Morphotectonics of fissure ridge travertines from geothermal areas of Mammoth Hot Springs (Wyoming) and Bridgeport (California)

Tectonophysics 548–549 (2012) 34–48

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Morphotectonics of fissure ridge travertines from geothermal areas of Mammoth Hot Springs (Wyoming) and Bridgeport (California) Luigi De Filippis a,⁎, Andrea Billi b a b

Dipartimento di Scienze Geologiche, Università Roma Tre, Rome, Italy Consiglio Nazionale delle Ricerche, IGAG, Rome, Italy

a r t i c l e

i n f o

Article history: Received 9 November 2011 Received in revised form 10 April 2012 Accepted 22 April 2012 Available online 3 May 2012 Keywords: Fissure ridge Travertine Active tectonics Geothermal field Fluid pressure

a b s t r a c t Eleven Quaternary fissure ridge travertines from Mammoth Hot Springs (Wyoming) and seventeen ones from Bridgeport (California) were mapped and studied with a morphotectonic approach to understand possible relationships between travertines and active versus passive tectonics. Results are compared with other known geothermal fissure ridges on the Earth. The studied fissure ridges are all located in the hangingwall of normal faults, but the fissure ridges appear as non-dislocated by faults, rather by axial fissures. Both in the two principal study areas and elsewhere, azimuthal analyses of faults and fissure ridges show that the distribution of fissure ridge long axis is rather dispersed around the strike of the local normal faults. No correlation occurs between the fissure ridge length and the angle between the strike of the normal fault and the strike of fissure ridges. The studied fissure ridges are 2 to 360 m long (mean length: 72.1 m), 1 to 15 m wide (mean width: 6.7 m), and 0.5 to 8 m high (mean height: 3.9 m). Fissure ridge aspect ratios show a moderate correlation between the length and both the width and the height of fissure ridges, whereas the correlation between width and height is less marked. The growth in height and width of ridges appears as much more inhibited than in length. A model is proposed in which fissure ridge travertines grow with enhanced elongation along one sub-horizontal direction, which seems moderately controlled by the associated normal fault and the regional extension. Other factors, such as the inherited fracture network and the geothermal and artesian pressure of fluids (fluid discharge) may be important in the development of the studied fissure ridges. Results from this study may contribute to the knowledge of factors that control the long-term geothermal circulation and also the long-term hermetic durability of CO2 subsurface repositories. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Due to the high mineralizing capability of several geothermal fluids, high-permeability pathways are necessary to bring these fluids to the surface. It follows that active faulting and, more generally, active tectonics are invoked as the main mechanism able to keep these pathways pervious to geothermal strongly-mineralizing fluids (Anderson and Fairley, 2008; Barton et al., 1995; Billi et al., 2007; Curewitz and Karson, 1997; Hancock et al., 1999; Micklethwaite and Cox, 2004; Newell et al., 2005; Norton and Knapp, 1977; Sibson, 1987). One problem in assessing the genetic relationships between geothermal fluid circulation and active tectonics consists of finding, at the Earth's surface, reliable markers for the long-term (e.g., tens of thousands of years) outflow of geothermal fluids to understand these relationships in a long-term perspective. Geothermal travertines are very good markers of geothermal circulation for temporal ranges up to several tens of thousands of years, they can be accurately ⁎ Corresponding author at: Dipartimento di Scienze Geologiche, Università Roma Tre, Largo S.L. Murialdo 1, Rome, 00146, Italy. E-mail address: ldefi[email protected] (L. De Filippis). 0040-1951/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2012.04.017

dated (the non-detrital sparitic travertines), and their development has been often associated with active tectonics (Barnes et al., 1978; Brogi, 2008; Brogi et al., 2010; Çakir, 1999; Crossey et al., 2006; Dockrill and Shipton, 2010; Evans et al., 2004; Hancock et al., 1999; Kele et al., 2011; Martinez-Diaz and Hernandez-Enrile, 2001; Ozkul et al., 2010; Uysal et al., 2007). For instance, Barnes et al. (1978), Çakir (1999), Hancock et al. (1999), and Uysal et al. (2007, 2009) claimed that seismic events partly control the growth of geothermal travertines that may therefore constitute a marker of earthquakes. Despite several studies, however, the genetic relationship between tectonics and travertine is still unclear in many ways. For instance, Sturchio et al. (1994), Rihs et al. (2000), and Faccenna et al. (2008), among others, in addition to active tectonics, highlighted the role of climate fluctuations in the deposition of geothermal travertine bodies, with emphasis on the water supply connected with the water table oscillations. Rihs et al. (2000) and Faccenna et al. (2008), in particular, related travertine deposition and tectonics through faulting likely promoted by pore pressure changes induced by fluctuations of the water table during climate oscillations (i.e., passive tectonics). On the other hand, Mesci et al. (2008) found that there are no straightforward relationships between geothermal

