Structural Controls of the Astor Pass Geothermal Field, northwestern Nevada
Thesis proposal for the degree ofMaster of Science in Geology
Brett Mayhew
Advisor: Dr. James Faulds
Mackay School of Earth Sciences and EngineeringUniversity of Nevada, Reno
April 25, 2012
Introduction
The Great Basin, USA, provides an unusual combination of geologic components that
foster geothermal activity, including an anomalously high geothermal gradient, active faulting,
and regional extension. Although the northern Basin and Range has experienced active
volcanism as recently as 10 to 3 Ma, this volcanism is thought to have little effect on the current
geothermal gradient (Blackwell, 1983; Faulds et al., 2006). Only a small number of systems
(Roosevelt Hot Springs, Coso, Steamboat) on the margins of the Great Basin show evidence of
active magmatism. Thus, the majority of the ~430 known geothermal systems in the Great Basin
are considered amagmatic, relying on elevated crustal heat flow of ~75±5 mWm-2 (Blackwell,
1983; Blackwell and Richards, 2004) and conduits for hydrothermal fluids along fault zones
(Faulds et al., 2006).
Recent studies of these amagmatic systems have shown that certain structural settings act
as more effective conduits. Steeply dipping normal faults that have discrete steps, terminate,
intersect other faults, intermesh with oppositely dipping faults, or reside in transtensional pull-
aparts are the preferred settings (Faulds et al., 2006, 2010, 2011; Vice et al., 2007; Hinz et al.,
2008, 2010, 2011). These fault geometries generate favorable sub-vertical, high fault and fracture
density zones that act as preferential fluid conduits. The fluid flow in these fault zones is further
increased when critically stressed with respect to the ambient stress conditions (Ferrill and
Morris, 2003; Moeck et al., 2009).
Astor Pass is a blind geothermal system that lies close to the California-Nevada border at
the margin of the northern Walker Lane and the western Basin and Range (Figure 1). The system
was identified by Coolbaugh et al. (2006) based on the presence of tufa mound lineaments. Tufa
can be precipitated when calcium rich geothermal fluids contact atmospheric CO2 dissolved in
lake water. The Astor Pass tufa is ~ 6 km northwest and along strike from a large modern
geothermal outflow at Needle Rocks, where boiling water emanates from springs and geothermal
wells drilled in the 1960s.
Purpose of Study
The purpose of this study is to analyze the structural controls of the Astor Pass
geothermal system. Geothermal activity in Astor Pass is evident from the linear tufa mounds and
moderately high temperatures shown from shallow temperature surveys (Kratt et al., 2010) and
deep wells. On the basis of detailed geologic mapping that showed the distribution of major
faults and distinct lineaments of tufa towers, Vice et al. (2007) and Vice (2008) suggested that a
zone of dilation occupied the southwest quadrant of a fault intersection between a northwest-
striking oblique dextral fault and north-northwest striking, west-dipping normal-dextral fault,
southwest of the tufa towers. Deeper wells (approximately 1300m) were drilled in the area of
hypothesized dilation in hopes of finding the upflow of the high temperature geothermal system.
These wells were unsuccessful in intercepting a high temperature upflow zone with peak well
temperatures of 94°C, lower than the estimated max temperature of 134°C from geothermometry
(Coolbaugh, unpublished data). The temperature versus depth plots for the three wells show a
nearly isothermal gradient with increasing depth.
To elucidate the relationship between faults and hydrothermal fluids at Astor Pass, I plan
to construct and analyze a detailed three-dimensional structural model of the faults and
stratigraphy of the study area. The model will incorporate 1) detailed geologic mapping of the
northern Virginia Mountains and Terraced Hills, 2) interpretations of 16 lines of seismic
reflection profiles, 3) detailed analysis of well cuttings, and 4) analysis of borehole stress and
kinematic data. The construction of this 3D model will allow for a detailed understanding of the
geothermal system and the structural controls therein. This will also allow for a more complete
understanding of geothermal areas with similar fault-intersection structural environments and aid
in exploration and development of future sites.
