Chapter 13: Styles of extension in the Nevada Test Site region, southern Walker Lane Belt; An integration of volcano-tectonic and detachment fault models

Abstract

Detailed geological information, supported by geophysical data and drill holes, indicates that several deformational styles characterize the Walker Lane Belt of the south-central Great Basin and Nevada Test Site region.The region is split by a north-trending major volcano-tectonic rift that includes several large calderas and is filled with more than 4 km of Miocene volcanic rocks. The rift lies in a large right-step in the Walker Lane Belt and separates areas to the west containing recognized detachment-style faults and metamorphic rocks from adjoining terrain on the east that contains no recognized major detachment faults or exposures of metamorphosed rocks.The volcano-tectonic trough was most active from about 16 to 10 Ma. The style and timing of Cenozoic faulting is different east and west of the rift, and well-dated volcanic sequences show that the principal faulting events occurred at different times and rates within adjacent structural domains. This variability suggests that no single tectonic process produced the Cenozoic structure of the region. Normal faulting east of the rift may have been controlled in part by extension accommodated by Mesozoic thrust faults.It is proposed that the volcanic rift represents a pull-apart at a right-step in the Walker Lane Belt and that the rift was the headwall or breakaway zone for detachment faulting to the west. In this model, extensional faults of Miocene age within and immediately east of the rift are largely gravitational responses to magmatism, rifting, and volcano-tectonic collapse.

P wave velocity structure in the Yucca Mountain, Nevada, region

“…[20] Although slower velocities tend to occur within the rift structure proposed by Carr [1990] or Wright [1989] at depths shallower than $3 km, the deeper velocity structure in general does not show a clear correlation with the rift. In regions such as the Funeral Mountains, where detachment faulting has been documented geologically [Scott, 1990], we do not image any clear evidence for detachment faults.…”

“…Because of this limitation, we cannot refute the detachment model; however, we may be able to provide indirect evidence if we image subhorizontal offsets across vertical structures. [7] Carr [1990] spatially associated Miocene deformation, caldera complexes, and Quaternary volcanic centers with what he interpreted to be the north-south oriented Kawich-Greenwater ''rift,'' bounded on the west by Bare Mountain and in eastern Amargosa Valley and in Jackass Flat by a gravity (Figure 4), aeromagnetic ( Figure 5), and seismogenically defined structure. The rift is envisioned as a north-south striking zone of crustal and/or upper mantle weakness acting as a focus of regional Miocene extension.…”

“…Hypocentral parameters are treated as unknown and are resolved in the Carr [1990] and reinterpreted here with respect to local seismicity and gravity gradients (roughly the 125 mGal gravity contour). The interpreted eastern boundary of the gravity trough is constrained by the continuity of the gravity gradient defined by Quaternary faults in eastern Amargosa Valley, the ''gravity fault'' in Jackass Flat, and the western physiographic extent of the Timber Mountain -Oasis Valley Caldera complex.…”

“…Death Valley, about 50 km west of YM, has some elevations just below sea [5] Tectonic models for the YM-Crater Flat structures have invoked low-angle, normal-fault mechanisms [Scott, 1990], a local caldera within a regional rift system [Carr, 1990], a broader half graben structure [Fridrich, 1999], and dextral shear on an approximately N-S striking fault system through Crater Flat [Schweickert and Lahren, 1997]. As a prelude to presenting our tomographic work, we summarize each of these models, indicate primary observations that support each, and surmise what structures would be present to evaluate each with respect to the tomographic model.…”

“…Despite advances in understanding many aspects of the mountain's geology, a consensus on a nearregional tectonic model remains elusive and several important questions still remain unanswered. Several tectonic models have been proposed (e.g., detachment mechanisms [Scott, 1990], a regional rift system [Carr, 1990], a half graben structure [Fridrich, 1999], and dextral shear on a local fault system [Schweickert and Lahren, 1997]), but mostly because of the lack of upper crustal information, little hard evidence is available to test these models. Also, these models lack a clear physical explanation for several important questions which include the following: Why is YM topographically elevated?…”