A Revolution in Plate Tectonics?

THE INTERACTION OF TECTONIC AND MAGMATIC PROCESSES IN THE LONG VALLEY CALDERA, CALIFORNIA

A DISSERTATION SUBMITTED TO THE DEPARTMENT OF GEOPHYSICS AND THE COMMITTEE ON GRADUATE STUDIES OF STANFORD UNIVERSITY IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY

Stephanie G. Prejean February 2002

Abstract
The Long Valley caldera of eastern California is a hazardous province where tectonic and magmatic processes interact to drive on-going seismicity and deformation. The caldera is located on the boundary between the Basin and Range province and the Sierra Nevada batholith along the actively extending Sierra Nevada range-bounding normal faults. It is not clear if Basin and Range tectonic extension drives magmatic intrusion in this area or if magmatic activity is independent of regional tectonic processes. Magmatic intrusion into the caldera and extensional faulting are temporally coupled, yet it is not clear how these pro- cesses mechanically interact and potentially trigger each other.

In this dissertation, I investigate the interaction between tectonic and mag- matic processes in the Long Valley caldera over a range of scales, with the goal of developing a comprehensive model for the observed activity in the caldera. To gain a first order understanding of the mechanics by which the Long Valley area deforms, local fault geometries need to be established. To this end I relocated seismicity in the greater Long Valley area. The resulting high-resolution locations reveal a systematic fabric of faults within the caldera and in the Sierra Nevada basement rock to its south. From the focal mechanisms associated with individ- ual faults, I developed a kinematic model for seismic deformation in the Long Valley caldera. Seismicity within the caldera occurs primarily on a set of east/ west-trending right-lateral faults in the caldera's south moat. Since the south moat is located in a left step of the Sierra Nevada range-bounding normal faults, the south moat shear zone in essence forms a "transform" zone between loci of Basin and Range extension. In the Sierra Nevada block, directly south of the caldera, tectonic extension is accommodated by an east-dipping oblique-normal fault and two left-lateral strike-slip faults in its hanging wall. The location of these faults in the footwall of the Sierra Nevada range-bounding normal fault at this latitude, suggests that Basin and Range extension is potentially cutting into the Sierra Nevada batholith in this area.

To understand better the mechanical interaction of tectonic and magmatic processes at the regional scale, I performed a series of focal mechanism stress inversions in the caldera area. The inversions show that around the caldera the minimum compressive stress is perturbed from the more regional WNW–ESE direction to a NE–SW orientation. Dislocation modeling of the mapped stress field reveals that the stress perturbation cannot be explained solely by the intru- sion of magma beneath the resurgent dome, but may reflect the large-scale left- stepping offset in the Sierran range-bounding normal faults. Thus, the direction of fault slip seems to be controlled by regional tectonic processes rather than local magmatic processes. This implies that Basin and Range extension governs activ- ity in the caldera and possibly provides conduits for ascending magma.

To understand the relationship between tectonic and magmatic processes at the scale of the earthquake source, I investigate the influence of magmatic activ- ity on earthquakes by examining the source processes of earthquakes and by studying the spatial and temporal development of seismicity during a crisis epi- sode. The great majority of earthquakes in the caldera region appear to be typical "tectonic" earthquakes with source parameters similar to those observed in non- volcanic regions. However, a small number of earthquakes show magmatic sig- natures. A close examination of a seismicity swarm on November 22, 1997 in the western south moat of the caldera reveals that the swarm was triggered directly by magmatically derived fluids. Thus, although earthquakes slip in accordance with the regional tectonic stress field, magmatic activity can trigger seismicity by decreasing the effective normal stress across faults.

Rockies Mystery Solved by New Mountain-Creation Theory?
Study "challenges this idea that we understand what's going on," expert says.

1. Scientific Objectives
This proposal is to drill to the crust-mantle boundary in a tectonic window at Atlantis
Bank on the ultraslow-spreading SW Indian Ridge. There are two principle objectives:[/url]
I. Test the hypothesis that the Moho beneath Atlantis Bank is a serpentinization
front.
II. Recover the igneous lower crust and the crust-mantle transition at an average
melt flux for slow and ultraslow-spreading ridges.
From this we seek to understand:
• The igneous stratigraphy of the lower crust
• How much mantle material is incorporated into the lower crust.
• How melt is transported through and emplaced into the lower crust
• How the lower crust shapes the composition of mid-ocean ridge basalt the
most abundant magma on Earth?
• The primary modes of accretion of the lower crust.
• Lateral heterogeneity of the lower crust at magmatic time scales.
• The distribution of strain in the lower crust and shallow mantle in the shallow
lithosphere during asymmetric seafloor spreading.
• The nature of magnetic anomaly transitions in the lower crust.
• The role of the lower crust and shallow mantle in the global carbon cycle.
• Life in the lower crust and hydrated mantle.

All published tomographic models of purported deep plumes are severely flawed, but I
discuss here only Hawaii, which provides the type example for rationalization of a “plume track”
while disregarding both observed tectonic controls of magmatism and failure of geophysical
predictions in plume speculations (Anderson, in press). Pro-plume tomographers Wolfe et al.
(2009) depicted a low-velocity plume rising through much of the upper mantle beneath the
Hawaiian region, and a disconnected narrow plume rising obliquely northwestward toward it
from a depth of 1500 km in the lower mantle. Wolfe et al. modeled only steeply rising
teleseismic S waves to calculate uppermantle structure, and only steeply rising SKS waves to
calculate midmantle structure with rays that came through the liquid core via phase conversions.
The narrow seismometer spread precluded sampling deep mantle beneath the islands with
moderately and gently inclined crossfire, and Wolfe et al. did not utilize any other steeply rising S
and P rays that would have increased coverage, nor did they incorporate any surface waves,
receiver functions, or Vp/Vs derivatives to constrain depths, amplitudes, and characters of
possible anomalies.Wolfe et al. truncated their published model downward at 2000 km, but the
narrow bundle of SKS rays that alone defined their purported lower-mantle plume rose
northwestward through a poorly known lowermost-mantle region of low velocity (likely
recording high iron content and high density, not high temperature), which they acknowledged
could be modeled as their plume — but they claimed a plume to provide the “simplest”
explanation. Wolfe et al. forced their S-wave time delay deep into the upper mantle by assuming
that only moderate retardation occurred within either the crust or a shallow magma-generating
system. Leahy et al. (2010; Wolfe was second author) showed, with receiver-function analysis of
the same seismometer records, that the upper-mantle “plume” of Wolfe et al. (2009) was the
product of downward smearing of the time delay within thickened Hawaiian Swell crust. Leahy et
al. acknowledged previous observational proof that the Hawaiian region lacked plume-predicted
high heat flow.