crustal structure and evolution of explosive silicic volcanic system at ...

© WGA, 2005 - Geology of Yellowstone Park Area; 33rd Annual Field Conference Guidebook, 1982

233 Thirty-Third Annual Field Conference - - 1982 Wyoming Geological Association Guidebook

CRUSTAL STRUCTURE AND E V O L U T I O N OF E X P L O S I V E SILICIC VOLCANIC SYSTEM AT YELLOWSTONE NATIONAL P A R K 3 ROBERT

B. S M I T H ~, L A W R E N C E

W. B R A I L E ~

ABSTRACT Large volumes of Quaternary silicic volcanics (,v6700 km3~, associated explosive caldera-forming eruptions, and high heat flow (in excess of 1800 m W m "2 infer the presence ofsilicic magmas within the crust and upper mantle beneath the Yellowstone Plateau. Seismic refraction-reflection data, analyses of earthquake hypocenters, and seismic attenuation have revealed a laterally inhomogeneous upper crust with lowP-wave velocities but a more seismically homogeneous lower crust. The upper crust beneath the Yellowstone camera is characterized by P-wave velocities of 5.7 km/sec and 4.0 km/sec -- values that are anomalously low compared with that of the surrounding thermally undisturbed crystalline basement of 6.0 km/sec. The 5.7 km/sec body generally underlies the Yellowstone cMdera (35 km x 65 km) and coincides with a regional -60 mgal gravity low, suggesting concomitant low density and low velocity. The 5.7 km/sec low-velocity body is interpreted to represent a hot but relatively solid body approximately 8 to 10 km thick that was probably the reservoir for the silicic magmas. A 4.0 km/sec low-velocity body located beneath the northeast boundary of the caldera coincides with a local -20 mgal gravity low and has a tenfold increase in seismic attenuation -- properties that can be interpreted to result from a steam-water-dominated system to a body of 10-50 percent silicic partial melt. The P-wave velocity of the upper 100 to 250 km of the mantle beneath the Yellowstone region, analyzed from teleseismic arrivals, is reduced by ,".,,,5 percent, suggesting the presence of a basaltic partial melt that is probably the source of heat that drives the Yellowstone hydrothermal system. In comparison, the lower crust of the Yellowstone region appears seismically homogeneous to the horizontally propagating refracted rays and similar to that of the thermally undisturbed lower crust of the surrounding Rocky Mountains. This suggests that the seismic properties of the lower crust were relatively unaffected by the ascension of the parental basaltic magmas that are hypothesized to have intruded and partially melted the crust producing the voluminous rhyolite and ash flow tufts of the Yellowstone Quaternary volcanic system. Maximum focal depths of earthquakes in Yellowstone systematically shallow from ~ 20 km outside the caldera to ,~' 5 km beneath the caldera, suggesting the influence of high-temperature abnormal pore pressure, compositional changes that restrict brittle failure to the upper crust. Orthometrically corrected reobservations of level lines across the Yellowstone camera show an area of crustal uplift, up to 15 mm/yr, that generally coincides with the outline of the 5.7 km/sec low velocity layer. These data are consistent with a model in which an ~University of Utah 2Purdue University

3Reprinted from "Explosive Volcanism" National Academy Press, Washington, D.C., in press through National Academy of Sciences, National Research Council.

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ROBERT B. SMITH AND LAWRENCE W. BRAILE

upper-crustal low-velocity~low-density layer, 75 km x 25 km, appears to be plastically deforming. Taken together with the geologic data this crustal model is interpreted to reflect the structure and properties of a thermally deforming Archean crust and the initial stages of the bimodal rhyolitic~basaltic volcanism of the Yellowstone- Snake River Plain volcano-tectonic system. While the interpretations are not unique; the youthfulness and volume of Quaternary volcanism, the high heat flow, the high rates of contemporary uplift, and the upper-crustal low-velocity layers infer the presence of hot crustal material and possible partial melts that underlie the Yellowstone Plateau. These properties cannot yet be evaluated to indicate temporal variations in volcanism, but the geologic record and the new geophysical models suggest future volcanic activity in the Yellowstone Plateau.

