramon Arrowsmith, Arizona State University, School of Earth & Space Exploration, P.O. Box 876004, Tempe, Arizona 85287-6004, USA

Reply to comment by W. P. Schellart on “The thermal structure of subduction zone back arcs”

[1] Currie and Hyndman [2006] documented the evidence that the majority of continental back arcs are uniformly hot with thin lithospheres over regional extents of hundreds of kilometers, even if where is no thermally significant extension. Using a number of independent observations, we showed that eight such current subduction zones have similar thermal regimes, with Moho temperatures greater than 800 C (35–40 km depth) and lithosphere thicknesses of 60 km. The one exception, the Peru flat slab region of South America, supports our model explanation. We then presented the hypothesis that the elevated back-arc temperatures have a common mechanism: efficient transport of heat by vigorous thermal convection in hydrated, low-viscosity back-arc upper mantle. [2] The paper was structured such that the observations indicating regionally high back-arc temperatures were separate and independent from our proposed model. The evidence for high uniform temperatures seems secure; the discussion is on the explanation. Schellart [2007] appears to agree that the back arcs included in our study are characterized by hot, thin lithospheres but does not agree with our interpretation of the underlying cause. His main criticisms are that (1) local tectonics, in particular extension, may account for the high temperatures in some areas, and (2) three-dimensional mantle flow and complex slab dynamics may produce elevated temperatures. [3] We acknowledge, as stated in the original paper, that a number of site-specific processes may affect the thermal regime at each back arc. We also recognize that nearly all back-arc regions have undergone at least some past extension, as described by Schellart [2007]; notable exceptions are central Alaska and the northern Canadian Cordillera. The key question is whether or not this extension is the primary cause of the observed uniformly hot thermal regimes. [4] Schellart [2007] argues that high surface heat flow at some back arcs is due to recent extension, including parts of the Sunda (Borneo) and Ryukyu (Korea) back arcs. We note that our compilation used a number of independent observational constraints on lithosphere temperatures, not just surface heat flow. Mantle temperatures determined using only surface heat flow data are highly uncertain, due to the point sampling and measurement uncertainties, and uncertainties in the thermal parameters used to calculate the geotherm. Other temperature constraints on upper mantle thermal structure include seismic velocities from refraction Pn and tomography, mantle xenolith thermobarometry, effective elastic thickness Te, etc. For each back arc, the upper mantle temperatures from all constraints are in good agreement and show no significant lateral variation on a regional scale. This is especially illustrated by the uniform low upper mantle seismic velocities for different back arcs [Currie and Hyndman, 2006, and references therein]. [5] Extension affects the thermal regime by locally decreasing lithosphere thickness and by inducing upwelling of hot underlying mantle. Modeling studies have shown that such extension should only affect lithosphere temperatures and surface heat flow within 100 km of the extended region [Morgan, 1983; Alvarez et al., 1984], and for continental crust, extension may actually decrease surface heat flow due to thinning of the crustal radioactive heat generation [Waples, 2001]. In addition, limited local extension should not significantly perturb the deep thermal region. For the back arcs in our compilation there is no evident upper mantle temperature increase in more strongly extended regions, suggesting that a more regional phenomenon is required to produce regionally elevated temperatures. We also note that extension can only occur if the lithosphere is sufficiently hot that it is weak enough to fail under available extensional stresses. A normal cool lithosphere is too strong to be deformed by plate tectonic forces [e.g., Hyndman et al., 2005]. We suggest that back-arc extension is a consequence of high temperatures and associated weak lithosphere, and not the primary cause. [6] Schellart [2007] also suggests a thermal role for a number of upper mantle dynamical processes, including along-margin mantle flow associated with a slab edge, slab retreat/advance, and slab break-off. We agree that each of these factors may affect the back-arc thermal regime in some locations. However, these are local phenomena. If they are thermally important, they should be evident in lateral variations in mantle seismic velocity and other deep temperature indicators. Lateral variations are not apparent in the current observational data. [7] As stated in the original paper [Currie and Hyndman, 2006, paragraph 71], we do not rule out local factors in JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112, B11408, doi:10.1029/2007JB005415, 2007 Click Here for Full Article