Study of tectonically controlled, time-dependent basaltic volcanism published

LA-UR-07-0914
August 2007
Greg Valentine (EES-6) and Frank Perry (EES-9)

Urbanization and construction of long-term facilities in basaltic volcanic fields creates an important class of volcanic risk assessment problems. Understanding the evolution of basaltic volcanic fields is critical to the understanding of basaltic magmatism and to volcanic risk assessment. Estimation of event probabilities in basaltic fields that are dominated by monogenetic volcanoes requires forecasts of both the recurrence rates (or timing) and locations of future events.

Greg Valentine (EES-6) and Frank Perry (EES-9) have published research regarding volcanism in the Southwestern Nevada Volcanic Field (SNVF), an example of an extremely low volume-flux end member of basaltic fields. The SNVF is the geographical area where the proposed Yucca Mountain radioactive waste repository is located (Figure 1 map). The researchers describe physical volcanological data that provide insight into the length scales of sources tapped for each volcano, the lengths of feeder dikes that transport magmas to the surface, and the relationship between lava effusion rates, dike lengths, and volumes at individual volcanoes. Relationships between age and cumulative eruptive volume indicate that the repose interval between eruptive episodes is determined by the volumes of prior episodes. Since approximately 3Ma, the field appears to be time-predictable.

Figure 1. Shaded relief map of the southwestern part of the Southwest Nevada Volcanic Shield showing Plio-Pleistocene volcanoes (buried volcanoes in the southern part of the map are indicated with white diagonal line patterns). Color patterns indicate basalts <3 Ma and gray scale patterns indicate>3 Ma.  4.6Ma episode — Thirsty Mountain (TM) and Anomalies C and D.   3.8 Ma episode — SE Crater Flat (SECF) and Anomalies B, F, G, and H.  2.9 Ma episode —Buckboard Mesa (BM).  1.1 Ma episode — Makani volcano (MC), Black Cone (BC), Red Cone (RC) and NE and SW Little Cones (LC).  350ka episode — Little Black Peak (LBP) and Hidden Cone (HC). 77 ka episode — Lathrop Wells volcano (LW).  Crater Flat and Amargosa Desert are major basins that host many of the Plio-Pleistocene basalts. Black lines are Miocene caldera boundaries.  

The data support a model wherein magmatism in the SNVF is a passive response to relatively slow regional tectonic strain.  Partial melt resides in pockets of lithospeheric mantle that are relatively enriched in hydrous minerals. Slow deformation focuses melt, occasionally resulting in sufficiently high melt pressure to drive dikes upward and feed eruptive episodes, which relieve local stresses. Larger events are followed by longer repose intervals required to recover crustal stresses. The scientists suggest that time-controlled predictability may be a fundamental property of tectonically controlled basaltic fields, where melt accumulation and ascent are controlled by tectonic strain rate. However, time-predictability in a tectonically controlled field does not, by itself, constrain the location or size of potential future episodes. The authors have previously shown that volcano location in the low-flux SNVF depends primarily upon the location and areal extent of the mantle source that is tapped (magma footprint) and secondarily upon shallow structure and surface typography. Because most of the basaltic episodes (except the ~2.9 Ma Buckboard Mesa event) have returned to the vicinity of the existing cluster, the researchers suggest that the magmatic footprints of future events will probably also occur near or within these clusters.  Furthermore, general similarities in volcano size and eruptive styles during the past ~1.1 Myr suggests that the future events during the next 100s of kyr will have similar characteristics. All of these factors form potentially important aspects of probabilistic risk assessment in the SNVF. The DOE Yucca Mountain Project supports the work. Reference: Valentine, G. A., Perry, F. V. “Tectonically Controlled, Time-predictable Basaltic Volcanism from a Lithospheric Mantle Source (central Basin and Range Province, USA), Earth Planet. Sci. Lett. (2007) doi, 10.1016/j.epsl2007.06.029

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