Tectonic Setting
Astor Pass is located in the northwest Great Basin in the western Basin and Range. Astor
Pass lies directly northwest of Pyramid Lake between the Virginia Mountains and Terraced Hills
to the west and east, respectively. Astor Pass lies along the northeastern boundary of the
Pyramid Lake domain of the Walker Lane structural belt (Stewart, 1988; Faulds and Henry,
2008). This places Astor Pass in the “transitional Walker Lane”, a transtensional structural
region involving northwest-striking, left-stepping, right-lateral faults of the Walker Lane and
kinematically linked north to north-northeast-striking Basin and Range normal faults (Figure 2)
(Faulds et. al.,, 2005a, b; Vice 2008). Along this boundary, strain is transferred between dextral
shear in the Walker Lane and west-northwest directed extension in the Basin and Range (Oldow,
1992; Faulds et al., 2005, 2006).
The Walker Lane accounts for ~20% of the strain between the Pacific and North
American plates (Hammond and Thatcher, 2004; Faulds and Henry, 2008; Hammond et al.,
2011). Geodetic studies currently suggest that the Pyramid Lake fault in the northern Walker
Lane accommodates ~1.0 ±0.3 mm/yr of dextral shear, whereas the structural blocks of the Basin
and Range (Pyramid Lake to Big Smokey Valley blocks) just east of Pyramid Lake fault are
extending ~N55W at a rate of 4.6 mm/yr relative to stable North America (Hammond et al.,
2011). Hammond et al. (2011) modeled the Pyramid Lake fault as the border between these two
structural areas, separating the Walker Lane transtensional and Basin and Range extensional
domains. A similar dextral slip rate of 2.6 ± 0.3 mm/year for the Pyramid Lake fault has been
measured by analyzing offset of geomorphic features along the fault trace (Briggs and
Wesnousky, 2004).
Basin and Range extension in this region began ~13-10 Ma, concurrently with a major
east to northeast extensional event that also occurred along much of the Walker Lane and eastern
flank of the Sierra Nevada (Trexler et al., 2000; Henry et al., 2007;). Dextral movement on the
major faults of the northern Walker Lane, including the Pyramid Lake and Warm Springs Valley
faults began ~ 9-3 Ma. (Henry et al., 2007; Faulds and Henry, 2008). Dextral faults in the
northern Walker Lane accommodated approximately 20-30 km of cumulative displacement
during this time (Faulds et al., 2005; Henry et al., 2007; Faulds and Henry, 2008; Hinz et al.,
2009).
Stratigraphy
Basement-
The basement of the northern Virginia Mountains and Astor Pass consists of Mesozoic
metasedimentary rocks and Cretaceous and/or Jurassic granodiorites. The Mesozoic
metasedimentary rocks are similar to those found in the Nightingale and Peavine sequences
(Bonham and Papke, 1969; Henry et al., 2004). These rocks do not crop out in the northern
Virginia Mountains but are identified in well cuttings at ~1200 m depth at Astor Pass and crop
out at Dogskin Mountain to the southwest and the southern Lake Range to the southeast (Henry
et al., 2004; Drakos, 2007).
Tertiary-
The majority of Astor Pass and to a lesser extent the northern Virginia Mountains are
composed of a thick section of middle Miocene volcanic rocks informally known as the Pyramid
sequence (Bonham and Papke, 1969; Faulds and Henry, 2002; Henry et al., 2004; Vice, 2008).
This thick sequence nonconformably overlies the Mesozoic basement. One source of this
sequence appears to be a Miocene shield volcano in the northern Virginia Mountains, possibly
associated with ancestral Cascade arc volcanism (Faulds and Henry, 2002). Locally, the Pyramid
sequence is broken into an upper and lower sequence, separated by the middle Miocene tuff of
Mullen Pass (Vice, 2008). This tuff crops out stratigraphically up-section of Miocene rocks
exposed at Astor Pass to the east in the Terraced Hills and does not appear in the drill cuttings.
