Project Description

Collaborative Research: UNR, UNLV & LDEO:

Lithospheric Architecture at a Magma Intrusion Event

Observed 30 km Below Lake Tahoe, California and Nevada

J. Louie, G. Biasi, R. Anooshehpoor, K. D. Smith, U. of Nevada, Reno

C. M. Snelson, U. of Nevada, Las Vegas

C. K. Wilson, Lamont-Doherty Earth Observatory

Motivation for This EarthScope FlexArray Project

The recent detection of magma movement deep below Lake Tahoe by K. D. Smith et al. (2004) exposed a critical shortage of information on the lithospheric structure of the region. This proposed NSF/EarthScope project will begin to address this deficiency. Study of this unusual event raised many questions about the structure, nature of the lower crust, most not well answered. We do not possess even baseline constraints on the configuration of the lithosphere in the northern Sierra Nevada. Nor do we have many constraints on the nature of the Sierra Nevada–Great Basin boundary zone (SNGBBZ). We must be able to describe the configuration of this boundary in at least a fundamental way before we can address questions such as whether the 2003 magmatic event was rare and isolated, or represents more pervasive lithospheric processes.

The need for focused work on the northern Sierra Nevada is clear when the region it is compared to the state of knowledge of the southern Sierra Nevada. Wernicke et al. (1996) summarized the results of the multi-year, multidisciplinary southern Sierra Nevada seismic experiments. Careful work such as by Jones et al. (1994) revealed the surprising lack of a deep crustal root below the topographically highest part of the range. New tectonic models for the southern Sierra Nevada resulted quickly, such as the delamination hypothesis of Ducea and Saleeby (1998). In contrast to the well-studied structure of the southern Sierra Nevada (Jones et al., 1994; Fleidner et al., 1996; Wernicke et al., 1996; Jones and Phinney, 1998; Boyd et al., 2004; etc.), information on the northern Sierra Nevada is relatively sparse[*]. Extrapolation of information from the southern to northern Sierra Nevada may not be supportable.

Compilations such as by Braile et al. (1989) show the dearth of seismic constraints on the northern Sierra Nevada region, with Eaton (1963) and Pakiser and Brune (1980) being the only experiments there until 2002. The 2002 refraction work of Louie et al. (2004) was a reconnaissance experiment lacking any blasts or on-line refraction sources in the Sierra Nevada. Although suggestive that a nonintuitively thicker (>50 km) crustal root may underlie the topographically muted northern Sierra Nevada, the 2002 survey could not constrain the upper crust without local shots. This project is partly intended to remedy the shortcomings of the reconnaissance results of Louie et al. (2004). With magma injection events observed below the northern Sierra, it is now imperative to collect baseline geophysical data there.

In characterizing the transition between the California Peninsular Ranges and the Gulf extensional province, Lewis et al. (2001) found tremendous dips on the Moho discontinuity, in places exceeding 20°. With the suggestion of a >50 km root for the northern Sierra Nevada by Louie et al. (2004), such radical lithospheric discontinuities may be a possibility for the SNGBBZ there. Yet the lack of baseline seismic characterization is even more severe in considering this fundamental tectonic boundary. The refraction results of Spieth et al. (1981) are too far west to address the transition, and the results of Catchings and Mooney (1991) and Gilbert and Sheehan (2004) are too far east. A COCORP survey crossed the northern Sierra Nevada, as reported in, for example, Nelson et al. (1986), Klemperer et al. (1986), Knuepfer et al. (1987), and Klemperer (1987). However, this COCORP line was 50 km north of Lake Tahoe where the SNGBBZ is complicated by interaction with right-lateral faulting of the Walker Lane. Thus we cannot determine whether the magma injection below Lake Tahoe was interior to a geotectonic Sierra Nevada block or a feature of the transition boundary zone itself.

