GRC Transactions, Vol. 39, 2015

Play Fairway Analysis of the Central Cascades Arc-Backarc Regime,
Oregon: Preliminary Indications

Philip E. Wannamaker¹, Andrew J. Meigs², B. Mack Kennedy³, Joseph N. Moore¹,
Eric L. Sonnenthal³, Virginie Maris¹, and John D. Trimble²

¹University of Utah/EGI, Salt Lake City UT
²Oregon State University, College of Earth, Ocean and Atmospheric Sciences, Corvallis OR
³Lawrence Berkeley National Laboratory, Center for Isotope Geochemistry, Berkeley CA

[email protected]

Keywords

Play Fairway Analysis, geothermal exploration, Cascades, andesitic volcanism, rift volcanism, magnetotellurics, LiDAR, geothermometry

ABSTRACT

We are assessing the geothermal potential including possible blind systems of the Central Cascades arc-backarc

regime of central Oregon through a Play Fairway Analysis (PFA) of existing geoscientific data. A PFA working model is adopted where MT low resistivity upwellings suggesting geothermal fluids may coincide with dilatent geological structural settings and observed thermal fluids with deep high-temperature contributions.Achallenge in the Central Cascades region

is to make useful Play assessments in the face of sparse data coverage. Magnetotelluric (MT) data from the relatively dense EMSLAB transect combined with regional Earthscope stations have undergone 3D inversion using a new edge

finite element formulation. Inversion shows that low resistivity upwellings are associated with known geothermal areas

Breitenbush and Kahneeta Hot Springs in the Mount Jefferson area, as well as others with no surface manifestations. At Earthscope sampling scales, several low-resistivity lineaments in the deep crust project from the east to the Cascades, most prominently perhaps beneath Three Sisters. Structural geology analysis facilitated by growing LiDAR coverage is

revealing numerous new faults confirming that seemingly regional NW-SE fault trends intersect N-S, Cascades graben-

related faults in areas of known hot springs including Breitenbush. Major element geochemical modeling of high-chloride thermal springs west of Three Sisters using Geo-T analytical software implies subsurface equilibration temperatures in

the 130-140^oC range. Subsurface temperatures and deep source contributions will be refined using ToughReact reactive

transport analysis including isotopic data. To date, results appear consistent with our PFAworking model described above.

Introduction

Play Fairway Analysis (PFA) in the geothermal context combines regional geological/geophysical understanding with knowledge of prospect control elements (e.g., origin of heat, source of fluids, pathways to heat up and concentrate fluids, accessible high permeability, and a sealing caprock). Its goal is to produce an inventory of prospect leads that

represent collocations of relatively high probabilities of elements (see Fraser, 2010, for an oil and gas analog). A play fairway that contains high-enthalpy systems ideally should reside in an extensional tectonic environment, at either region or local scales, to promote permeability. The potential for new discoveries may be increased dramatically in regions where

active arc magmatism due to andesitic volcanism and subduction fluid fluxing occurs together with basaltic or bimodal

extensional volcanism within the fairway.
Thus we have been drawn to examine the Central Cascadia arc segment and its near backarc area in central Oregon. Here, active Basin and Range extension with bimodal volcanism is superimposed upon and contemporaneous with an active subduction arc (Keach et al., 1989; Schmidt et al., 2008, 2011; Wannamaker et al., 2014) (Figure 1), foster-

ing both high heat flow and prodigious thermal spring activity (Ingebritson and Mariner, 2010). Regional hydrological

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models of the High Cascades have emphasized topographically driven flow where fluids that

encounter low permeability at shallow levels emerge as cold springs while those successfully penetrating to deeper levels may be substantially warmed (Ingebritsen et al., 1992; Jefferson et

al., 2006). A significant hydrological element

controlling spring emergence is the boundary between relatively permeable High Cascades rocks and the less permeable Western Cascades rocks (Jefferson et al., 2006). However, while

this outflow-dominated model explains major

physical characteristics of the hydrology, isotopic data particularly elevated ³He argue for deep permeability and magmatic contributions to the

fluids (van Soest et al., 2002; Evans et al., 2004).

Volcanic products in the Central Cascades segment demonstrate that extensional basaltic magma recharge increasingly dominates their evolution, while lack of thinned crust under the arc is a sign

that extension and ductile flow balance magmatic

underplating (Schmidt et al., 2011).

