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https://doi.org/10.1130/G45394.1

Manuscript received 17 July 2018 Revised manuscript received 11 October 2018 Manuscript accepted 14 October 2018

© 2018 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. Published online XX Month 2018

Penetration depth of meteoric water in orogenic geothermal systems Larryn W. Diamond*, Christoph Wanner, and H. Niklaus Waber Institute of Geological Sciences, University of Bern, Baltzerstrasse 3, CH-3012 Bern, Switzerland

ABSTRACT contrast, other workers (e.g., Raimondo et al., Warm springs emanating from deep-reaching faults in orogenic belts with high topography 2013) have argued that the isotopic signatures and orographic precipitation attest to circulation of meteoric water through crystalline bed- are inherited from pre-orogenic, near-surface rock. The depth to which this circulation occurs is unclear, yet it is important for the cooling alteration of the rocks and that metamorphic history of exhuming orogens, for the exploitation potential of orogenic geothermal systems, dehydration upon deep burial has liberated the and for the seismicity of regional faults. The orogenic geothermal system at Grimsel Pass, isotopically depleted water. Hence, it is cur- Swiss Alps, is manifested by warm springs with a clear isotopic fingerprint of high-altitude rently unclear how deep meteoric water actually meteoric recharge. Their water composition and their occurrence within a 3 Ma fossil upflow penetrates in such systems. zone render them particularly favorable for estimating the temperature along the deep flow As an alternative approach to this ques- path via geochemical modeling. Because the background geotherm has remained stable tion, we performed geochemical modeling on at 25 °C/km and other heat sources are unavailable, the penetration depth can be derived chemically and isotopically well-characterized from the deep-water temperature. We thus estimated the base of the Grimsel system to be thermal waters currently discharging from the at 230–250 °C and 9–10 km depth. We propose that deep temperatures in such systems, par- Grimsel Pass orogenic geothermal system in the ticularly those with normal background geotherms (<30 °C/km), have been systematically Swiss Alps. This approach bypasses many of underestimated. Consequently, far more enthalpy may be accessible for geothermal energy the assumptions inherent in the petrologic stud- exploitation around the upflow zones than previously thought. Further, the prevalence of ies and provides robust evidence that meteoric recent earthquake foci at ≤10 km beneath Grimsel suggests that meteoric water is involved in water has penetrated at least 9–10 km deep into the seismicity of the host faults. Our results therefore call for reappraisal of the heat budget the continental crust to attain temperatures of and the role of meteoric water in seismogenesis in uplifting orogens. 230–250 °C.

INTRODUCTION water penetration defines the maximum tem- SITE DESCRIPTION Meteoric water circulation in orogenic belts perature attainable by surface springs and their The Grimsel Pass geothermal system (Fig. 1) is expressed by thermal springs discharging at upflow zones, thereby setting limits on their is manifested by several low-flow (<10 L/min) temperatures up to 80 °C from deep-reaching potential for geothermal energy exploitation. warm springs with temperatures up to 28 °C faults, e.g., in the Canadian Rocky Mountains The δ18O and δ2H signatures of minerals and (Pfeifer et al., 1992; Waber et al., 2017) and by (Grasby and Hutcheon, 2001), the Southern Alps fluid inclusions in metamorphic and hydrother- two fossil mineralized breccias, each of which is of New Zealand (Menzies et al., 2014), and the mal rocks in orogenic belts occasionally indi- ~0.25 km2. The springs discharge through one of European Alps (Sonney and Vuataz, 2009). The cate equilibration with isotopically depleted the breccias into a north-south pipeline tunnel that hydraulic gradients that drive circulation arise water that can only have had a meteoric origin. lies 250 m beneath Grimsel Pass (Figs. 1B and from the conjunction of high orographic pre- Petro­logic calculations and structural argu- 1C). At 1910 m above sea level (a.s.l.), they are cipitation, mountainous topography, and per- ments are typically used to constrain the pen- the highest known warm springs in the entire Alps. meable faults that link topographic highs with etration depths, leading to estimates of 5–23 km The mineralized breccias are subvertical, valley floors via the hot bedrock. Knowledge of (for references, see Table DR1 in GSA Data pipe-like bodies that overprint and link a set of the maximum possible depth to which meteoric Repository1). If these depths are accurate, then older, subvertical east-northeast–trending duc- water can infiltrate bedrocks is fundamental to meteoric water would attain temperatures up tile shear zones (Fig. 1C). These shear zones are our understanding of how uplifting orogens cool, to ~400 °C. The studies that report the great- major strike-slip structures in the central Alps, of how groundwater chemistry evolves in oro- est depths explicitly invoke penetration into the with lengths of several tens of kilometers and genic belts, and of how water catalyzes deforma- ductile deformation regime, for which active seismically indicated vertical extents of >20 km tion and seismicity in active orogens. In addition, processes such as fault-valve behavior would (Belgrano et al., 2016; Herwegh et al., 2017). since the bedrock geotherm supplies the heat be necessary, since hydraulic gradients can- The brecciated faults are hosted by monoto- to the circulating water, the maximum depth of not drive flow through disconnected paths. In nous granitoids and minor quartzofeldspathic

