deMartin, Brian

Seismic structure of the Yellowstone Hotspot

Known primarily for its world renown display of geysers, hot springs, and fumaroles, Yellowstone National Park is also characterized by recent, (2.0, 1.2 and 0.6 Ma) cataclysmic silicic volcanism. Numerous postcaldera silicic and basaltic flows, some as recent as 70 ka, emphasize Yellowstone's youthful volcanism. Large normal faults associated with Basin-Range extension, wide spread earthquake swarms, exceptional crustal deformation of up to 1 m of caldera uplift from 1923 to 1984, reversing to subsidence of up to 15 cm by 1995, and high thermal fluxes of up to ~ 2000 mW m-1 confirm the fundamental role of active igneous and tectonic processes at Yellowstone. In order to understand the surface manifestations of subsurface igneous and tectonic phenomena numerous studies have attempted to determine the seismic structure of the Yellowstone region (e.g. Lehman et al., 1982). The most recent study (Miller and Smith, 1999) has derived a three dimensional P wave velocity model for the region below the Yellowstone caldera using a total of 66, 627 P travel times. Five low-velocity zones were identified in the model (see Figure) and were well correlated with dominant surface features (e.g. Mallard Lake resurgent dome and Hot Springs Basin).

The first low-velocity zone, associated with the caldera, coincides with a prominent 60 mGal negative Bouguer gravity anomaly. P velocities beneath the caldera average 5-5.6 km s-1 compared to the 6-7 km s-1 modelled outside of the caldera boundary within the thermally undisturbed basement rocks. Velocity reductions within this region are interpreted to be associated with low-density, granitic caldera fill, increased temperatures, higher fracture density and the presence of hydrothermal fluids, partial melts and magma.

In the northeastern section of the caldera lies Hot Springs Basin, the largest and most active of the Yellowstone hydrothermal systems. An ~10 km ´ ~20 km, pear shaped low-velocity zone underlines Hot Springs Basin. Increased fluid pressures supplying Hot Springs Basin as well as hydrothermally altered rocks at depths inhibit the propagation of compressional waves, reducing P velocity. Compressional wave velocities within this zone are as low as 3.4 km s-1 at 4 km depth. NW striking faults that dip SW towards the caldera extend beneath the Hot Springs Basin area and coincide with a more diffuse low-velocity zone that merges with the third low velocity zone, roughly below the Sour Creek (SC) resurgent dome.

The Sour Creek resurgent dome lacks a strong negative gravity anomaly, implying density variations at depth are small. Two possible explanations are consistent with the observed decreased in seismic velocity. One, a moderate volume of partially melted rock is present beneath the Sour Creek resurgent dome. Two, hydrothermal fluids produced by the cooling of partially melted rocks could locally increase pore pressure and create micorcracks, as is hypothesized for the Hot Springs Basin anomaly. Miller and Smith (1999) interpreted the velocity model to imply a local hydrothermal convection cell beneath the Sour Creek dome that is recharged with meteoric water from within the caldera. Convection is driven by a region of partial melt and flow is along the SW dipping faults running thought the region. Velocities within this ~20 km ´ ~20 km zone are as low as 4.0 km s-1.

A fourth, but less distinct low-velocity zone is present beneath the southwestern caldera near the Mallard Lake (ML) resurgent dome. The low P velocities are also interpreted as a zone of partially melted rock despite an observable gravity anomaly. The smaller size of the Mallard Lake P velocity anomaly implies a smaller volume of partial melt than that predicted below the Sour Creek resurgent dome.

The final low-velocity anomaly imaged lies along the Norris-Mammoth corridor and is not shown in the figure. Miller and Smith (1999) concur with previous studies (e.g. Lehman et al., 1982) and interpret the low-velocity zone to be associated with a deep graben bounded on the west by the Gallatin fault and filled with low-velocity alluvium and colluvium. In addition, as with all Yellowstone low-velocity features, fluid or gas saturated sediments associated with hydrothermal systems in the region are also suspected to contribute to lower observed velocities.

The seismic structure of the Yellowstone caldera shows low-velocity anomalies in the upper crust related to Yellowstone's active volcanism. Within the entire caldera region, P velocities are 1-2 km s-1 lower than the thermally undisturbed basement rocks immediately in contact with the caldera. Low velocity, density, and gravity anomalies beneath the northeast rim of the caldera near Hot Springs Basin imply a saturated and hydrothermally altered zone characterized by high pore pressures. Low-velocities within the caldera imply two distinct zones (Sour Creek and Mallard Lake resurgent domes) of partial melting near regions of crustal deformation. In the northern section of the caldera tectonic processes influence crustal velocity structure. The strong correlation between surface features and subsurface velocity anomalies leads to a greater understanding of the process that have formed, and continue to transform, Yellowstone National Park.

References:

Lehman, J. A., R. B. Smith, M. M. Schilly, and L. W. Braile (1982) Upper crustal structure of the Yellowstone caldera from delay time analyses and gravity correlations, J. Geophys. Res., 87, 2713-2730.

Miller, D. S., and R. B. Smith (1999) P and S velocity structure of the Yellowstone volcanic field from local earthquake and controlled-source tomography. J. Geophys. Res., 104, 15105-15121.