Figure 1. Location maps showing Play Fairway Analysis area (cyan insets). Left (a): GPS geodetic motion estimate arrows are yellow after McCaffrey et al. (2013), faults after Hildreth (2007), Stewart and Carlson (1978), Blakely et al. (2011). Contours of rhyolitic volcanism in southeast Oregon approaching the central Cascades are red dashes (Hildreth, 2007). Green bands enclose terranes Siletzia, Klamath and Blue Mtns (Schmidt et al., 2008; Humphreys, 2008). Olympic Mtns is Oly. Pink bands enclose active terranes Basin&Range and Modoc Plateau. Mt Jefferson is JF. Volcanic chain segment boundaries after Schmidt et al. (2008). Plate contours are in blue from McCrory et al. (2012). DEM base layer is from GeoMapApp utility. Right (b): Enlargement shows faulting in more detail, plus Newberry (NB), Mt Mazama (MZ) and Medicine Lake (ML) volcanic centers. The EMSLAB MT transect is yellow circles and blue squares, and Earthscope MT transportable array (TA) stations are inverted green triangles.

The pursuit of blind geothermal sys-

tems necessitates being able to “see” into the

third dimension: depth. Magnetotelluric (MT) surveying in the Great Basin has revealed that high-temperature geothermal systems appear to be underlain by steep low-resistivity (conduc-

tive) structures interpreted to be fluidized fault zones that typically are connected to deep crustal low-resistivity bodies representing magmatic underplating and fluid release (Wannamaker et al., 2007, 2008, 2011; Siler et al., 2014). Hence, for PFA application, existing MT data deserve reanalysis with modern methods to seek such fluidized conductive fault zones

whose surface evidence at present may be obscure. Geothermal systems of course also are expected to reside in favorably

dilatent structures to create permeability, and provide a pathway connecting heat and fluid sources to a reservoir (e.g.,

Faulds et al., 2013). Furthermore, the high-temperature systems examined with 3D MT in the Great Basin (Dixie Valley,

McGinness Hills) show evidence through soil gas or spring/well fluid chemistry of magmatic or high-grade metamorphic volatile components including elevated ³He (as R/Ra) (Wannamaker et al., 2013a,b; Siler et al., 2014). Thus, the confluence

of favorability of these three lines of evidence may be taken to imply that an area deserves further exploration assessment.

We propose that a similar confluence of indicators may exist for geothermal resources in Central Cascadia. This will be our approach for assessing presence of deep permeability and upflow in the face of obscure surface evidence. Re-

analysis of existing data in the region is intended to verify and calibrate a PFA working model in that regard. A challenge in the Central Cascades region is to make useful Play recommendations in the face of sparse data coverage. The PFA

approach will define common risk segments (CRS) (Fraser, 2010) for the elements of source, fluid pathways, reservoir

volumes, and seal using the three primary geoscience data sets. Where the downside risk of any of the elements is deemed

confidently to be high that geographic area is considered to be of poor prospectivity. Only where all elements in an area are considered to be of low risk can that area be considered firmly prospective. However, confidence in assigning risk to a sub-region may be challenging in the face of sparse data such that prospectivity can be neither confirmed nor overruled without obtaining new geoscientific observations.

Analysis of Available Magnetotelluric Data

High quality MT data in the public domain are relatively limited in this project area. They consist of the E-W oriented
EMSLAB transect initially acquired to understand the Juan de Fuca subduction system at this latitude (Wannamaker et al., 2014) plus regional Earthscope Transportable Array (TA) sites at ~70 km spacing for orogenic scale resistivity imaging (Meqbel et al., 2014). These data were combined for a total of 60 sites over the period range of 0.11 through 2560 s for 3D inversion, and all twelve data types (four complex impedance elements and two complex tipper elements) were included. Inversion was performed with a new algorithm developed in-house under DOE support that implements the

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Figure 2. Central section of finite element model inversion mesh for the Central Cascades publically available MT data set. Edge of Pacific Ocean is dark blue at western edge. Topography is represented through gradual distortion of the hexahedral elements and color coded for elevation. Convergence plot is given to lower left, and model parameters and run times in text box to lower right.