*E-mail: diamond@​geo​.unibe.ch 1GSA Data Repository item 2018412, chemical analyses of springs and details of analytical and computational methods, is available online at http://​www.geosociety​ ​ .org​/datarepository​/2018/, or on request from [email protected] CITATION: Diamond, L.W., Wanner, C., and Waber, H.N., 2018, Penetration depth of meteoric water in orogenic geothermal systems: Geology, v. 46, p. 1–4, https://​doi​.org​/10​.1130​/G45394.1

Geological Society of America | GEOLOGY | Volume 46 | Number 12 | www.gsapubs.org 1 Downloaded from https://pubs.geoscienceworld.org/gsa/geology/article-pdf/doi/10.1130/G45394.1/4543509/g45394.pdf by guest on 13 November 2018 TABLE 1. COMPOSITION OF Switzerland combined with the current rock uplift rate of A Regional strike- REPRESENTATIVE SPRINGS slip shear zone ~0.9 km/m.y. (Hofmann et al., 2004) yield a Zurich Spring type Warm Cold Thermal* Bern Lake mean recent denudation rate of ~0.75 km/m.y., end Grimsel implying the breccias formed at ~2.5 km depth Sample S10S25 Pass Cold/warm spring member S10 Water sample at 3.3 Ma. Primary fluid inclusions in hydro- Year sampled 2016 2016 thermal quartz homogenize at ≤152 °C and Discharge (L/min)3.0 n.m. n.m. Grimsel Pass B s show that the hydrothermal minerals precipi- Temperature (°C)283.0 57 46°33′45″N Granitoid tated from single-phase water containing 3000 Eh (mV) –125 330–125 Springs pH (–)8.867.788.59 Gneisses Tunnel mg/L total dissolved solids (TDS; Hofmann in tunnel Na+ (mg/L) 79.7 0.453172.7 46°33′20″N Hydrothedrothermals et al., 2004). Intersection of the inclusion iso- K+ (mg/L)6.6 0.67 13.6 breccia N 1 km 8°18′E 8°19′E chores with hydrostatic pressure at ~2.5 km Mg2+ (mg/L) 0.16 0.201<0.02 NSdepth yields trapping temperatures of 165 ± Ca2+ (mg/L) 3.94.153.6 C Hydrothermal + breccia 5 °C. Stable isotope analyses of the hydrother- Li (mg/L)0.2 <0.001 0.4 2200 m TIC (mg/L) 14.1 3.326.7 Granitoids mal minerals revealed the breccias were miner- SO 2– (mg/L)85.81.32185.0 2000 m S11S12 Gneisses 4 alized by chemically modified meteoric water F– (mg/L) 8.60.2118.5 Tunnel S3,3A S25 S10 S20 2 18 Cl– (mg/L) 13.8 0.09 29.9 1800 m 8.0 km 8.5 km 9.0 km 9.5 km (δ H of –111‰ to –137‰ and δ O of –7.5‰ Si (mg/L) 35.3 1. 674.9 Oberaarhorn to –11‰ relative to Vienna standard mean ocean D 3630 m TDS (mg/L)248 12 525 water [VSMOW]; Hofmann et al., 2004). 18 δ OSMOW (‰)–14.7 –14.2–15.3 Meteor Discharge at The close spatial and temporal relationships δ2H (‰)–105.8–100.7–111.8 recha i 3 rgec Grimsel Pass between the warm springs and the breccias allow H (TU) 2.67.5 0.0 Faul 2164 m S.I. (–)–0.1–1. 87 0.15 Hydro us to use the mineralogy of the breccia cements as calcite plant therma e b Cold-water fraction (vol%)*54100 0 reccia l s a window into the current state of the geothermal system at ~2.5 km depth. In the absence of any Note: Calculations are described in the Data Reposi- 25 °C/ km tory (see text footnote 1).TIC—total inorganic carbon; Pliocene–Pleistocene igneous activity in the cen- TDS—total dissolved solids; SMOW—standard mean Figure 1. A: Location of Grimsel Pass, Swiss tral Alps, the only source of heat for the thermal ocean water; S.I.—saturation index. *Corrected for cold-water fraction (Table DR4 in the Alps. B: Geological map with tunnel trace water is its wall rocks, which have maintained a showing spatial coincidence among regional Data Repository [see text footnote 1]). strike-slip faults, 3.3 Ma hydrothermal brec- regional geothermal gradient of 25 °C/km over cias, and currently active warm springs (after the past several million years (Hofmann et al., Belgrano et al., 2016). C: Geological cross sec- 2004). The 165 ± 5 °C formation temperature of tion along tunnel showing sampled springs the breccia at relatively shallow depth thus repre- The warm springs are of reduced (Eh > (labeled Sxx; after Pfeifer et al., 1992). X axis Ag/AgCl sents a thermal anomaly due to prolonged ascent –250 mV) Na–SO –HCO type, carrying 170– is distance from north portal. D: Regional 4 3 view to north-northwest showing