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Precise chemical abrasion--thermal ionization mass spectrometry (CA-TIMS) U-Pb zircon ages in combination with detailed field mapping, [sup.40]Ar/[sup.39]Ar thermochronology, and finite difference thermal modeling in the magmatic lobes of the Tuolumne batholith characterize these 10-60 [km.sup.2] bodies as shorter-lived, simpler magmatic systems that represent increments of batholith growth. Lobes provide shorter-term records of internal and external processes that are potentially obliterated in the main body of long-lived, composite batholiths. Zircon ages complemented by thermal modeling indicate that lobe-sized magma chambers were present between ~0.2 and 1 m.y., representing only a small fraction of the total duration of melt presence in the main body. During these shorter intervals, a concentric pattern of normal compositional zoning formed during inward crystallization and widespread zircon recycling in the lobes. Lobes largely evolved as individual magma bodies that did not interact significantly with the main, more complex magma chamber(s). Antecrystic zircons and the range of autocrysts, used to track the extent of interconnected melt, record only a limited range of ages and have contrasting zircon populations to those found in the same units in the main batholith. We consider lobes to either be single batches formed during continuous magma flow or multiple, quickly coalescing pulses that in either case formed separate magma chambers that failed to amalgamate with other compositionally distinct pulses such as those occurring in the central batholith. Zircon age comparisons between all four lobes and the main body imply that growth of the Tuolumne intrusion was not stationary, but that the locus of magmatism shifted both inward and northwestward. doi: 10.1130/B30004.1

Finite strain analysis and thermal modeling of magmatically folded leucocratic dikes in the Mount Stuart Batholith, Washington and the Tuolumne Intrusive Suite, California, yield strain rates in the range of [10.sup.-2] to [10.sup.-13] [s.sup.-1]. Compared to published regional strain rates ([10.sup.-13] to [10.sup.-15] [s.sup.-1]), wall-rock strain rates associated with dike-fed expansion ([10.sup.-7] [s.sup.-1]), and rates resulting from numerical modeling of crystal-plastic creep in aureoles ([10.sup.-11] [s.sup.-1]), our fast rates are several orders of magnitude higher. Field and microstructural observations suggest that multiple material transfer processes, including rigid rotation, ductile flow, cracking, and potentially melt-assisted granular flow operated in the aureoles to accommodate emplacement of these two plutons. Calculated durations of pluton construction and bulk shortening in the aureoles indicate that aureole deformation requires only slow, long-term strain rates of [10.sup.-14] [s.sup.-1]. Thus our local fast strain rates indicate that aureoles may be characterized by pulsating high strain rate surges. We suggest that magmatically folded dikes in these and other pluton aureoles around the world may be used as evidence for last host rock strain rates during pluton emplacement. Keywords: dikes, aureole, strain rates, plutons, emplacement, cooling.

We have constructed a composite image of the fault systems of the M 6.7 San Fernando (1971) and Northridge (1994), California, earthquakes, using industry reflection and oil test well data in the upper few kilometers of the crust, relocated aftershocks in the seismogenic crust, and LARSE II (Los Angeles Region Seismic Experiment, Phase II) reflection data in the middle and lower crust, In this image, the San Fernando fault system appears to consist of a decollement that extends 50 km northward at a dip of ~25[degrees] from near the surface at the Northridge Hills fault, in the northern San Fernando Valley, to the San Andreas fault in the middle to lower crust. It follows a prominent aseismic reflective zone below and northward of the main-shock hypocenter. Interpreted upward splays off this decollement include the Mission Hills and San Gabriel faults and the two main rupture planes of the San Fernando earthquake, which appear to divide the hanging wall into shingleor wedge-like blocks. In contrast, the fault system for the Northridge earthquake appears simple, at least east of the LARSE II transect, consisting of a fault that extends 20 km southward at a dip of ~33[degrees] from ~7 km depth beneath the Santa Susana Mountains, where it abuts the interpreted San Fernando decollement, to ~20 km depth beneath the Santa Monica Mountains. It follows a weak aseismic reflective zone below and southward of the mainshock hypocenter. The middle crustal reflective zone along the interpreted San Fernando decollement appears similar to a reflective zone imaged beneath the San Gabriel Mountains along the LARSE I transect, to the east, in that it appears to connect major reverse or thrust faults in the Los Angeles region to the San Andreas fault. However, it differs in having a moderate versus a gentle dip and in containing no mid-crustal bright reflections. Keywords: crustal structure, tectonics, earthquakes, seismic imaging, southern California.

A region of low seismic-wave speed is detected beneath the central Southern Alps of New Zealand on the basis of traveltime delays for both wide-angle reflections and P-waves from teleseismic events. Respective ray paths for these P-waves are mutually perpendicular, ruling out anisotropy as a cause of the delays. The low-speed region measures about 25 km by 40 km, has a speed reduction of 6%-10%, and is largely above the downward projection of the Alpine fault. The most likely cause of the low-speed zone is high fluid pressure due to excess water being released by prograde and strain-induced metamorphism into the lower crust. Because enhanced fluid pressure reduces the work required for deformation, the existence of the central Southern Alps low-speed zone implies that this part of the Australian-Pacific plate boundary is relatively weak. Keywords: fluids, seismic velocity, seismic reflection, Southern Alps, Alpine fault.

A macroscopic geological structure can geometrically map a local rock material anisotropy into a larger volume that may have different net anisotropic properties on a scale to which seismic waves respond. The bulk structure's anisotropy intensity, symmetry type and orientation of symmetry axes will generally be different from the local rock; a typical crustal rock with material fabric showing slow-axis transverse isotropy can be converted, for example, into a bulk structure that is weaker fast-axis orthorhombic or lower symmetry. We define this modification as "structural geometric anisotropy" (SGA). The seismic anisotropy signals produced by this structure are influenced by the length scale of seismic waves: shorter wavelengths respond to each larger part of the structure (path integration) whereas longer wavelengths respond to just the bulk average of all parts (effective medium). We present a tensor formulation that under certain conditions can decompose an anisotropy-filled structure into its macroscale structural geometry separated from infilling rock types. When a single representative rock material can be substituted for local rocks with fabric, the orientation operators that describe the structure's geometry can be separately volume averaged to produce a unique "structural geometry operator" that can then be used to define the equivalent structure's effective medium. We illustrate these principles using common geometrical structures and show as an example the progressive modification of seismic anisotropy produced by cylindrical folding. Due to the widespread distribution of crustal tectonic structures, their effects on seismic anisotropy should be incorporated into interpretations of seismic anisotropy. The assumption of slow-axis transverse isotropy in crustal volumes is not always valid. [ABSTRACT FROM AUTHOR]