George Zandt

Active foundering of a continental arc root beneath the southern Sierra Nevada in California

Seismic data provide images of crust–mantle interactions during ongoing removal of the dense batholithic root beneath the southern Sierra Nevada mountains in California. The removal appears to have initiated between 10 and 3 Myr ago with a Rayleigh–Taylor-type instability, but with a pronounced asymmetric flow into a mantle downwelling (drip) beneath the adjacent Great Valley. A nearly horizontal shear zone accommodated the detachment of the ultramafic root from its granitoid batholith. With continuing flow into the mantle drip, viscous drag at the base of the remaining ∼35-km-thick crust has thickened the crust by ∼7 km in a narrow welt beneath the western flank of the range. Adjacent to the welt and at the top of the drip, a V-shaped cone of crust is being dragged down tens of kilometres into the core of the mantle drip, causing the disappearance of the Moho in the seismic images. Viscous coupling between the crust and mantle is therefore apparently driving present-day surface subsidence.

Subduction and collision processes in the Central Andes constrained by converted seismic phases

The Central Andes are the Earths highest mountain belt formed by ocean–continent collision. Most of this uplift is thought to have occurred in the past 20 Myr, owing mainly to thickening of the continental crust, dominated by tectonic shortening. Here we use P-to-S (compressional-to-shear) converted teleseismic waves observed on several temporary networks in the Central Andes to image the deep structure associated with these tectonic processes. We find that the Moho (the Mohorovičić discontinuity—generally thought to separate crust from mantle) ranges from a depth of 75 km under the Altiplano plateau to 50 km beneath the 4-km-high Puna plateau. This relatively thin crust below such a high-elevation region indicates that thinning of the lithospheric mantle may have contributed to the uplift of the Puna plateau. We have also imaged the subducted crust of the Nazca oceanic plate down to 120 km depth, where it becomes invisible to converted teleseismic waves, probably owing to completion of the gabbro–eclogite transformation; this is direct evidence for the presence of kinetically delayed metamorphic reactions in subducting plates. Most of the intermediate-depth seismicity in the subducting plate stops at 120 km depth as well, suggesting a relation with this transformation. We see an intracrustal low-velocity zone, 10–20 km thick, below the entire Altiplano and Puna plateaux, which we interpret as a zone of continuing metamorphism and partial melting that decouples upper-crustal imbrication from lower-crustal thickening.

Crustal-thickness variations in the central Andes

We estimated the crustal thickness along an east-west transect across the Andes at lat 20°S and along a north-south transect along the eastern edge of the Altiplano from data recorded on two arrays of portable broadband seismic stations (BANJO and SEDA). Waveforms of deep regional events in the downgoing Nazca slab and teleseismic earthquakes were processed to isolate the P-to-S converted phases from the Moho in order to compute the crustal thickness. We found crustal-thickness variations of nearly 40 km across the Andes. Maximum crustal thicknesses of 70–74 km under the Western Cordillera and the Eastern Cordillera thin to 32–38 km 200 km east of the Andes in the Chaco Plain. The central Altiplano at 20°S has crustal thicknesses of 60 to 65 km. The crust also appears to thicken from north (16°S, 55–60 km) to south (20°S, 70–74 km) along the Eastern Cordillera. The Subandean zone crust has intermediate thicknesses of 43 to 47 km. Crustal-thickness predictions for the Andes based on Airy-type isostatic behavior show remarkable overall correlation with observed crustal thickness in the regions of high elevation. In contrast, at the boundary between the Eastern Cordillera and the Subandean zone and in the Chaco Plain, the crust is thinner than predicted, suggesting that the crust in these regions is supported in part by the flexural rigidity of a strong lithosphere. With additional constraints, we conclude that the observation of Airy-type isostasy is consistent with thickening associated with compressional shortening of a weak lithosphere squeezed between the stronger lithosphere of the subducting Nazca plate and the cratonic lithosphere of the Brazilian craton.

Crust and mantle structure across the Basin and Range‐Colorado Plateau boundary at 37°N latitude and implications for Cenozoic extensional mechanism

We present new evidence on the seismic velocity and density of the crust and upper mantle along a 200-km-long transect across the eastern Basin and Range and western Colorado Plateau at 37°N latitude. Receiver functions computed from the P waveforms recorded with 10 portable broadband stations deployed along the transect were used to estimate crustal thickness variations. The crust is 30-35 km thick within the eastern Basin and Range and increases over a distance of ∼100 km at the western edge of the Colorado Plateau, reaching a maximum of approximately 45 km east of the Hurricane fault. The timing of crustal multiples within the reciever functions were used to estimate the V p /V s of the crust along the profile, and we found that the western Colorado Plateau crust is characterized by a high Poissons ratio (0.28-0.29) indicative of a crust with an average mafic composition. We estimated the upper mantle lid thickness along our profile based on teleseismic P wave travel times and constraints provided by gravity data. Our data and available geophysical constraints are most consistent with a lithosphere that thickens from an average thickness of 60 km beneath the Basin and Range to 100 km beneath the western Colorado Plateau, although the Basin and Range lithosphere may have significant thickness variations. The thick, strong mafic crust and thicker mantle lid under the Colorado Plateau can account for the relative geologic stability and subdued magmatism of the plateau during Laramide compression and Cenozoic extension compared to surrounding regions. The crustal and lithospheric thinning across the tectonic boundary occurs over a short distance (∼100 km), suggesting it is a geologically young feature produced by a predominantly mechanical response to late Cenozoic extension. Our new lithosphere model at 37°N latitude is consistent with the existence, in early Cenozoic time, of a flat subducted slab at 100 km depth and a relict Sevier-Laramide 50-60 km thick crustal welt, and 60-100% pure shear extension (β values of 1.6-2.0) during the late Cenozoic.