The seismic refraction, tomography, and teleseismic scattering experiments proposed here for the Lake Tahoe area are targeted to understand the structures and context of the 2003 deep magmatic event. Is the shallow mantle unusual near north Lake Tahoe, or are similar conditions prevalent throughout the eastern Sierra Nevada? Are there lithospheric scatterers associated with the magma injection area? How are these scatterers or other associated anomalies distributed? Are they a feature of the Sierra Nevada block, or of the SNGBBZ transition? Is there a deeper mantle source for the magma, and if so, what is its distribution? If lithospheric anomalies are limited to just the area of the 2003 injection, then it may have been an isolated and rare event or potentially associated with focused extensional strain within the north-south structural transition expressed in the surface faults at Lake Tahoe. The Lake Tahoe area is a region is of relatively high extensional strain (Smith et al., 2004). If these anomalies are part of a broader pattern, though, magma injection of the lower crust could be a typical feature of the boundary between the Sierras and the extensional Great Basin, and perhaps of the boundaries of extensional provinces in general.

Background and Rationale

Geodetic data indicate that the Sierra Nevada block is moving about 13 mm/yr N40-45W relative to stable North America. This motion accounts for about 20-25% of the western North American plate motion budget and is generally oblique to north-striking down-to-the-east active normal faults along the SNGBBZ in northern Nevada and eastern California near the latitude of Lake Tahoe. This motion drives the earthquake hazard of the region. In view of this hazard, the Nevada Seismological Laboratory (NSL) has operated seismic stations in the Lake Tahoe area for over 30 years.

As the SNGBBZ boundary has evolved westward since 15 Ma (Dilles and Gans, 1995) in a process that incorporates Sierra granites into the Basin Range Province (Surpless et al., 2002), the normal fault systems at this boundary have accommodated the eastward collapse of the Sierra block and development of Basin and Range physiography. Along the Sierra Nevada front, north-striking normal faults have developed in a left-stepping geometry from about the latitude of Long Valley, extending northward through the northern end of Lake Tahoe. The Lake Tahoe basin represents the westward, and youngest, expression of the SBGBBZ having developed over the past 3 Ma. To the east the Walker Lane Belt (Stewart, 1988) primarily accommodates dextral plate-boundary shear including extensional faulting in overall transtensional tectonics. In other words, whereas the eastern front of the Sierra Nevada is the focus of down-to-the-east normal faulting reflecting the westward evolution of the Basin and Range into the Sierra Nevada block, strike-slip faulting, more representative of plate boundary shear, characterizes the Walker Lane Belt.

The normal and strike-slip regimes in the upper crust along the Sierra Nevada front operate under regionally consistent E-W directed extension (T-axis from focal mechanisms; Ichinose et al., 2001), with the P-axis rotating locally to reflect either normal or strike-slip faulting. Along the SNGBBZ at the latitude to of Lake Tahoe, Quaternary moment release is dominated by the normal fault systems, with strike-slip faulting apparently playing a role in slip transfer between these primary normal-fault systems (Schweickert et al., 2004).

Large volume Tertiary ash-flow tuff sequences that initiated around 30 Ma extend throughout northern Nevada and eastern California near Lake Tahoe. Volcanism progressed throughout the Miocene and by about 3 Ma had evolved to arc-related andesitic magmas and then waned. The most recent mapped volcanics comprise a number of 1-2 Ma basaltic volcanic sources in the north Lake Tahoe area, and additional source areas including the rhyolitic domes of the Steamboat Hill geothermal area in metropolitan Reno east of the lake (Henry and Faulds, 2004). There is an ongoing debate concerning the most recent age of volcanism in the north Lake Tahoe area (Sylvestor 2004, personal communications); and age dating is underway for some suspected young basalts at Tahoe. Only the Long Valley caldera and Lassen Peak volcanic areas show volcanics younger than 1 Ma along the eastern front of the Sierra.

Evidence for magma injection in the SNGBBZ at lower crustal depth beneath north Lake Tahoe in late 2003 has recently been published in Science (Smith et al., 2004). A sequence of 1600 earthquakes (Figure 1) from August 2003 through February 2004 define a planar structure at a depth of roughly 30 km below the lake that was coeval with a 10 mm permanent displacement observed on a nearby GPS instrument at Slide Mountain, Nevada (Figure 2). The observations were modeled as a 40 km2, 1-m thick, lower-crustal dike injection event; and therefore the process accommodated about 1 m of extension in the lower crust. The structure is oriented approximately N30ºW and dips 50º to the northeast. Observed E-W extension in the Lake Tahoe region would support tensile failure for a structure with this geometry. In addition, the volcanic dike in map view is situated within 4 mapped locations of 1-2 Ma basalts in the north Lake Tahoe area (Henry and Faulds, 2004). Remarkably, the dike injection event apparently triggered an increase in shallow seismicity (< 15 km) throughout the northern Lake Tahoe area that included an M 4.2 earthquake in June 2004 at about 10 km epicentral distance from the deep crustal sequence.