L. De Filippis, A. Billi / Tectonophysics 548–549 (2012) 34–48

fissure ridge travertines of the Sivas area (central Turkey) and paleoclimate. With the aim of contributing to the knowledge of the genetic relationship between travertine deposition and active versus passive tectonic processes, in this paper, we present results from a morphotectonic study conducted in two geothermal areas of Northern America: Mammoth Hot Springs (Yellowstone, Wyoming) and Bridgeport (California) (Fig. 1). We mapped morphotectonic structures from these sites through field surveys (mapping at the 1:500 and larger scales) and aerial image analysis (various scales), and then collected morphometric and structural data during field work and, subsequently, during laboratory analysis on aerial images. Mammoth Hot Springs and Bridgeport are outstanding sites for the study of geothermal travertines (Chesterman and Kleinhampl, 1991; Fouke, 2011), yet their morphotectonic features remain to be substantially studied. Morphotectonic results from this study may constitute a starting point for future hydrological and geochemical researches aimed at understanding the tectonically-controlled long-term geothermal circulation in the two study areas. At Mammoth Hot Springs and Bridgeport, we focused our attention to fissure ridge travertines, which are whale-back-shaped or elongate mound-shaped deposits of travertines (Fig. 2) developed along open fissures (Altunel and Hancock, 1996; Bargar, 1978; Chafetz and Folk, 1984). In the attempt of generalizing our results, we compared them with morphotectonic evidence from fissure ridges studied in the Denizli basin, Turkey, and also in other sites on the Earth's surface (Atabey, 2002; Bargar, 1978; Brogi, 2004; Brogi and Capezzuoli, 2009; Çakir, 1999; Chafetz and Folk, 1984; Chafetz and Guidry, 2003; Chesterman and Kleinhampl, 1991; Fouke, 2011; Goff and Shevenell, 1987; Guo and Riding, 1999; Altunel and Hancock, 1993, 1996; Hancock et al., 1999; Haluk Selim and Yanik, 2009; Temiz et al., 2009; Uysal et al., 2007, 2009). The findings of this study suggest that (active) tectonic control on the growth of geothermal fissure ridge travertines is moderate. Evidence of the type presented here may be useful for developing conceptual models of long-term geothermal fluid circulation or also leaking from failed geologic sequestration reservoirs (e.g., Anderson and Fairley, 2008; Dockrill and Shipton, 2010; Gilfillan et al., 2011; Nelson et al., 2009; Shipton et al., 2004, 2005). In particular, to assess the risk of leakage from reservoirs used for long-term underground CO2 storage, possible natural analogs as those ones presented in this paper should be carefully considered because these travertine deposits are surface markers of a long-lasting (tens or even hundreds of thousands of years) leakage of CO2 from a substratum rich of carbonate rocks (e.g., De Filippis et al., in press; Faccenna et al., 2008; Uysal et al., 2007; Uysal et al., 2009). Also for these reasons, below,

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Fig. 2. Schematic three-dimensional representation of a typical fissure ridge travertine deposit. For dimension nomenclature (length, width, and height), we follow previous examples (Altunel and Hancock, 1993; Chafetz and Folk, 1984).

we attempt to understand the causes of travertine deposition at Mammoth Hot Springs and Bridgeport. 2. Geological settings Mammoth Hot Springs is located c. 8 km south of the north entrance of the Yellowstone National Park (YNP), near the Montana– Wyoming border (Figs. 1, 3a and b). The subsurface geology of the YNP is primarily composed of a thick (3 km) sequence of Paleozoic and Mesozoic sedimentary rocks deposited in marine settings and subsequently folded and faulted during late Cretaceous and early Tertiary times (Harris et al., 1997). Afterward, during late Tertiary (Pliocene) time, the whole region underwent continued uplift, faulting, and erosion. Volcanic activity, the latest phase of which began about 1.2 Ma, is a surface expression of a magma chamber that presently lies at a shallow depth beneath the YNP. Recent major periods of eruptions (dated 640 ka and 160 ka) were followed by the caldera collapse (Eaton et al., 1975) forming the present Yellowstone volcanic caldera (Christiansen et al., 2007). In the YNP, there are more than 12,000 active or recently-active hot springs, geysers, fumaroles, and mud pots, which are fed or stimulated by the Yellowstone supervolcano (Fournier, 1989, 2005; Rye and Truesdell, 2007). Generally, the hot spring water is meteoric, coming from the mountains to the north and northwest of the

Fig. 1. Location of the study areas in USA: Mammoth Hot Springs (Yellowstone National Park, Wyoming) and Bridgeport (California).

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Fig. 3. Mammoth Hot Springs (Wyoming, USA). (a) Yellowstone National Park image produced using elevation data by USGS (available on http://en.wikipedia.org/wiki/File:YellowstoneTF.jpg, by Martin D. Adamiker, copyleft by Creative Commons License). (b) Yellowstone National Park area (MHS = Mammoth Hot Springs, N–M = Norris–Mammoth corridor, NGB = Norris Geyser Basin, LGB = Lower Geyser Basin, UGB = Upper Geyser Basin). (c) Simplified geological map of Mammoth Hot Springs area. (d) Study area and main trails. (e) Morphotectonic map for the fissure ridge area of Mammoth Hot Springs (Table 1). Rose diagram shows the azimuthal distribution of fissure ridge long axes and the strike of the master normal fault. Note that all fissure ridges occur on the hangingwall of the NE-striking SE-dipping normal fault (see the related lower-hemisphere Schmidt diagram) at different altitudes along a terraced slope. Fissure ridges are classified (red tones) by height, h. No sharp correlation occurs between fissure ridge height and orientation. Rose diagram is produced with Daisy software (Salvini et al., 1999).