topogra- of hot water. It follows that the maximum circula- 280 mg/L TDS, including elevated contents of phy and conceptual present-day flow path of tion depth of the modified meteoric water must be Si, K, Li, and F. In contrast, the cold springs meteoric water along subvertical strike-slip much greater than 2.5 km, where the background are weakly mineralized (<100 mg/L TDS) with fault. Reconstructed deep-water temperature rock temperature is well above 170 °C. oxidized, Ca–HCO character typical of modern of 230–250 C corresponds to 9–10 km pen- 3 ° 3 etration depth (see text). surface waters at Grimsel Pass. The H, Na, and METHODS Cl contents reveal that most of the warm waters Cold and warm springs in the pipeline tunnel have mixed with 50–70 vol% modern surface gneisses of the Aar Massif, an ~20-km-thick were sampled in March 2014, September 2015, water prior to discharging in the 250-m-deep block of Variscan lower continental crust that and July 2016 (Fig. 1C) and analyzed for their tunnel. The remainder of this paper deals with was overprinted by greenschist-facies metamor- chemical composition, δ2H and δ18O values, and the thermal end-member waters that have been phism and uplifted during the Alpine orogeny 3H (bomb-tritium) activity (see the Data Reposi- corrected for this mixing (Table 1; Table DR4), (Herwegh et al., 2017). Hence, the upflow path tory for data and methods). The analyses of the referred to hereafter as “thermal waters.” of the thermal waters is dominated to great warm springs were then corrected for mixing The δ2H and δ18O values of the thermal depths by microcline K-feldspar, completely with modern surface waters (exemplified by waters fall on the local meteoric water line albitized plagioclase, quartz, biotite, muscovite, the cold springs) based on their 3H, Na, and and are depleted compared to the cold springs and accessory chlorite, epidote, calcite, fluorite, Cl contents (explained in the Data Repository). (Fig. 2). Evidently, surface water has been and apatite. Slivers of anhydrite-bearing Trias- The chemical evolution of the upwelling ther- recharging the geothermal system from an infil- sic dolomite may also be present along the flow mal end-member water was simulated numeri- tration site at a higher altitude than Grimsel path, as indicated by the high sulfate concentra- cally using the reactive-transport software Pass, and possibly during a cooler climate in the tion in the thermal waters. TOUGHREACT V3 (Xu et al., 2014; see the past. The local topography rises continuously to The granitic clasts of the fault breccias are Data Repository for details). >1000 m above the discharge site ~12–14 km hydrothermally altered and cemented by an epi- to the west along the strike of the water-con- thermal assemblage of quartz, microcline, and RESULTS AND DISCUSSION ducting faults in the Oberaarhorn area (Fig. 1D). sulfides (mainly pyrite), locally overgrown by Thus, assuming recharge in this western region, small amounts of fibrous chalcedony, illite, and Water Analyses and Fluid Flow Path the topographic head differential provides the celadonite (Hofmann et al., 2004). The hydro- Example analyses of warm and cold springs hydraulic force to drive isotopically depleted thermal microcline has been dated at 3.3 ± are given in Table 1, and futher analyses (loca- meteoric water deep into the fault zone, where 0.06 Ma (Hofmann et al., 2004), demonstrating tions in Fig. 1C) are given in Data Repository it is heated and acquires solutes via reaction that the geothermal system has been active at Tables DR2 and DR3, including analyses from with the wall rocks and then ascends through the same site, albeit perhaps intermittently, for Pfeifer et al. (1992). These demonstrate that the breccia pipes, cooling along the way and millions of years. Fission-track and U-Th/He the properties of the springs have remained finally discharging as mineralized thermal ages in the distal wall rocks (Egli et al., 2018) remarkably stable over the 25 yr of monitoring. water at Grimsel Pass. Because the thermal