Thus, the volcanic and volcaniclastic units found in Astor Pass appear to correlate with the lower
Pyramid sequence. This lower part of the sequence is primarily composed of interbedded basalt
and basaltic andesite flows, volcaniclastic rocks, as well as sparse rhyolitic flows (Faulds et al.,
2007; Henry et al., 2004). The northern Virginia Mountains also contain a large rhyolitic dome,
presumably coeval with the Pyramid sequence. Cuttings from wells drilled at Astor Pass show
that the Pyramid sequence volcanic rocks are approximately 1100 m thick. The mafic lavas
include a variety of textures, including aphyric to finely porphyritic, porphyritic with phenocrysts
up to 9 mm long, and aphanitic flows, some of which are intercalated with volcaniclastic rocks.
The clastic deposits found in the Astor Pass cuttings are matrix supported, poorly to moderately
sorted, light gray conglomerates, breccias, and sandstones. The clasts found in the sedimentary
rocks were derived primarily from the Pyramid sequence volcanic rocks, with the sandstone and
matrix in the conglomerates also consisting of feldspars and lithics sourced from the Pyramid
sequence. These findings are in accord with previous descriptions of the Pyramid sequence
volcanic and sedimentary units in the surrounding area (Faulds et al., 2003, 2007; Henry et al.,
2004; Drakos, 2007; Vice, 2008).
Quaternary-
The majority of Astor Pass and some parts of the northern Virginia Mountains are
overlain by Quaternary deposits, primarily lacustrine deposits, tufa deposits, and young alluvial
fans (Vice, 2008). The tufa in Astor Pass is of particular importance. The tufa mounds that crop
out on the floor of Astor Pass occur in linear trends, suggesting a fault-controlled origin and
recent geothermal activity. These tufa mounds can also grow to over 90 m in height as seen at
the Needle Rocks ~6 km to the southeast of Astor Pass (Coolbaugh et al., 2006).
Structural Framework
Astor Pass occupies a narrow ~0.5 km wide pass between the Terraced Hills to the east
and north and the Virginia Mountains to the southwest. The Terraced Hills are composed of a
series of moderately east-tilted fault blocks composed primarily of the middle Miocene Pyramid
sequence (Vice, 2008). Based on kinematic and seismic reflection data, along with offset
stratigraphy, the north-northwest-striking faults appear to have accommodated both normal and
right-lateral motion, whereas the WNW-striking faults probably accommodated primarily dextral
displacement (Vice, 2008).
Fault blocks in the northern-most Virginia Mountains also have a moderate easterly tilt.
To the south, the Virginia Mountains contain a north-trending extensional anticline with NNW-
striking normal faults (Faulds and Henry, 2002; Faulds et al., 2003; Henry et al., 2004). The
limbs of the anticline dip ~30º, similar to the east-tilted fault blocks in the Terraced Hills.
Preliminary analysis of Astor Pass reveals a similar structural framework as the
surrounding area; seismic reflection data shows multiple east- and west-dipping normal faults
linked with apparent sub-vertical right-lateral or oblique-slip faults. The sub-vertical northwest
striking fault shows oblique motion, whereas the north-striking faults appear to be primarily
normal faults based on fault dip (Vice, 2008). Paleomagnetic data also show that at least part of
the Terraced Hills have been rotated slightly clockwise (~12º) (Vice, 2008).
Objectives
The objective of this project is to develop a conceptual structural model of the Astor Pass
geothermal system by: 1) constructing a detailed 3D structural model, 2) identifying faults and
fault intersections conducive to fluid flow, and 3) continuing to develop the stratigraphic,
kinematic, and stress models of the area.
This study will involve: 1) detailed 1:24,000 geologic mapping in the northern Virginia
mountains, 2) construction of detailed cross sections using the detailed mapping and
interpretations of seismic reflection data, 3) incorporation of these cross sections into a new fault
trace map for Astor Pass, 4) a detailed analysis of well cuttings and borehole stress data, 5)
integration of new and previous mapping, cross sections, petrographic analysis, and borehole
breakout data into a 3D structural model using Earthvision software, and 6) GIS compilation of
an updated geologic map of the Astor Pass geothermal area.
Methods
The methods involved in this study include geologic mapping, interpretation of seismic
reflection data, petrographic analysis, stress data analysis, 3D structural modeling, and GIS
compilations.