caldera, and interacting with subsurface rocks at temperatures as high as 300 °C (Lewis et al., 1997). The hottest and most active geysers and springs of the YNP occur in correspondence of the ring-

fracture zone of the Yellowstone caldera (Johnson et al., 2003; Morgan et al., 2003a, 2003b) and in the Norris–Mammoth corridor, north of the caldera (Fig. 3a and b). The Mammoth Hot Springs

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geothermal area is located within the Norris–Mammoth corridor (Fig. 3a), a complex N–S subsiding structure, extending 40 km from the Yellowstone caldera to the northern portion of the YNP (e.g., Pierce et al., 1991). There are several N–S striking normal faults and lineaments in the corridor, which control fluid flow and related geysers, hot springs, and similar geothermal structures. The sedimentary section of Mammoth Hot Springs area consists of Cambrian-toCretaceous marine sandstones, shales, and limestones, including the Mississippian Mission Canyon Limestone, which is an important regional aquifer. Mammoth Hot Springs is the second largest site on Earth for active travertine deposition after Pamukkale in Turkey (Pentecost, 2005). The travertine in Mammoth Hot Springs has been deposited during the last 8000 years, with a deposition rate of about 1 cm/a (Sturchio, 1992; Sturchio et al., 1994) over an area of 4 km 2 and for a total thickness of about 73 m (Allen and Day, 1935; Bargar, 1978; White et al., 1975). At present, discharge from the entire hot thermal springs complex totals 590 l/s, of which 10% erupts from the travertine terraces and 90% flows directly into the Gardiner River (Sorey, 1991; Sorey and Colvard, 1997). Regional groundwater is heated by the near-surface magma chamber and mostly uprises along the Mammoth and Swan Lake faults (Sorey, 1991). The surface temperature of the springs at Mammoth Hot Springs is 73 °C, whereas the temperature at reservoir depths is c. 100 °C (Kharaka et al., 2000). Active deposition of travertine forms terraced and fissure ridge deposits, which are located at different altitudes along a terraced slope (Fig. 4). Terraced deposits are the volumetrically dominant morphology at Mammoth Hot Springs (Bargar, 1978). The fissure ridges are mostly located in the southwestern portion of Mammoth Hot Springs (Fig. 3b, c, and d) and are explained in detail in the next section (see also Fouke, 2011). In this site, we did not collect samples because sampling is illegal in the entire YNP. Active deformation in the Yellowstone region is mostly connected with magmatic and geothermal processes that provoke cycles of inflation and deflation. The area is monitored by local seismic and geodetic networks. GPS data from the northern sector of YNP (including Mammoth Hot Springs) show that this area is undergoing extensional tectonics with a maximum extension trending between ENE–WSW and NE–SW and an extension rate around 2 mm/a (Aly and Cochran, 2011; Puskas and Smith, 2009; Puskas et al., 2007; Smith et al., 2009). Travertine Hot Springs at Bridgeport (Mono County, eastern California) is located c. 17 km southwest of the California–Nevada border (Figs. 1 and 5a), within the larger Mono Basin, a sediment-filled structural depression created by extensional faulting and tectonics mostly during Quaternary time (Bursik and Sieh, 1989; Bursik et al., 2002). Mono Basin is surrounded, to the west, by the massive Mesozoic granite and Paleozoic metamorphic rocks of the Sierra Nevada escarpment, to the north and east, by the highly fractured Tertiary volcanic rocks of the Bodie Hills, Anchorite Hills, and Cowtrack Mountain, and, to the south, by the Quaternary volcanic rocks of the Mono Craters and Glass Mountains (Gilbert et al., 1968). The complexly faulted western part of the Bodie Hills topographically dominates, from the east, the structural basin of the Bridgeport Valley (Fig. 5b), an alluvium-filled basin of Quaternary age (Chesterman and Kleinhampl, 1991), about 10 km wide and 12 km long, at an elevation of 2000 m. More in detail, the Bodie Hills comprise a complex mass of late Tertiary volcanic rocks consisting of lava flows, tuff breccias, intrusive dikes, plugs, and domes erupted and emplaced in late Miocene time. Toward the west, these volcanic rocks are overlain by Pleistocene glacial till and outwash deposits (Kleinhampl et al., 1975). A distinct northeast-trending gravity low crossing the Bodie Hills may indicate the presence of a buried graben structure, partially filled with low-density Tertiary volcanic rocks and alluvium. The graben includes the Bridgeport Valley, some hot spring areas, and Big Alkali (Fig. 5b), a 2.4 km diameter caldera (Chesterman, 1968). The Travertine Hot Springs geothermal area, which is our study area, is located about 2 km to the southeast of the Bridgeport village