INTRODUCTION The heat from the numerous geysers, hot springs, and fumaroles of Yellowstone National Park represents only a small fraction of the total thermal energy of this major silicic volcanic system. Voluminous Quaternary volcanism with rhyolite flows as young as 50,000 yr., three explosive caldera-forming eruptions, and very high heat flow (in excess of 1800 mW/m2) infer that large magma bodies have existed within the upper crust beneath the Yellowstone Plateau which now provides the heat and energy that drives the hydrothermal and tectonic systems of Yellowstone (see Christiansen, in press, for the Quaternary volcanic history of Yellowstone). However, the longevity and large volumes of the Quaternary silicic volcanics suggest early and continuous involvement of basaltic magmas from lithospheric sources. An evaluation of earthquakes, crustal structure, and contemporary deformation of the Yellowstone Plateau when interpreted within the framework of Quaternary history can provide an insight into the dynamics of a thermally evolving continental crust. Interpretations of these data can be used to assess the locations and physical state of possible magma bodies -information required to identify precursors that could precede eruptions. The Quaternary history of the Yellowstone Plateau is characterized by rather continuous eruptions of rhyolites and ash-flow tufts that accumulated over 6500 km 3 but was marked by three explosive eruptions and subsequent caldera-forming collapses of the crust 2.0, 1.3, and 0.6 million years (m.y.) ago (Christiansen, in pressl. While the young volcanism and high heat flow reflect a major thermal episode, they are but the primary phase of a temporal-spatial propagating silicic system -- the Yellowstone-Snake River Plain (Y-SRP) volcano-tectonic province that is now hydrothermally active at Yellowstone. It is generally agreed that the Yellowstone-Snake River Plain province, which is characterized by a bimodal suite of

rhyolites and basalts, is related to the systematic progression of silicic volcanic centers that migrated northeasterly at about 4 cm/yr (Armstrong et al., 1075; Christiansen and McKee, 1978) beginning about 15 m.y. ago from southwestern Idaho to the present location of the Quaternary volcanism at the Yellowstone Plateau. Several hypothesis have been proposed to explain the spatialtemporal progression of the silicic volcanism along the axis of the Y-SRP, including (1) the passage of the North American Plate across a convective mantle plume or a hot spot related to heat-generating radioactive material in the upper mantle, (2) the propagation of a lithospherie fracture, (3) the spatial progression of thermal instabilities produced by friction at the base of the lithosphere, and (4) temporal progressive volcanism along a transform fault. An understanding of the dynamics of the Yellowstone volcanic system is crucial to an evaluation of the thermal modification of a continental Archean crust, particularly to an understanding of the kinematics of lithospheric deformation accompanying crustal differentiation from intrusion and partial melting. Models derived from such properties as size, depth, and physical state of magma-generating bodies may be inferred from seismic velocities, seismic-wave attenuation, and density. These properties, when interpreted within the geologic framework can be useful in predicting locations and future volcanic eruptions. The presence of shallow crustal magmas in other continental settings has been inferred on the basis of seismic P-wave delays, crustal structure, and the attenuation of seismic waves, for example, beneath the Rio Grand Rift, New Mexico (Sanford et al., 1977); beneath the Long Valley caldera, California (HiU, 1976); and beneath the Coso geothermal area, California (Reasenberg et al., 1980); whereas there was a lack of distinct seismic evidence for a shallow magma chamber beneath the Mount St. Helen's volcano prior to the 1980 eruption IS. Malone, University of Washington, personal communication, 1982), which is

CRUSTAL STRUCTURE AND EVOLUTION OF AN EXPLOSIVE SILICIC VOLCANIC SYSTEM AT YELLOWSTONE NATIONAL PARK "