Geologic Mapping - Detailed geologic mapping of ~25 km2 of the northern Virginia Mountains
will be conducted at a scale of 1:24,000. This map area will lie primarily within the Astor Pass
7.5 minute topographic quadrangle. Contacts and faults will be mapped in the field on
stereographic air photos and transferred into digital form using Vr software. The Vr software
allows the stereo air photos to be georeferenced and ortho-rectified yielding an accurate 3D
terrain model overlain with geologic data. These data will then be exported to ArcGIS 10.1 to
produce a final cartographic publication. The GIS production will then be added to the ongoing
compilation of a geologic map for the Pyramid Lake area. The geologic map and available
subsurface data (e.g., well cuttings and seismic reflection data) will be used to construct six
detailed cross-sections for both the Astor Pass area and across the entire study area.
Logging of drill holes - Three potential geothermal production wells and several shallow
temperature gradient holes will be logged for lithology and faults using a research-grade
binocular microscope and hand lens. Cuttings of particular importance are those showing
evidence of faults, including gouge and zones of lost circulation during drilling. Lithology from
the logs will then be used to constrain the subsurface geology in the 3D geologic model.
Structural Analysis - Analysis of the geometry and kinematics of faults at Astor Pass and the
Virginia Mountains will be conducted using surface and borehole indicators. Borehole breakout
data will be the primary method of analyzing stress and dilation at Astor Pass. Borehole imaging
was collected during drilling of the APS-2 and APS-3 wells. By using wellbore images,
directions of the maximum and least principle horizontal stresses can be determined using
breakouts (Zoback et al., 2003). Breakouts occur at the azimuth of the minimum horizontal
stress, appearing in an unwrapped image as a dark band of low reflectance on opposing sides of
the well. The width of the breakout (opening angle) and the orientation are then easy to
determine. Tensile fractures may also exist 90° from the breakout showing failure in
compression as well as tension (Zoback et al., 2003). Particular attention will be paid to the
orientation of the least principle stress and if possible, its measured magnitude. The orientation
and magnitudes of the principal stress allows for determination of the slip and dilation tendency
of individual faults or fault segments (e.g., Moeck et al., 2009). The analysis of the images and
calculations of stresses from the breakout data will most likely be conducted by colleagues at
GNS Science in New Zealand, but I will then synthesize this work with the other data sets to
develop a comprehensive stress model for Astor Pass.
Structural features, including bedding attitudes, flow foliations, and compaction
foliations, will all be recorded while mapping where available. Preliminary mapping indicates
that well-exposed fault surfaces are scarce throughout Astor Pass and the Virginia Mountains,
but fault traces and offsets can be mapped accurately and will constrain fault attitudes and
kinematics. Fault kinematic indicators, such as striations, accretionary mineral steps, Riedel
shears, and rough facets will also be measured if available (e.g., Angelier et al., 1985; 1994).
Accretionary mineral steps are a simple way of determining the slip direction on fault surfaces,
with surface crystal growth during fault movement forming steps in the same direction as
motion. Riedel shears also act as an excellent kinematic indicator, forming at narrow acute
angles to the fault plane (15-25o). The acute angle of the Riedel points in the direction of
movement of the host block (Figure 3). If enough fault kinematic data can be obtained, principle
stress directions will be calculated and compared to those from the borehole breakout data using
the methodology described by Marrett and Allmendinger (1990).
Seismic Reflection Interpretations - During the summer of 2010, seismic reflection data were
collected and/or reprocessed in the Astor Pass area. Previous seismic data collected by
ZAPATA-Blackhawk in 2006, 5 lines totaling ~8.4 km, was reprocessed by Optim in 2010. In
addition, 11 new profiles totaling ~40 km in length were collected. All 16 seismic lines were
processed by Optim, including first-arrival velocity optimization and pre-stack depth migration
analysis (Louie et al., 2011).