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(Fig. 5b). The Travertine Hot Springs area is limited to the north, east, and south by a complex system of intersecting normal faults with associated warm and hot springs. Although there is no certain dating, the travertine begun its deposition during Pleistocene time and it is still active over some fissure ridges. The poorly exposed bedrock–travertine contact in the Travertine Hot Springs area appears to be a depositional lap onto late Miocene dacitic flows, some of which are weakly hydrothermally altered (Chesterman and Kleinhampl, 1991). At present, the springs and seeps discharge a total of 0.8 l/s at 50 to 65 °C (Hannah, 1975). The presence of lithium in the spring waters may indicate the interaction with magmatic components. Moreover, the large volume of travertine deposits implies the presence of subsurface carbonates (Chesterman and Kleinhampl, 1991). The studied fissure ridges are located in the south-east portion of the travertine area and are explained in detail in one of the next sections. All fissure ridges developed at about the same altitude on a caliche plain (Fig. 5c). The studied site used to be a quarry area in the 1890s, but it is now a national protected area where sampling is prohibited. The travertine quarried from Travertine Hot Springs (Bridgeport) was used as an ornamental stone in the rotunda of City Hall, San Francisco (Chesterman and Kleinhampl, 1991). The presence of artificial exposures created during quarrying activities allowed us to study also some inner sections of the fissure ridges. Concerning active deformation, Bridgeport is located in a transitional region between the extensional domain (E–W-oriented maximum extension) of Basin and Range, to the east, and the right-lateral strikeslip domain (strike-slip motion principally along N140°-striking faults) of western California, to the west (Puskas and Smith, 2009). GPS data from the Mono County area (including Bridgeport) show that this region is undergoing extensional tectonics with a maximum extension trending between N90 and N110° and an extension rate around 1 mm/a or even less (Hammond et al., 2009; Kreemer et al., 2009; Puskas et al., 2007). 3. Results 3.1. Fissure ridge travertines from Mammoth Hot Springs Mammoth Hot Springs (MHS) occupies the central section of a N– S-trending, subsiding, quadrangular structure (Fig. 3c), which is part of the larger Mammoth–Norris corridor (e.g. Pierce et al., 1991). NW–SE and NE–SW striking normal faults constitute the boundaries of this structure (Fig. 3a). These tectonic elements together with the associated fractures are likely responsible for the main upflow of fluids in the area. The travertine mass at MHS developed with an elongate NE–SW trend along the c. 4 km long slope between Terrace Mountain, to the southwest, and the Gardiner River, to the northeast (Fig. 3c). The travertine deposit of MHS is mainly a terraced mound deposit (Chafetz and Folk, 1984) comprehending step-like travertine terraces, fissure ridges, and hot-spring cones located at elevations between 1725 and 2085 m. Other travertine deposits (older than the MHS ones) are found toward the southwest (Terrace Mountain) at higher elevations (Bargar, 1978). The terraced mound accumulations of MHS contribute to form the large MHS travertine deposit. Terraced mounds are often associated with one or more spring orifices (Chafetz and Guidry, 2003). Well known active examples are the Main Terrace (Fig. 3e), Minerva Terrace, Highland Terrace, and Jupiter Terrace. More in detail, each flat floored, shallow pool (rimstone pool of Warwick, 1953; terracettes of Bargar, 1978) receives water from higher altitude springs or is directly fed by local springs (Chafetz and Folk, 1984). At MHS, about 100 active hot springs were documented by Bargar (1978). The travertine deposit of MHS is located on the hangingwall of a N45°-striking master normal fault (Fig. 3e). Although the terraces are morphologically dominant at MHS, the most marked topographic

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L. De Filippis, A. Billi / Tectonophysics 548–549 (2012) 34–48

Fig. 4. Photographs from the fissure ridge area of Mammoth Hot Springs (Wyoming, USA). See Table 1 for fissure ridge data. (a) Panorama including the Main Terrace, the MHS7 fissure ridge, and the New Blue Spring. (b) Crest area of the MHS7a fissure ridge with the axial fissure. (c) Banded travertine exposed along the axial fissure of the MHS7a fissure ridge. (d) Lateral view of the active MHS1a fissure ridge, also known as White Elephant Back Terrace. (e) Frontal view of the MHS1a fissure ridge with a hot spring emerging from the axial fissure. (f) The MHS1b fissure ridge with his axial fissure. (g) Detail from the hot spring of panel (e). An orifice is visible in the crestal region (see Supplemental Material 1). (h) The Orange Spring Mound and Tangerine Spring form the active MHS5 fissure ridge (see Supplemental Material 2). (i) The MHS4 fissure ridge with a tree grown on its crest. (j) Northwestern tip of the active MHS6 fissure ridge, also known as Narrow Gauge Terrace. A narrow fissure is visible along the crest. (k) Southeastern tip of the MHS6 fissure ridge with the axial fissure.

features on the upper Mammoth terraces are the numerous fissure ridges of the Upper Terrace Area (Fig. 3d). The term fissure ridge was used, for the first time, by Hayden (1883) at MHS to indicate whale-back-shaped or elongate mound-shaped deposits of travertines (Bargar, 1978). Fissure ridge travertines grow where hot spring water emerges along a fracture feeding elongate symmetrical or asymmetrical travertine mounds, which then evolve to elongate ridges.