This inferred magmatic event represents a unique opportunity, possibly not to be repeated anytime soon, to study a coherent magmatic intrusion in the northern Sierra Nevada. Thus it is reasonable to combine the detailed analysis of the deep crustal context of the 2003 event with a broader look at the structure of the northern Sierra Nevada at the latitude of Lake Tahoe, as proposed here. We also propose to apply state-of-the-art imaging methods to the zone of the deep earthquake cluster (and presumed magmatic intrusion) and to the problem of defining deep fault zones of the Tahoe Basin. The observation of magma injection with the SNGBBZ also brings up fundamental questions regarding the evolution and westward encroachment of the Basin and Range province that could contribute to our understanding of volcanic hazards and public safety.

Broadly, this proposal will address two scales, one identified with the SNGBBZ faulting and magmatism in the Tahoe Basin, and one more regional in scope that will provide the larger context. The proposed work will address:

  1. Is the dike related to the SNGBBZ frontal fault system or another of the Tahoe Basin faults? Association with the frontal fault zone would require a fairly high angle in the mid-crust and a more listric geometry eastward to join the surface expression of the frontal fault system.
  2. Can we image lower crustal magma similar to those interpreted from the COCORP Sierra Valley line (Klemperer et al., 1986; Knuepfer et al., 1987)? Are lower crustal magmas pervasive?
  3. Can we specifically image a deeper magma source for the 2003 magma injection event under Lake Tahoe? What is the depth of this source?
  4. What is the nature of the boundary between the Sierra Nevada and the Great Basin? The presence of volcanism in the physiographic Sierra Nevada suggests that the boundary is offset between the crust and mantle as it is in the southern Sierra, but this remains to be demonstrated.
  5. What is the structure of the Moho and crustal thickness through the SNGBBZ?
  6. Are there other regions of the lower crust and upper mantle that suggest potential future magma movement at depth?

Passive Tomography Experiments and Analysis

Objectives: 1) Image the crustal and upper mantle structure in the Lake Tahoe area, including the region of the inferred deep crustal magmatic injection in 2003;2)Develop a velocity model for the structure under the northern Sierra Nevada and Lake Tahoe that can be used to guide the wave-equation imaging at depth and to improve hypocentral locations for deep seismicity in this area; 3) Identify shallow mantle regions with similar physical properties and presumably similar conditions for magmatic activity; and 4) Link the study area to the broader context of subduction and evolution of the Sierra Nevada.

Motivation: The crustal structure across the Sierra Nevada at the latitude of Lake Tahoe is poorly known. Figure 3a provides the larger context of the northern Sierra Nevada and the study area. P-wave velocity variations are shown for California and western Nevada at a depth of 50-70 km using the combined California and Nevada permanent seismic arrays. Known effects of crustal thickening (Oppenheimer and Eaton, 1984) were removed before inversion to improve resolution of the mantle contribution. The western half of the Sierra Nevada range and eastern half of the Great Valley are high velocity relative to model average. At these depths a silicic root would be lower velocity, so it appears rather to be cold and perhaps eclogitic in composition. Its extent along strike indicates that it formed by subduction-related processes, either with the Mesozoic construction of the arc or by modification during Cenozoic subduction. Figure 3b shows volcanism since 13 Ma on an enlarged portion of the regional image. Volcanism clearly concentrates east of the Sierra Nevada, but extensive volcanism north and west of Lake Tahoe, which lies well within the physiographic Sierra, suggest that the mantle definition of the SNGBBZ is 15-30 km to the west.

The detailed context for volcanism in Tahoe Basin and SNGBBZ is essentially unknown. The recent magmatic event makes clear that at some level the system is active, but work proposed here will be required before it can be known whether it was simply a local anomaly, whether it is part of a system with similar source conditions, or whether it is the general state of the upper mantle beneath the eastern Sierra Nevada. The active source portion of the proposed experiment discussed below is designed to evaluate whether master faults of the Tahoe region or perhaps other fine-scale features work to localize magmatic events.