2 www.gsapubs.org | Volume 46 | Number 12 | GEOLOGY | Geological Society of America Downloaded from https://pubs.geoscienceworld.org/gsa/geology/article-pdf/doi/10.1130/G45394.1/4543509/g45394.pdf by guest on 13 November 2018 –60 at a unique temperature, min-T , below which Temperature (°C) Modern waters L eq silicate mineral dissolution is suppressed but 50 100150 200250 –70 Thermal endmember

A

Warm spring precipitation is permitted (over a certain tem- ) l 24 Tqtz b A i Cold spring Global MWL t –80 Grimsel MW perature interval) according to the computed e – saturation indices within the multicomponent, m 22 i –90 c r multiphase chemical system. Calcite, which Observed mean 211 °C o c ‰) Pliocene water l Na/K in springs Min-T i eq n –100 reacts much faster than the silicates, is allowed 20 e H ( in fossil system

e 2 q δ at 3.3 Ma to precipitate and dissolve at all temperatures. u

i –110 l 18 i Eventually, cooling of the water will kinetically b r i u tal aqueous Na/K (molal Silicate diss. halted m –120 inhibit precipitation of the silicate minerals (e.g., 230 °C Simmons and Browne, 2000). We suppressed To 16 Silicate pptn. halted –130

silicate precipitation at the temperature at which ) 3 0.4 m T –140 the cooling water departs from equilibrium with B qtz –20–18 –16 –14–12 –10–8–6 quartz, denoted Tqtz. This temperature was iden- 18 0.3 Min-Teq δ OSMOW (‰) tified by matching the solubility of quartz in the 211 °C simulations with the mean concentration of Si(aq) 0.2 Quartz Figure 2. Stable O-H isotope signatures of in the discharging thermal waters (79.8 mg/L; cold and warm springs and reconstructed Muscovite x 10 end-member thermal waters, all showing analogously to classical “quartz thermometry”), 0.1 Microcline x 10 + albite close association with meteoric water lines yielding a mean Tqtz = 165 °C. (MWL). Rectangle denotes isotope signatures 0.0 Under these constraints, min-Teq was found Minerals precipitated (mol/ Calcite of fossil system at 3.3 Ma (Hofmann et al., by iterative forward simulations of the ascend- 2004), consistent with meteoric water origin. ing thermal water, such that the final Na/K 0.8 SMOW—standard mean ocean water. T Min-T ratio in the model water at 20 °C matched that C qtz eq 0.6 211 °C observed in the end-member thermal springs at 14 Quartz waters are C-free, they must have resided for 20 °C. Thus, the mean molal Na/K of 22.7 in 0.4 Microcline

>30 k.y. within the subsurface flow path (Waber the thermal water was obtained when min-Teq Muscovite et al., 2017). was set to 211 °C (Fig. 3A). Quartz and minor 0.2