At MHS, we studied the morphotectonic features of eleven main fissure ridges (Table 1). Fissure ridges are mapped in Fig. 3(e) to understand their spatial distribution and geometrical relationship with the master normal fault. Fissure ridges vary between 30 and 235 m in length, 5 and 15 m in width, and 0.5 and 8 m in height. Fissure ridges are mapped by classes of height to appreciate their size frequency and distribution (Fig. 3e). Analytical diagrams of fissure ridge morphotectonic features are proposed and depicted in one of

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Fig. 5. Bridgeport (California, USA). (a) Site map. (b) Simplified geological map of the Bridgeport area. (c) Morphotectonic map for the fissure ridge area of Bridgeport (Table 1). The rose diagram shows the azimuthal distribution of fissure ridge long axes and the strike of the master normal faults (see the related lower-hemisphere Schmidt diagram). Note that all fissure ridges occur on the hangingwall of the normal faults at about the same altitude. Fissure ridges are classified (red tones) by height, h. Also in this case, no sharp correlation occurs between fissure ridge height and orientation. Rose diagram is produced with Daisy software (Salvini et al., 1999).

the following sections. Most fissure ridges are symmetrical, although some asymmetrical ones are present (e.g., the MHS6 fissure ridge, also known as Narrow Gauge; Fig. 4j and k). In one case, we were able to observe a banded travertine along the crestal region of one of the studied fissure ridges. This banded travertine is c. 20 cm in width, partly fills the axial fissure of the ridge (Fig. 4c), and is characterized by sparitic CaCO3 consisting of a series of parallel white–gray bands alternated with thinner (less than 0.5 cm) dark brownish bands. At MHS, we did not observe faults cutting through the fissure ridges or other travertine deposits. We observed both active and inactive fissure ridges. To better understand and show the present growth of fissure ridges from the groundwater upflowing through their axial fissures, we recorded three movies (Supplemental Materials 1, 2, and 3) showing the thermal activity on active fissure ridges and some of their structural features: (1) Supplemental Material 1 shows the present growth of the southwestern tip of the MHS1a fissure ridge, also

known as White Elephant Back Terrace; (2) Supplemental Material 2 shows the growth of the southern flank of the MHS5 fissure ridge, which is composed by two coalescent mounds, namely the Orange Spring Mound and the Tangerine Spring; and (3) Supplemental Material 3 shows both lateral and axial growth of the MHS6 fissure ridge, also known as Narrow Gauge Terrace, where hot water emerges from an orifice located on the central-crestal portion of the fissure ridge. 3.2. Fissure ridge travertines from Bridgeport Travertine Hot Springs at Bridgeport (BRP) (Fig. 5a and b) consists of a travertine terrace deposit forming a compact tabular mass area with numerous hot but also cold springs, covering an area of c. 0.2 km 2 at elevations between 2010 and 2065 m. The caliche plain (i.e., a flat area covered by a hardened deposit of calcium carbonate precipitated from geothermal waters in the semiarid climate of

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Table 1 Data for fissure ridges from Mammoth Hot Springs (Wyoming), Bridgeport (California), and other well known fissure ridges on the Earth. (1) In the names given to fissure ridges (e.g., BRP1_1 or MHS1a), the symbol underscore (“_”) is for branches of main fissure ridges, whereas “a”, “b”, “c” are for groups of fissure ridges partly coalesced or very close. Fissure ridge (1)

Locality

Country

Lat

Long

Average axis orientation (°)

Length (m)

MHS1a MHS1b MHS2 MHS3 MHS4 MHS5 MHS6 MHS7a MHS7b MHS7c MHS8 BRP1_1 BRP1_2 BRP2 BRP3 BRP4 BRP5 BRP6 BRP7_1 BRP7_2 BRP7_3 BRP7_4 BRP7_5 BRP8 BRP9_1 BRP9_2 BRP10 BRP11 BRP12 BRP13_1 BRP13_2 BRP14 BRP15 BRP16 BRP17 Soda Dam Kamara Çukurbaga Çukurbagb Akköy_1 Akköy_2 Akköy_3 Karahayit Kizilseki Hill_1 Kizilseki Hill_2 Kizilseki Hill_3 Hanife Hill_1 Hanife Hill_2 Hanife Hill_3 Hanife Hill_4 Hanife Hill_5 Kocabaş_1 Kocabaş_2 Kocabaş_3 Kocabaş_4 Bal Cambazli Kirsehir Terme S. Giovanni

Mammoth Hot Springs, Wyoming Mammoth Hot Springs, Wyoming Mammoth Hot Springs, Wyoming Mammoth Hot Springs, Wyoming Mammoth Hot Springs, Wyoming Mammoth Hot Springs, Wyoming Mammoth Hot Springs, Wyoming Mammoth Hot Springs, Wyoming Mammoth Hot Springs, Wyoming Mammoth Hot Springs, Wyoming Mammoth Hot Springs, Wyoming Bridgeport, California Bridgeport, California Bridgeport, California Bridgeport, California Bridgeport, California Bridgeport, California Bridgeport, California Bridgeport, California Bridgeport, California Bridgeport, California Bridgeport, California Bridgeport, California Bridgeport, California Bridgeport, California Bridgeport, California Bridgeport, California Bridgeport, California Bridgeport, California Bridgeport, California Bridgeport, California Bridgeport, California Bridgeport, California Bridgeport, California Bridgeport, California Yemez Springs, New Mexico Yenice (Denizli basin) Pamukkale (Denizli basin) Pamukkale (Denizli basin) Karakaya Hill (Denizli basin) Karakaya Hill (Denizli basin) Karakaya Hill (Denizli basin) Karahayit (Denizli basin) Kizilseki Hill (Denizli basin) Kizilseki Hill (Denizli basin) Kizilseki Hill (Denizli basin) Hanife Hill (Denizli basin) Hanife Hill (Denizli basin) Hanife Hill (Denizli basin) Hanife Hill (Denizli basin) Hanife Hill (Denizli basin) Kocabaş (Denizli basin) Kocabaş (Denizli basin) Kocabaş (Denizli basin) Kocabaş (Denizli basin) Balkayasi (Gediz graben) Cambazli (Gediz graben) Kirsehir (Central Anatolia) Rapolano Terme, Siena (Tuscany)

