The Southwest Nevada Volcanic Field As An Upper Mantle Strain Marker

• Roots of the SWNVF are sharply defined:
• 50 km in diameter
• Preserved to over 300 km depth or more
• Vertical over the upper 200 km, with a slight NE trend beneath.
• The root is preserved within 15 km or less of the surface expression
as a multiple-caldera complex.
• Magma extraction is gravitationally driven, so this root must have begun in a near-vertical orientation.
• Upper mantle flow above 200 km has been negligible.
• Between 200 km and 350 km about 40 km of southwestward flow parallel to NA APM has occurred.
• The top-to-the-west shearing predicted from post-15 Ma B&R expansion does not shear the root structure.

The Style and Timing of Volcanism

• Onset of large volume explosive caldera activity somewhat predates 15 Ma.
• Several thousand cubic kilometers of material, mostly tuffs, with small volume basalt flows, were produced over about 3.5 My.
• Following the Timber Mountain Caldera eruption at 11.7 Ma, volcanic volumes dropped significantly.
• Subsequent volcanic activity has been subdued in style, and basaltic in dominant style.

Origin of the Southwest Nevada Volcanic Field

• The origin of the SWNVF root must be deeper than 300 km, and in central Nevada.
• Melt production requires supersolidus conditions.
• Magma ascent requires it be buoyant at all upper mantle depths imaged. There does not appear to have been a density inversion trapping melt in the upper 300+ km.
• No mechanism suggests itself if this were to be a purely thermally driven anomaly. How does one get this much heat so deep?
• Introduction of fluids from (the now) Gorda Slab dewatering would explain the root structure, the brevity of volcanism, and the residual high velocity of the root.

Implications

• The SWNVF is not crustal in origin, and cannot be attributed to its location in a right step of the east Walker Lane strike-slip structures.
• Crust and mantle have not been displaced relative to one another. Long-offset Death Valley-like detachments have not disrupted the SWNVF structure.
The asthenosphere, if it is indeed present, has not been highly mobile, or accommodated counterflows such as top-to-the-west shallow and bottom-to-the-east at depth.
• Shear in the upper 200 km is much smaller than crustal extension east of the SWNVF since 15 Ma, indicating that extension was concentrated in a narrow zone to the east.
• The lack of upper mantle flow indicates any Yellowstone plume had no effect on structures shallower than 200 km, and thus is unlikely to account for the gravity and topographic gradient in southern Nevada.

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