microcline precipitate upon cooling between Oversat. Calcite Geochemical Modeling 211 °C and 165 °C (Fig. 3B). The precipitation 0.0 Undersat. Given the slow flow rates, the long residence of microcline lowers the activity of aqueous Al, Normalized saturation index Albite –0.2 time of the circulating water, and the relatively such that albite remains undersaturated (Fig. 3C) 50 100150 200250 rapid reaction kinetics of silicate minerals at tem- despite its prograde solubility behavior. In con- Temperature (°C) perature T >> 170 °C (Giggenbach, 1988), it is trast, calcite in the wall rocks dissolves along the Figure 3. Numerical simulation of chemi- expected that the thermal water was buffered by entire cooling path. cal evolution of thermal water upon upflow equilibrium with its wall rocks along the deep Our simulations are consistent with numer- and cooling (explained in text). A: Observed reaches of its flow path. The high-temperature ous observations and hence appear to be robust. Na/K ratio of thermal end-member water at history and chemical maturity of the water are The computed min-T of 211 °C is well above its discharge site is reproduced if minimum eq temperature of equilibrium between water and in fact well corroborated by its high Na and K the formation temperature of the breccia, as wall rock (min-Teq) is set at 211 °C. Tqtz—tem- and negligible Mg contents (Table 1; Table DR4). expected. The simulation also reproduces the perature at which cooling water departs from On the other hand, it is expected that the water occurrence of quartz and microcline in the brec- equilibrium with quartz; diss.—dissolution; departed from equilibrium with its wall rocks as it cia at ~165 °C, and it predicts pH = 9 at the pptn.—precipitation. B: Amounts of minerals cooled upon ascent, due to increasingly sluggish discharge site, in accord with the actual measure- precipitated in a nominal 40 yr period of upflow in mol per m3 of porous rock. C: Saturation indi- dissolution-precipitation kinetics, and possibly ments (Table 1; Table DR2). The reconstructed ces normalized to amount of Si in each mineral. armoring of the wall rock by quartz and precipita- Na/K ratio of the thermal end-member water is tion of low-temperature minerals. In line with this insensitive to the mixing ratio used to correct for expectation, the only wall-rock minerals that also cold-water input, because the cold springs con- proven by the vastly weaker mineralization of occur as hydrothermal precipitates in the breccia tain so little Na and K. In contrast, the Na/K ratio the thermal springs (TDS ≤ 0.03 wt%). It fol-

are microcline and quartz, corresponding to an is a very sensitive monitor of min-Teq (Wanner lows that albite dissolution is the only source of intermediate level in the upflow path. et al., 2014), as demonstrated by the simulation Na in today’s thermal waters. Finally, although

Solute geothermometry, as routinely applied in which min-Teq was set to 230 °C (Fig. 3A), we assumed in the simulations that the departure to geothermal fluids, aims to find the tempera- resulting in a predicted Na/K ratio well below from complete water-rock equilibrium occurs

ture at which the ascending water departs from that measured in the thermal water. Fluid inclu- at a unique temperature, the calculated min-Teq equilibrium with its wall rocks, interpreted to sion analyses show that a high-salinity pore water remains a valid minimum even if, in reality, the

represent the minimum reservoir temperature of 6.5–7 wt% NaClequiv. was present in the gra- kinetically controlled departure occurs over a in mature systems (Giggenbach, 1988). Rather nitic host rock during late Alpine metamorphism temperature interval or if flow paths at several than relying on geothermometers calibrated in (Hofmann et al., 2004). In principle, mixing of levels within the host fault contribute to the sam- other geological environments (the results of this water with a hotter upwelling meteoric water pled spring waters.

which are nevertheless given in Table DR4 for could lower the apparent min-Teq. However, comparison), we numerically simulated the spe- owing to the long lifetime of the Grimsel Pass IMPLICATIONS FOR DEEP METEORIC cific evolution of the Grimsel thermal water as system (3.3. m.y.) and its high total flux (indi- WATER INFILTRATION it rises and cools through its granitic wall rocks. cated by the abundance of hydrothermal miner- Because the minimum deep temperature is We assumed in the model that the water departs als in the breccias), this metamorphic fluid has 211 °C, the maximum temperature along the from complete equilibrium with the wall rock evidently been flushed out of the flow path, as deep flow path of the Grimsel system is very