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48 48 11 37 77 65 125 14 155 166 54 125 165 105 53 67 38 38 40 65 40 71 110 150 60 135 60 120 123 53 53 49 41 40 177 60 125 85 77 146 140 116 145 148 157 18 126 161 135 113 120 126 145 115 130 135

235 50 105 150 90 30 100 60 90 100 160 110 25 45 40 360 145 25 84 3 2 3 5 11 5 2 2 80 140 80 4 105 54 15 10 100 63 350 130 1000 1000 900 600 410 600 130 750 300 155 235 245 1900 390 910 660 350 200 800 250

44°57′49″ 44°57′50″ 44°57′47″ 44°57′54″ 44°57′57″ 44°57′59″ 44°58′10″ 44°58′08″ 44°58′06″ 44°58′07″ 44°57′58″ 38°14′41″ 38°14′41″ 38°14′41″ 38°14′42″ 38°14′43″ 38°14′46″ 38°14′44″ 38°14′45″ 38°14′45″ 38°14′45″ 38°14′45″ 38°14′45″ 38°14′43″ 38°14′43″ 38°14′43″ 38°14′44″ 38°14′46″ 38°14′46″ 38°14′45″ 38°14′45″ 38°14′47″ 38°14′46″ 38°14′47″ 38°14′45″ 35°47′29″ 38°03′24″ 37°55′53″ 37°55′51″ 37°56′58″ 37°56′40″ 37°56′43″ 37°57′26″ 37°57′17″ 37°57′12″ 37°56′54″ 37°56′39″ 37°56′50″ 37°56′44″ 37°56′42″ 37°56′39″ 37°48′39″ 37°48′28″ 37°49′10″ 37°48′55″ 38°22′07″ 38°34′14″ 39°07′50″ 43°16′46″

Bridgeport; Figs. 5c and 6d) hosting the studied fissure ridges is at an elevation of about 2065 m (Fig. 5c). About 0.09 km 2 of this area is occupied by a series of finger-like fissure ridges and mounds of various sizes, in places associated with pools and springs. Two intersecting normal faults, striking N30° and N145°, respectively, delimit the travertine area to north-east and south-east, at the foot hill of the Bodie Hills (Figs. 5b and c). In the study area, the N30°-striking fault is the master normal fault, close to which (in the hagingwall) the studied fissure ridges grew. Hot waters at BRP, with a surface temperature between 50 and 65 °C (Hannah, 1975), ascend along

170 117

Width (m)

Height (m)

15 10 11 9 7 10 15 5 6

8 0.5 4 4 4 6 5 3 4

11 5 4 5 3 7 7 4 7

5 5 4 6 4 6 5 3 4.5

2 4

1.2 0.9

1 5 7 7

0.5 2 5 5

7 5 4 5 25 15 36 14 150 120 200 200 100 220

6 3 3 0.9 15 6 11 1 15 25 14 7 25 35

160

10

300

20

25 5 30 30

15 4 10

Master normal fault striking (°) 45 45 45 45 45 45 45 45 45 45 45 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 50 125 125 125 125 125 125 125 125 125 125 125 125 125 125 125 125 125 125 125 140 90 170 165

fractures through Tertiary volcanic rocks (Fig. 5b) and are enriched in calcium bicarbonate. At BRP, we studied the morphotectonic features of 17 fissure ridges (Table 1). These structures are mapped in Fig. 5(c) to understand their spatial distribution and geometrical relationship with the master normal fault. An evident morphotectonic feature is the alignment of some fissure ridges along the same fracture (i.e. BRP7, BRP15 and BRP16). Fissure ridges vary between 2 m and 360 m in length, 1 and 7 m in width, and 0.5 and 6 m in height. Fissure ridges are mapped by classes of height to appreciate their size frequency

L. De Filippis, A. Billi / Tectonophysics 548–549 (2012) 34–48

and distribution (Fig. 5c). Analytical diagrams of these morphotectonic features are shown and depicted in the following section. Most fissure ridges are symmetrical although some asymmetrical ones are present (i.e. BRP1 and BRP7). The crestal fissure of BRP7 diverges forming a small parasitic fissure ridge on the northwestern flank (Figs. 5c and 6e). Another small parasitic fissure ridge was observed on the southeastern flank of BRP13 (Fig. 5c). Generally, the fissure ridges at BRP are parallel to the original fractures along which they developed. The old quarry activity at BRP and related artificial exposures allowed us to study also some inner structures of the fissure ridges.