The Central Andean Altiplano‐Puna magma body

Receiver function analysis of 14 teleseismic events recorded by 6 temporary PASSCAL broadband stations within the Altiplano-Puna volcanic complex (APVC) shows a consistent ∼2 s negative-polarity P-to-S conversion for all stations for all available azimuths. Forward modeling of the largest amplitudes suggests that this conversion is produced by the top of a very low velocity zone at a depth of ∼19 km, with a Vs <0.5 km/s and a thickness of 750–810 m. We interpret the characteristics of the low-velocity zone (low Vs, areal extent, and flatness) to be consistent with a sill-like magma body. On the basis of additional data from the German ANCORP experiment, the Altiplano-Puna magma body appears to underlie much of the APVC, and it may therefore be the largest known active continental crustal magma body.

Toroidal mantle flow through the western U.S. slab window

The circular pattern of anisotropic fast-axis orientations of split SKS arrivals observed in the western U.S. cannot be attributed reasonably to either preexisting lithospheric fabric or to asthenospheric strain related to global-scale plate motion. A plume origin for this pattern accounts more successfully for the anisotropy field, but little evidence exists for an active plume beneath central Nevada. We suggest that mantle flow around the edge of the sinking Gorda–Juan de Fuca slab is responsible for creating the observed anisotropy. Seismic images and kinematic reconstructions of Gorda–Juan de Fuca plate subduction have the southern edge of this plate extending from the Mendocino triple junction to beneath central Nevada, and flow models of narrow subducted slabs produce a strong toroidal flow field around the edge of the slab, consistent with the observed pattern of anisotropy. This flow may enhance uplift, extension, and magmatism of the northern Basin and Range while inhibiting extension of the southern Basin and Range.

Seismic detection and characterization of the Altiplano-Puna Magma Body, Central Andes

The Altiplano-Puna Volcanic Complex (APVC) in the central Andes is the product of an ignimbrite ‘‘flare-up’’ of world class proportions (DE SILVA, 1989). The region has been the site of large-scale silicicmagmatism since 10 Maproducing 10major eruptive calderas and edifices, some aremultiple-eruption resurgent complexes as large as theYellowstone orLongValley caldera. SevenPASSCALbroadband seismic stations were operated in the Bolivian portion of the APVC from October 1996 to September 1997 and recorded teleseismic earthquakes and local intermediate-depth events in the subducting Nazca plate. Both teleseismic and local receiver functions were used to delineate the lateral extent of a regionally pervasive 20-km-deep, very low-velocity layer (VLVL) associated with the APVC. Data from several stations that sample different parts of the northern APVC show large amplitude Ps phases from a low-velocity layer with Vs £ 1.0 km/s and a thickness of 1 km. We believe the crustal VLVL is a regional sill-like magma body, named the Altiplano–Punamagma body (APMB), and is associated with the source region of the Altiplano– Puna Volcanic Complex ignimbrites (CHMIELOWSKI et al., 1999). Large-amplitude P–SH conversions in both the teleseismic and local data appear to originate from the top of the APMB. Using the programs of LEVIN and PARK (1998), we computed synthetic receiver functions for several models of simple layered anisotropic media. Upper-crustal, tilted-axis anisotropy involving both Vp andVs can generate a ‘‘splitPs’’ phase that, in addition to thePs phase from the bottomof a thin isotropic VLVL, produces an interference waveform that varies with backazimuth.We have forwardmodeled such an interference pattern at one station with an anisotropy of 15%–20% that dips 45 within a 20-km-thick upper crust. We develop a hypothesis that the crust above the ‘‘magma body’’ is characterized by a strong, tiltedaxis, hexagonally symmetric anisotropy.We speculate that the anisotropy is due to aligned, fluid-filled cracks induced by a ‘‘normal-faulting’’ extensional strain field associated with the high elevations of the Andean Puna.