edge finite element method solving for the vector electric (E) field (Kordy et al., 2015a,b). The solution of all system

matrices is direct (non iterative), making use of the Intel

MKL matrix packages PARDISO for the sparse finite

element system and PLASMA for the full parameter step matrix. The latter matrix is formulated in data space for

compactness. The solutions are parallelized on a single-

box, 24-core Linux workstation with 0.5 TB RAM. The

central portion of the finite element mesh, and the inversion

parameters and performance are given in Figure 2.
Plan views through the 3D resistivity model appear in Figure 3.At the shallowest levels one sees patchy low-resistivity areas associated with Willamette Valley sediments and possible low-grade alteration of vol-

canic clastic and flow units.As depth exceeds ~2 km, a “butterfly” pattern

of four conductors is seen centered on Mount Jefferson. The northwesterly one of these underlies Breitenbush

Hot Springs, which may reflect pre-

viously inferred interaction between deep heat sources and upper crustal groundwater circulation (Ingebritsen et al., 1992). The other three have no reported surface features. A further prominent conductor ~50 km ENE of Mount Jefferson is nearly coincident with Kahneeta Hot Springs. Away from the relatively dense EMSLAB

profile, only broad deep crustal features are confidently resolvable from

the Earthscope stations. However, below ~10 km, a series of pronounced E-W oriented conductors underlying the northwestern Basin and Range and terminating under the Cascades volcanic arc become clear. Perhaps the strongest of these ends beneath the Three Sisters volcanic complex.

Figure 3. MT electrical resistivity plan views derived from EMSLAB and Earthscope TA data inverted as described in the text. Volcanic physiological features include Mount Hood (MH), Mount Jefferson (MJ), Three Sisters (TS), Newberry Volcano (NV), Crater Lake (CL), Mount McLoughlin (MMc), Medicine Lake (ML) and Mount Shasta (MS).

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These are essentially the same features resolved in the independent Earthscope inversion of Meqbel et al. (2014) using a different algorithm and not including the EMSLAB data. At these station spacings, there is some question as to whether this E-W conductive streaking is a manifestation of regional anisotropy with E-W being the more conductive orientation (op. cit.). Finer data sampling would resolve that issue. Nevertheless, the correspondence between the conductive axes

and the volcanic centers is tantalizing.

Structural Geology Analysis

Geothermal systems are inherently 3D and occupy reservoirs created through brittle deformation. Recognized

structural settings favorable to dilatency for geothermal reservoirs of the Great Basin include relay ramps, horse-tailing

terminations, accommodation zones, and fault intersections (e.g., Faulds et al., 2013). We expect these structural styles to be candidates for geothermal systems in the Central Cascades region, but identification of faults in the Cascades has traditionally has been difficult due to forest cover. High-resolution ‘LiDAR’, remotely-sensed images of the Earth’s surface, allow for creation of topography with vertical resolution of centimeters (Zielke et al., 2012; Madden et al., 2013; Meigs,

2013). Faults and volcanic features can be imaged in remarkable detail even where obscured by dense vegetation. LiDAR coverage of the region has greatly increased the ability to identify faults, which allows for a commensurate increase in the number of candidate settings favorable for geothermal systems. To identify and assess these sites, a database of digital topographic, geologic, hot and cold spring locations, and fault locations has been assembled using USGS, Oregon DOGAMI and other sources.
Figure 4 shows progress in understanding the structural setting of the Central Cascades by our group (Trimble,

2015). Advancements in understanding the structural setting of the central Cascades are exemplified by new fault identifi- cation and setting integration in the Mt Jefferson – Three Sisters corridor. Numerous new faults south of Mount Jefferson are evident in the 1 m scale definition LiDAR data that have not been visible in older 10 m and coarser-scale DEM data.

Integration with recent fault databases shows families of N-S, NW-SE and NE-SW trending faults which raise the potential of fault trend intersections and dilatency favorable to geothermal systems. In particular, there is the possibility that NW-SE trending faults in the Breitenbush-Austin Hot Springs areas link through the N-S trending High Cascades

graben to the Sisters fault zone

to the southeast. The location of this potential cross-arc

fault zone roughly coincides

with the step-like northward decrease in hydrothermal heat discharge observed by Ingebritsen and Mariner (2010). The latter appear to control the eruption of young cinder

cones and lava flow. However,

thermal springs directly to the west of Three Sisters (e.g., Belknap) appear to lie along predominantly N-S trending faults of the western boundary of the High Cascades graben that cross-cuts older faults of variable orientation.

Figure 4. Left: compilation of faults from recent databases for the Mount Jefferson – Three Sisters corridor together with known thermal springs. Right: New young faults (red) recognized from LiDAR data analysis in the Cascades graben just south of Mount Jefferson.