41

Porous and stratified travertine (bedded travertine) forms the bulk of the terraces, fissure ridges, and other hot spring features, whereas a vertical, nonporous, sparitic travertine (banded travertine) fills the interior veins of the fissure ridges (Fig. 6f–k). Crystal pattern forming the banded travertine is shown in Fig. 7. Most ridge crests slope gently downward, generally in a southwesterly direction. The flank of some fissure ridges (i.e. BRP1 and BRP12) shows a characteristic progressive bed steepening (Fig. 6f). The contact between bedded and banded travertine is primarily at right angle, in places with a zig-zag-like pattern (Fig. 6k). At least in one case (the BRP4 fissure ridge), we observed a cross-section of a fissure filled by banded

Fig. 6. Photographs from the fissure ridge area of Bridgeport (California, USA). (a) Panoramic view of the BRP4 fissure ridge, also known as Long Ridge (note his curved shape). (b) A low fissure mound in the Travertine Hot Springs area. Similar structures are rather frequent in this area. (c) Frontal view of the BRP7 fissure ridge showing the asymmetrical shape of the southwestern tip. (d) A northeastern panoramic view of the fissure ridge area at Bridgeport taken from the crest of the BRP7 fissure ridge. The caliche plain and BRP13 fissure ridge are visible in the center of the photograph. (e) Detail from panel (d) showing a parasitic fissure ridge grown on the northwestern flank of the BRP7 fissure ridge. (f) Southwestern flank of the BRP1 fissure ridge showing a progressive upward steepening of beds in the flank of the fissure ridge. (g) The banded travertine filling the crestal fissure of the BRP1 fissure ridge shows some fluid-like structures. (h) At the northeastern tip of the BRP4 fissure ridge, the banded travertine filling the main fissure shows a V-like shape. (i) Vertical banded travertine exposed on an artificial cut of the BRP4 fissure ridge. (j) A chemical weathering contact between bedded and banded travertines within the BRP4 fissure ridge. (k) A zig-zag-like sub-perpendicular contact between the bedded and banded travertines within the BRP3 fissure ridge.

42

L. De Filippis, A. Billi / Tectonophysics 548–549 (2012) 34–48

travertine tending to widen upward in a characteristic V-like fashion (Fig. 6h). In the central part of the travertine area and between the fissure ridges BRP7 and BRP13, a thin crust of white fine salt (mostly CaCO3) covers the flat floor (i.e., the caliche plain; Fig. 6c and d). Above and around the caliche plain, small travertine shield-like structures (i.e., very flat and small fissure ridges less than 2 m in length) characterized by banded travertine filling short sinuous axial fractures are exposed (Fig. 6b). On the northwestern slope of the travertine area (i.e., named caliche slope for the presence of a hardened deposit of calcium carbonate precipitated over a shallow slope; Fig. 5c), long linear emergences of naked banded travertine (i.e., with no flanks of bedded travertine) are exposed. Both at MHS and at BRP as well as in other fissure ridge areas on the Earth (Altunel and Hancock, 1996; Atabey, 2002; Chesterman and Kleinhampl, 1991; Mesci et al., 2008), curved fissure ridges of various sizes occur (Figs. 3e, 5c and 6a). These fissure ridges, instead of being linear (in map view) as most fissure ridges (Figs. 3e, 4d, j, 5c and 6d), consist of two roughly linear wings (ridges) joined in a curved hinge zone. The inter-wings angle at MHS and at BRP is up to 130°. Further particular structures are bifurcated fissure ridges (in map view; Fig. 5c), with bifurcation angles, at Bridgeport, up to 95°. Bifurcation may occur as closure structures at the tips of fissure ridges (Fig. 6c) or as large structures involving a significant portion of fissure ridges (Fig. 5c). Bifurcated fissure ridges are present at BRP, but also elsewhere (e.g., Altunel and Hancock, 1996; Kirsehir region, Turkey, Atabey, 2002; Sivas area, Turkey, Mesci et al., 2008). At BRP, we did not observe faults cutting through the fissure ridges or other travertine deposits. We observed both active and inactive fissure ridges. To better understand and show the present growth of fissure ridges from groundwater flowing upward through their axial fissures, we recorded two movies showing the thermal activity on an active fissure ridge (BRP7) and his main structural features (Supplemental Materials 4 and 5): (1) Supplemental Material 4 shows the present growth of the southwestern tip of the BRP7 fissure ridge, also known as Hot Tube Ridge (Chesterman and Kleinhampl, 1991), with hot water flowing longitudinally on the ridge crest; (2) Supplemental Material 5 shows, in detail, the axial growth of BRP7, with the hot water emerging directly from the top of the fissure ridge and flowing away along the axial fissure. 3.3. Fissure ridge morphotectonics In this section, we have synthesized the main morphotectonic data from fissure ridges of MHS and BRP (Figs. 8 and 9). In the attempt of generalizing our results, we have also considered the same