In an attempt to more clearly image the steeply dipping normal faults in the area,
acquisition of the 11 new seismic lines used long offsets (i.e., larger distances from sources to
receivers). These long offsets focused the survey toward imaging structure and stratigraphy at
predicted reservoir depths of 600-1500 m instead of higher resolution data at shallower depths as
would be the case with shorter offsets. All data collected showed reflections and terminations to
1500 m, as well as velocity control to 900 m. Processing was conducted using Optim’s
proprietary Advanced Seismic Imaging, as well as SeisOpt®. Depth-migrated images were then
interpreted for faults and stratigraphy using OpendTect software. Fault picks were based
primarily on obvious trends in terminations and offsets of sub-horizontal to dipping reflections,
as well as fault-plane reflections (Louie et al., 2011).
Petrographic analysis - Along with the cuttings analysis, hand samples and thin sections will be
described in detail. Hand samples will be analyzed to correlate units across the region. Thin
sections will also be used for correlating units, as well as for selecting samples for
geochronologic dating.
3D modeling – A three-dimensional model will be constructed using a combination of geologic
mapping, seismic reflection interpretations, well cuttings analysis, and the structural analysis of
the borehole breakout data. The Astor Pass model will be focused on an approximately 10 km2
area centered on the tufa mounds. The fault model is built primarily using fault traces from the
seismic interpretation in combination with mapped faults in the model area. The fault model is
also correlated with data from the well cuttings, specifically lost circulation zones and cuttings
showing fault gouge or slickenlines. The stratigraphic horizon model is then built into the
completed fault model, drawing data from well cuttings, geologic mapping, and a cross section
produced by Vice (2008). This completed 3D model may then be used to visualize and detect
structural zones important to a more detailed understanding of the geothermal system. Specific
fault segments and fault intersections and their locations can be studied in detail.
Structural analysis of the fault segments and fault intersections can also be performed by
exporting the modeling data into 3D Stress. The stress measurements calculated from the
borehole breakout data are then applied to the fault segments and intersections, showing
localized areas of slip and dilation tendency.
GIS compilation – Line work from the detailed field mapping will be digitized using Vr
software to display the work in three-dimensional space. The resulting shape files will then be
exported into ArcGIS for further modification of attributes and formatting. Field mapping
datasets will be combined with temperature, magnetic, and other geophysical data from Astor
Pass to complement the existing GIS geologic database for the Pyramid Lake area.
PRELIMINARY SCHEDULE
Semester GoalSpring 2011 Choose project
Begin literature reviewSummer 2011 Field reconnaissance and detailed geologic mapping
Begin logging well cuttingsBegin interpreting seismic reflection profiles
Fall 2011 Complete seismic reflection profilesComplete logging well cuttingsComplete cross sectionsUpdate current geologic map
Spring 2012 Submit program of studyComplete preliminary 3D model
Summer 2012 Complete field workFinish digitizing mapFinish 3D model
Fall 2012 Finalize mapSubmit first thesis draftPresent paper at Geothermal Resources Council MeetingAGU Poster with Drew Siler
Spring 2013 Complete thesisDefend ThesisPresent work at Exxon Global Geoscience meetingPresent work at Nevada Petroleum and Geothermal Society meeting
EXPECTED PRODUCTS
A GIS database that includes a geologic map and well data of the Astor Pass area.
A geologic map of the northernmost Virginia Mountains. This map may be published as
a Nevada Bureau of Mines and Geology open-file report.
A completed 3D structural model of Astor Pass and immediate surrounding area.
Publication of a paper in the Geothermal Resources Council Transactions.
Publication of an article in a peer-reviewed journal.
References Cited
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Figure 1 - Distribution of geothermal systems in the Great Basin. Astor Pass is shown on the boundary of Walker Lane in the Black Rock Desert group of systems (from Faulds et al., 2011).
Figure 2 - Regional map of the greater Pyramid Lake area, showing major northwest-striking dextral faults associated with the Walker Lane and north-striking normal faults of the Basin and Range. Box outlines the current 3D model area. Larger red box shows the entire study area.
Figure 3 - Cross section of a fault showing direction of motion of the missing block relative to preserved surface. Riedel shears (small arrows) point in the direction of movement of the block they are found in (from Angelier et al., 1985).
