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. A challenge 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 130-140oC 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 PFA working 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 Or- egon. 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 785 Wannamaker, et al. 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, isoto- pic 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 seg- 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 ment demonstrate that extensional basaltic magma after Hildreth (2007), Stewart and Carlson (1978), Blakely et al. (2011). Contours of recharge increasingly dominates their evolution, rhyolitic volcanism in southeast Oregon approaching the central Cascades are red while lack of thinned crust under the arc is a sign dashes (Hildreth, 2007). Green bands enclose terranes Siletzia, Klamath and Blue that extension and ductile flow balance magmatic 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. Vol- underplating (Schmidt et al., 2011). canic chain segment boundaries after Schmidt et al. (2008). Plate contours are in The pursuit of blind geothermal sys- blue from McCrory et al. (2012). DEM base layer is from GeoMapApp utility. Right tems necessitates being able to “see” into the (b): Enlargement shows faulting in more detail, plus Newberry (NB), Mt Mazama third dimension: depth. Magnetotelluric (MT) (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 surveying in the Great Basin has revealed that inverted green triangles. 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 imag- ing (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 786 Wannamaker, et al. 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