suggestive of a different mechanism and volume for upper-crustal magma emplacement. A relative perspective of Quaternary volcanism at Yellowstone is shown in Figure 1, where volumes of several well-known eruptions (Williams and McBirney, 1979; Decker and Decker, 1981~ are shown for comparison. The 1980 Mt. St. Helens eruption, which dominates our current concept of explosive eruptions, is on the order of a ten thousandth of the volume of the large explosive Yellowstone eruptions. And while the distribution of airborne ash from Mt. St. Helens produced millions of dollars of damage, airborne ash flows from the Quaternary Yellowstone eruptions were measured as far away as Saskatchewan and Mississippi. A comparison with other large explosive eruptions, such as Katmai in 1920, Krakatoa in 1882, Tambora in 1815, and Mazama (about 7000 years b.p.) dramatizes the fact that these catastrophic eruptions were hundred's of times smaller than the Quaternary Yellowstone explosive eruptions and emphasizes the importance to understand this major silicic volcanic system. The Quaternary tectonic framework of the Yellowstone Plateau is portrayed in Figure 2, which shows the distribution of volcanic flows, collapse calderas (including the most recent 600,000-year-old calderaj, the Sour Creek (northeastj and the Mallard Lake (southwest~ resurgent domes, and Cenozoic faulting related both to tumescence during thermal expansion and collapse and contraction of the crust following the explosive eruptions. The youthfulness and large volumes of Quaternary volcanics at Yellowstone infer that intermediate- to upper-crustal heat sources may still be present. Geophysical experiments designed to characterize the crustal properties of the Yellowstone system have been conducted through the last decade. Earthquake monitoring on both a regional and local scale have been conducted by the University of Utah and the U.S. Geological Survey (Eaton et al., 1975; Trimble and Smith, 1975; Smith et al., 1977; Pitt, 1980L Aeromagnetic surveys and gravity compilations have been used to infer the distribution of shallow silicic melts and caldera infill (Blank and Gettings, 1974; Eaton et al., 1975; Smith et al., 1974, 1977). P-wave delays of up to 1.8 sec from teleseismic arrivals were interpreted by Iyer et al. (1981) to represent up to a 20 percent velocity reduction in the crust and a 5 percent reduction in the upper mantle to depths of 250 km. These delays were interpreted to reflect zones of possible melt, deep in the lithosphere beneath the Yellowstone caldera. While conductive heat-flow measurements are difficult to obtain in Yellowstone, because of the widespread shallow-water circulation, inferences of conductive heat flow from marine-type probes in Yellowstone Lake (Morgan et al., 1977}, and the C1 geochemical budget

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(Fournier et al., 1976~ show that the average heat flux through the Yellowstone camera is approximately 1800 mW/m2--about 60 times the world average. These data and inferences from the geologic record suggest that crustal sources of heat are driving the Yellowstone hydrothermal systems and provide much of the energy for the contemporary tectonic deformation. In 1978 a major seismic refraction/reflection experiment was conducted in the eastern Snake River Plain-Yellowstone region as a means of evaluating the lithospheric structure of this continental tectono-volcanic province (Braile et al., 1982; Smith et ai., 1982}. In this project up to 225 vertical-component seismographs were located on profiles and in two-dimensional arrays throughout the region. Within Yellowstone, five explosions external to the caldera were recorded across the Yellowstone Plateau. Interpretations of P-wave refraction and wide-angle reflections from these profiles yielded average crustal velocity structures (Schilly et al., 1982j. Three-dimensional velocity distributions of the upper crust were determined from delay-time analyses (Lehman et al., 1982), from simultaneous inversions of travel times from local earthquake and refraction data measurements for the upper-crustal structure (Benz and Smith, 19811, and from University of Utah unpublished refraction data of the southwest Yellowstone Plateau recorded in 1980. Information on the attenuation of P-waves was determined from inversion of spectral amplitude ratios obtained from the refraction data [Clawson and Smith, 1981). Together these studies provided a seismic image of the crustal velocity structure, with a highest resolution of a structure of a few kilometers -- near the scale of surface tectonic features. Longer-range refraction/wide-angle reflection profiles (Smith et al., 1982) and S-wave delay and surface-wave dispersion investigations (Daniel and Boore, 1982~ were used to evaluate the averaged P- and S-wave structure of the crust and upper mantle. Interpretations of the new seismic refraction-reflection data served as the basis for this discussion and together with other geophysical and geological data provide an evaluation of the locations of possible bodies of hot rock, zones of partial melt, and magma in the crust beneath the Yellowstone Plateau. Seismic models are synthesized with new hypocenter data, with evaluations of contemporary crustal deformation from level-line re-observations (Pelton and Smith, 1979, 1982) and the Quaternary geologic record (Christiansen, in press). Inferences of physical state including composition and temperature are then used to evaluate the sources of the Quaternary volcanism and to speculate on future volcanism at Yellowstone.

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