Fluid Geochemistry

Important geothermal resource questions that geochemistry can address include reservoir fluid equilibration tempera-

tures, origin depths of spring waters, and proportions of magmatic or deep metamorphic input. To this end we have compiled

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three geochemical databases for modeling subsurface conditions and processes. One contains He isotope data for all thermal and non-thermal springs sampled to date throughout the Cascades, most of which reside within the Play area. A second contains major, minor and trace chemistry, free and dissolved gases, water isotopes, isotopes of dissolved species and gas isotope compositions acquired from a USGS compilation collected prior to 2000. The third database contains chemistry and isotope data for the thermal and non-

thermal springs analyzed post

2000, with an emphasis on the Three Sisters hydrologic system (Figure 5) (Evans et al., 2004).

Figure 5. Thermal and cold spring sample coverage in the Three Sisters area, Central Cascades or Oregon. Data obtained from Evans et al. (2004).

There are two lines of attack in the interpretation. First, there will be a focus on the Three Sisters complex where

most of the geochemical and isotopic data are concentrated. Second, there will be focus on the Mount Jefferson/Breitenbush area because it represents a confluence of good structural geology knowledge, prior modeling of hydrothermal discharge

and subsurface temperatures (Ingebritsen et al., 1992), and the most detailed MT coverage to address the goal of verifying our working PFA model described previously. The data will be interpreted using new state-of-the-art software Geo-T and

ToughReact (Xu et al., 2011; Spycher et al., 2014), the latter now parallelized for computation. Geo-T uses thermodynamic

data for multiple mineral-water pairs to determine water-rock equilibration temperatures, and can incorporate (and correct

for) the impact on fluid chem-

istry related to dilution, boiling, and gas loss. Currently, we are developing a 2-D reactive transport model using ToughReact coupled to Geo-T for the Three Sisters hydrothermal system. The model will be used to assess

fluid flow pathways, fluid-rock interactions and fluid flow rates

through the system (e.g. Wanner et al., 2014).
An example interpretation using Geo-T is given in Figure 6. High chloride and alkalinity springs tend to lie to the west of the Three Sisters summit area whereas lower chloride cold springs occur in the summit area. For the Belknap and Terwilliger (Cougar) hot springs, mineral reaction pairs point to subsurface equilibration temperatures of 130-140^oC. This is the temperature of last equilibration whereas other temperatures may have been experienced previously in the

Figure 6. Example calculations using Geo-T of subsurface equilibrium temperatures at two hot spring areas in the High Cascades graben bounding fault zone to the west of Three Sisters.

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fluid path history. We plan to assess fluid condition history using ToughReact modeling including isotopes (e.g., Wanner

3

et al., 2014). The Cl-rich thermal springs exhibit moderately high He anomalies (R/Ra of 4-5) while the low-Cl cold

3

springs have higher R/Ra (7-9) (van Soest et al., 2002; Evans et al., 2004). Both the Cl and He behavior may reflect the possibility that the two fluid types have traveled through different rock compositions including possible older buried

marine sediments for the former (van Soest et al., 2004).

Preliminary Synthesis and Conclusions

Ideally, we would identify one or more locales in the Central Cascades region where there is a confluence of upwelling low MT resistivity, geological structures pointing to dilatency and permeability, and fluid geochemistry indicative

of prospective high temperatures at depth. At the moment, the Breitenbush area appears the most promising from a data coverage standpoint although interpretation continues. Given that the MT data only have reasonable sampling at the latitude of Mount Jefferson, we will be using this area in an attempt to perform a calibration of our Play model having greater reliance upon structure and geochemistry with the intent to translate it to other parts of the project area. However, interesting MT geophysical indications exist in the area of Kahneeta hot springs also. At larger scales, the Three Sisters

area is of interest due to the presence of cross structural trends, high R/Ra values, and deep conductive geophysical lineaments. The geophysical data of the project as a whole are recognized to be sparse; one of the main project purposes will be to define new data needs as required to achieve a successful analysis of the future geothermal potential of the Central

Cascades arc-backarc environment.

Acknowledgements

This research was supported under U.S. Dept of Energy contract DE-EE0006727 to Wannamaker under the Play

Fairway Analysis funding opportunity. Development of the edge finite element MT inversion code was supported by U.S.

Dept of Energy contract DE-EE0002750 to Wannamaker. The LBNL portion of this work was supported by the Assistant

Secretary for Energy Efficiency and Renewable Energy, Geothermal Technologies Program of the U.S. Department of

Energy under Contract No. DE-AC02-05CH11231.

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