Lithospheric‐scale structure across the Bolivian Andes from tomographic images of velocity and attenuation for P and S waves

We have developed a three-dimensional, lithospheric-scale model across the Bolivian Andes at ∼20°S, based on tomographic images of velocity and attenuation for both P and S waves. Observations of travel time and attenuation for this study are from regional, mantle earthquakes in the subducted Nazca plate recorded on a portable, broadband seismic array (Broadband Andean Joint Experiment and Seismic Exploration of the Deep Andes) in Bolivia and Chile. The shallow mantle under the Altiplano from ∼18°S to ∼21°S is high-velocity and moderately high Q (Vp ≈ 8.3,Vs ≈ 4.7, Qp ≈ 500, and Qs ≈ 200), suggesting lithospheric mantle. High-velocity material in the Altiplano extends to a depth of ∼125–150 km. The shallow mantle of the Western Cordillera is characterized by high Vp/Vs (∼1.83), suggesting a correlation between Vp/Vs and arc volcanism. Seismic velocity in the Western Cordillera mantle is, on average, only slightly reduced from global averages; however, velocity and attenuation anomalies are locally strong ( Vp ≈ 7.8, Vs ≈ 4.3, Qp ≈ 200, and Qs ≈ 100), consistent with partial melt conditions. Under the Los Frailes volcanic field, in the Eastern Cordillera, shallow mantle velocity and Q decrease drastically from the neighboring Altiplano ( Vp ≈ 7.8, Vs ≈ 4.3, Qp ≈ 300, Qs ≈ 100); however, high Vp/Vs is not as pervasive as it is in the Western Cordillera. We believe that slab-derived water, and perhaps other volatiles, strongly influence the Western Cordillera, while the Eastern Cordillera low-velocity region is more affected by partial melt and/or compositional changes. Average velocity and Q in the shallow mantle across the Bolivian Andes, where the tomographic images are best resolved, are significantly higher than in most mantle wedge environments where corresponding images are available. This is likely the result of a compressional “back arc” setting in the Andes. This implies that lithospheric shortening and thickening associated with the formation of the Andes has profoundly influenced the shallow mantle structure across the range. Shallow mantle structure is locally influenced by the subduction processes, particularly under the Western Cordillera; however, the differing volcanism and seismic character under the two Cordilleras suggest that the volcanic process in the Eastern Cordillera may be distinct from arc volcanism. Tertiary volcanism in the Eastern Cordillera is located in the region where mantle shortening is suspected to be greatest. Both the timing and location of volcanism are consistent with upward migration of mantle wedge asthenosphere following the removal of over thickened lithosphere.

Lithospheric structure of northern California from teleseismic images of the upper mantle

Teleseismic P wave travel time residuals from 120 earthquakes recorded across the U.S. Geological Survey California seismic network were used to determine the lithosphere P wave velocity structure beneath northern California, a region characterized by complex interactions between the Pacific, North American, and Gorda plates. Lateral P wave velocity variations beneath the array were determined by inversion of 9383 travel time residuals. Inversion results for the crust show strong correlations to volcanic features. The active volcanic fields, Shasta-Medicine Lake, Lassen, and Clear Lake, are characterized by crustal low-velocity anomalies that average approximately −6%, possibly identifying partially molten magma bodies. Cooled, solidified magma bodies beneath the extinct volcanic fields, Sonoma, southern Clear Lake, and Sutler Buttes, are denoted by relative velocity highs averaging +3%. The largest upper mantle velocity variations occur in the depth range 30–110 km, where velocities vary from −5.5% to +9.5%. These velocity variations reflect changes in the thickness and geometry of the Pacific, North American, and Gorda plates where they interact at the Mendocino Triple Junction. North of the Mendocino Triple Junction, the steep 70° east dipping portion of the Gorda plate is imaged as a +5% velocity high to depths near 270 km. A presumed segment of the Gorda plate, observed beneath the northern Great Valley and south of the inferred edge of the plate, is characterized by a +9% velocity high in the depth range 30–70 km. Beneath the northern Coast Ranges, shallow asthenosphere is imaged in the depth range 30–100 km as a pronounced southward tapering −4% low-velocity zone, which we interpret as the slab window. Results from this study provide improved constraints on Gorda plate subduction, evolution of the San Andreas fault system, and development of the lithosphere beneath western North America.

Anisotropy and mantle flow in the Chile-Argentina subduction zone from shear wave splitting analysis

[1] We examine shear wave splitting in teleseismic phases to observe seismic anisotropy in the South American subduction zone. Data is from the CHARGE network, which traversed Chile and western Argentina across two transects between 30� S and 36� S. Beneath the southern and northwestern parts of the network, fast polarization direction (j) is consistently trench-parallel, while in the northeast j is trench-normal; the transition between these two zones is gradual. We infer that anisotropy sampled by teleseismic phases is localized within or below the subducting slab. We explain our observations with a model in which eastward, Nazca-entrained asthenospheric flow is deflected by retrograde motion of the subducting Nazca plate. Resulting southward flow through this area produces N-S j observed in the south and northwest; E-W j result from interaction of this flow with the local slab geometry producing eastward mantle flow under the actively flattening part of the slab. INDEX TERMS: 7203 Seismology: Body wave propagation; 7218 Seismology: Lithosphere and upper mantle; 8123 Tectonophysics: Dynamics, seismotectonics; 8150 Tectonophysics: Plate boundary—general (3040); 9360 Information Related to Geographic Region: South America. Citation: Anderson, M. L., G. Zandt, E. Triep, M. Fouch, and S. Beck (2004), Anisotropy and mantle flow in the Chile-Argentina subduction zone from shear wave splitting analysis, Geophys. Res. Lett., 31, L23608, doi:10.1029/ 2004GL020906.