Geological Society of America | GEOLOGY | Volume 46 | Number 12 | www.gsapubs.org 3 Downloaded from https://pubs.geoscienceworld.org/gsa/geology/article-pdf/doi/10.1130/G45394.1/4543509/g45394.pdf by guest on 13 November 2018 probably 230–250 °C or even higher. Given shallower than 10 km (Belgrano et al., 2016, and Peiffer, L., Wanner, C., Spycher, N., Sonnenthal, E.L., that the wall rocks along the flow path are the references therein). Involvement of meteoric Kennedy, B.M., and Iovenitti, J., 2014, Multicom- ponent vs. classical geothermometry: Insights only heat source, their geothermal gradient of water in seismicity therefore seems very likely, from modeling studies at the Dixie Valley geo- 25 °C/km implies that meteoric water must although with the local brittle-ductile transition thermal area: Geothermics, v. 51, p. 154–169, have penetrated to at least 9–10 km depth. To situated at 15–20 km depth (Viganò and Martin, https://​doi​.org​/10​.1016​/j​.geothermics​.2013​.12​ our knowledge, both this depth and deep-fluid 2007), there is no evidence for penetration of .002​. temperature are the highest inferred from spring meteoric water into the ductile regime below Pfeifer, H.R., Sanchez, A., and Degueldre, C., 1992, Thermal springs in granitic rocks from the Grim- analyses at any active orogenic geothermal sys- ­Grimsel Pass. sel Pass (Swiss Alps): The late stage of a hydro­ tem worldwide. Hot springs elsewhere in similar thermal system related to Alpine orogeny, in structural and topographic settings with normal ACKNOWLEDGMENTS Kharaka, Y.K., and Maest, A.S., eds., Proceed- background geotherms (<30 °C/km) typically We acknowledge support from the Swiss Competence ings of Water-Rock Interaction WRI–7, Park City, Utah: Rotterdam, Netherlands, A.A. Balkema, reveal min-T < 150 °C and hence lower pen- Center for Energy Research–Supply of Electricity eq (SCCER-SoE) and Swiss National Science Founda- p. 1327–1330. etration depths, such as in the Canadian Rocky tion NRP70 Grant 407040_153889 to Diamond. Grant Raimondo, T., Clark, C., Hand, M., Cliff, J., and Mountains (Grasby and Hutcheon, 2001), the Ferguson, Mark Reed, and an anonymous reviewer Anczkie­wicz, R., 2013, A simple mechanism for Qilian Mountains in China (Stober et al., 2016), kindly provided constructive comments. mid-crustal shear zones to record surface-derived the Pyrenees in Spain (Asta et al., 2010), and fluid signatures: Geology, v. 41, p. 711–714, https://​doi​.org​/10​.1130​/G34043​.1​. other locations in the Swiss Alps (Sonney and REFERENCES CITED Reyes, A.G., Christenson, B.W., and Faure, K., 2010, Vuataz, 2009). 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Similarly, sites Canadian Cordillera: Canadian Journal of Earth Wanner, C., 2017, Constraints on evolution and Sciences, v. 38, p. 427–440, https://​doi​.org​/10​ residence time of geothermal water in granitic with higher background geotherms will yield .1139​/e00​-091​. shallower minimum constraints on the depth of rocks at Grimsel (Switzerland): Procedia Earth Herwegh, M., Berger, A., Baumberger, R., Wehrens, and Planetary Science, v. 17, p. 774–777, https://​ water penetration. P., and Kissling, E., 2017, Large-scale crustal- doi​.org​/10​.1016​/j​.proeps​.2017​.01​.026​. 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Far more enthalpy may I.M., Frei, R., and Eikenberg, J., 2004, Topog- Geothermics, v. 51, p. 130–141, https://​doi​.org​ be accessible for geothermal energy exploitation raphy-driven hydrothermal breccia mineraliza- /10​.1016​/j​.geothermics​.2013​.12​.003​. within the thermal plumes around the upflow tion of Pliocene age at Grimsel Pass, Aar massif, Xu, T., Sonnenthal, E.L., Spycher, N., and Zheng, L., central Swiss Alps: Schweizerische Mineralo- 2014, TOUGHREACT V3.0-OMP Reference zones than previously thought. Second, our gische und Petrographische Mitteilungen, v. 84, evidence that meteoric water penetrates down Manual: A Parallel Simulation Program for p. 271–302, https://doi​ .org​ /10​ .5169​ /seals​ -63750​ .​ Non-Isothermal Multiphase Geochemical Reac- faults to 9–10 km depth calls for reassessment Menzies, C.D., Teagle, D.A.H., Craw, D., Cox, S.C., tive Transport. LBNL Manual: http://esd1​ .lbl​ .gov​ ​ of the way in which such incursion influences Boyce, A.J., Barrie, C.D., and Roberts, S., 2014, /FILES​/research​/projects​/tough​/documentation​ Incursion of meteoric waters into the ductile re- seismicity. In the Grimsel area, for example, /TOUGHREACT_V3​-OMP_RefManual​.pdf ​ gime in an active orogen: Earth and Planetary (accessed July 2018).​ 33 of the 38 magnitude 0.1–2.5 earthquakes Science Letters, v. 399, p. 1–13, https://​doi​.org​ recorded since A.D. 1971 had focal depths /10​.1016​/j​.epsl​.2014​.04​.046​. Printed in USA

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