Figure 3. (a) P-wave tomographic image of California and Nevada at a depth of 50-70 km. Teleseismic delays to permanent network stations of California and Nevada plus a local temporary array in the southern Sierra Nevada (Biasi and Humphreys, 1992; Jones et al., 1994) were inverted in a model with a total depth of 650 km. Block size is 30x35 km (EWxNS), extrapolated after inversion to 10x10 km for plotting purposes. Outer red line encloses the region at this depth with crossing-ray constraint, which can be interpreted as the resolved map region. Holes in the coverage exist in both the southern and northern Sierra Nevada, generally corresponding to gaps between regional networks. LVC: Long Valley Caldera; MTJ: Mendocino Triple Junction; SGVA: Southern Great Valley Anomaly; TRA: Transverse Ranges Anomaly. Stratovolcanos labeled with white triangles. (b) Enlargement of (a) around the study area. The prominent upper mantle velocity high velocity region is offset to the west some 15 to 30 km from the eastern bound of the physiographic Sierra Nevada. A similar offset in the southern Sierras, evident in (a), has been extensively studied (Jones et al., 1998; Ducea and Saleeby, 1998). Volcanism since 13 Ma (C. Henry, pers comm.;circles: dated samples; stars: interpreted locations of volcanic centers) indicate a fundamental difference in mantle conditions is present in shallow mantle depths. Inversion block sizes and native station density are too coarse to resolve questions raised in this proposal.

Proposed Work:

Briefly, we propose an 18-month deployment of 40 broadband seismic stations (Figure 4). Data acquired during this experiment will be supplemented by network station coverage of the California and Nevada regional seismic networks. Where needed to fill in azimuth or ray-parameter coverage, recordings during the experiment will be supplemented by archive data of the permanent seismic networks. The proposed temporary array will entail relatively high density coverage in the central area to ensure adequate resolution at upper mantle depths.

Figure 4. Proposed layout of the passive teleseismic array. Note that current “Digital” and “Analog” sites can be used for additional passive array instruments.

The usefulness of high-density teleseismic recording is shown in the Vp images shown in Figure 5. Magma was erupted from five small centers (green triangles), four at about 1 Ma, and the fifth (LC) approximately 120 Ka. Melt extraction increases seismic velocities, of itself and as it removes water from the solid phases (esp. olivine) in the mantle (Karato and Jung, 1998; Hammond and Humphreys, 2000). The only depth beneath the volcanic centers showing the predicted velocity increase is about 50 km. Petrologic evidence shows the magma originated at about 53 km (the center of the layer shown in Fig. 5a, black oval in b). Of note as well, the lowest velocity regions in the 45-60 km depth slice (south and east of LSM) are and have been substantially amagmatic. For reasons developed elsewhere (Biasi, in prep.) this lowest velocity region is likely to be hydrated and subsolidus. Finally, the scale of the upper mantle structures involved to produce the Quaternary volcanism here imaged are so small that imaging with teleseismic shear waves could be difficult. At, say, five second periods, the wavelength is of order 20 km, and structures large enough to likely be imaged are probably just wet (Karato and Jung, 1998). The southern Nevada example in Figure 5 shows that low velocities are not certain beneath regions of recent volcanism, and that a nuanced interpretation that considers all available data may be necessary to unravel the northern Sierra Nevada teleseismic images.

Figure 5. (a) Direct image of a melt extraction region using P-wave tomography. Melt extraction to Quaternary volcanic centers of Crater Flat (green triangles) should cause a 1-2% increase in Vp in part by melt extraction and in part by dehydration of olivine. The one depth at which velocities increase is imaged in the 45-60 km depth layer. One year of portable and network station teleseismic data from southern Nevada was used to produce the image. Net station density is comparable to the proposed northern Sierra Nevada experiment – 5-8 km in the center of the imaging area. This model shows that 4.5x4.5 km blocks can be resolved to a model depth of 80 to 100 km. Note that lowest mantle Vp regions have not been volcanically active; these regions are likely to be sub-solidus and hydrated. LSM: Little Skull Mountain; ESF: Exploratory Studies Facility, Yucca Mountain. (b) North-south cross-section beneath the Crater Flat volcanic cones. The magmatic source is centered on ~50 km depth, consistent with petrologic estimates.