morphotectonic data from other important fissure ridge areas on the Earth (Table 1). As observed in the previous sections, fissure ridges from MHS and BRP (Figs. 3e and 5c), as well as from other well known fissure ridge areas, usually develop on the hangingwall of normal faults (Altunel and Hancock, 1996). This notion per se indicates that fissure ridges are at least in part tectonically controlled; however, to better understand the degree and mode of tectonic control, in Fig. 8, we have plotted the azimuth of the fissure ridge long axes and that of the related master normal faults (rose diagrams in Fig. 8a to c). We have also plotted the angular separation between the strike of the master normal fault and the strike of the fissure ridge long axis (Δα) versus the length of fissure ridges to understand whether the tectonic control on these structures is dependent on their size (Fig. 8d). As comparison terms, we have considered the same type of morphotectonic data for the fissure ridges of Denizli basin in southwestern Turkey (Altunel and Hancock, 1996; De Filippis et al., in press). In synthesis, Fig. 8 shows a weak (Mammoth Hot Springs) to moderate (Bridgeport and Denizli basin) parallelism between the master normal faults and the related fissure ridges, whose orientation is rather disperse and whose length seems substantially independent on Δα. Regardless of the master normal faults, in each site, also the parallelism among fissure ridges is moderate (Fig. 8a to c). Figs. 3 and 5 show that the height of fissure ridges (see red tones of fissure ridges in Figs. 3e and 5c) is also substantially independent of Δα. All these observations are valid for MHS, BRP, and also for the Denizli basin. Fig. 9 shows aspect ratios (i.e., in log–log spaces, to the left, and in frequency histograms, to the right) of the studied fissure ridges. To better understand these morphological features, we have also plotted the aspect ratios of other well known fissure ridges on the Earth (Table 1). In particular, in addition to those ones from MHS and BRP, we have considered fissure ridges from Soda Dam, USA (Chafetz and Folk, 1984; Goff and Shevenell, 1987), Denizli basin, Turkey (Altunel and Hancock, 1996; Çakir, 1999; De Filippis et al., in press; Hancock et al., 1999; Uysal et al., 2007, 2009), Bal, Turkey (Çakir, 1999), Cambazli, Turkey (Haluk Selim and Yanik, 2009), Kirsehir, Turkey (Atabey, 2002; Temiz, 2004; Temiz et al., 2009; Uysal et al., 2009), and Terme S. Giovanni, Italy (Brogi, 2004; Brogi and Capezzuoli, 2009; Guo and Riding, 1999). In synthesis, the left diagrams of Fig. 9(a) and (b) show a moderate correlation between length and both width and height of fissure ridges, whereas the left diagram of Fig. 9(c) shows a correlation less marked than those observed in Fig. 9(a) and (b) between width and height of fissure ridges. These diagrams suggest that fissure ridges grow much more easily in length rather than in width and, above all, in height. This notion is also supported by frequency histograms

Fig. 7. Calcite growth texture (photograph, on the left, and related line drawing, on the right) within the banded travertine of the BRP3 fissure ridge. The texture is characterized by elongate blocky to fibrous crystals forming moderate competition textures identified by the fan-shaped arrangement (i.e., widening in the growth direction) developed toward the center of the vein with progressive lateral superposition of adjacent crystal fans. The calcite texture is visible at the naked eye thanks to the wind selective erosion in a semi-arid environment (Bridgeport, California). The presence of hiatus zones (accretion pauses) transversally interrupting the elongate crystal fans suggests step wise crack-and-seal growth (Bons, 2000; De Filippis et al., in press; Nuriel, 2011; Ramsay, 1980; Urai et al., 1991; Uysal et al., 2007).

L. De Filippis, A. Billi / Tectonophysics 548–549 (2012) 34–48

(a)

(b)

43

(c) (d)

Fig. 8. Azimuthal relationship between master normal faults and associated fissure ridges from Mammoth Hot Springs (USA), Bridgeport (USA), and Denizli basin (Turkey). Rose diagrams of fissure ridge azimuth for: (a) Mammoth Hot Springs, (b) Bridgeport, and (c) Denizli basin. In the rose diagrams, the strike of the master normal faults (long dashed arrows) and the direction of present extensions derived from GPS data (short double arrows) are also plotted. GPS data are from Allmendinger et al. (2007), Kreemer et al. (2009), and Puskas and Smith (2009) for Denizli, Bridgeport, and Mammoth Hot Springs, respectively. (d) Diagram of Δα versus length of fissure ridges. Δα is the angular separation between the strike of the master normal fault and the strike of fissure ridges.

of Fig. 9, where the length of fissure ridges is usually 15 and 20 times larger than their width and height, respectively, and the width is usually 5 times larger than the related height. The left diagram of Fig. 9(b) shows that data from Bridgeport (red squares) tend to flat for a fissure ridge height of c. 6 m. As fissure ridges from Bridgeport developed all at about the same altitude, the height of 6 m may represent an altitude threshold connected with the local artesian pressure. In other words, this evidence may indicate that, over this threshold (6 m), at Bridgeport, fissure ridges cannot grow further in height. 4. Discussion The fissure ridges studied at Mammoth Hot Springs and Bridgeport are very similar to other fissure ridges previously studied in other active geothermal areas (e.g., Altunel and Hancock, 1996; Brogi and Capezzuoli, 2009; De Filippis et al., in press; Hancock et al., 1999; Mesci et al., 2008; Temiz et al., 2009; Uysal et al., 2007, 2009). In synthesis, these structures are composed by bedded travertine, which constitutes the bulk of fissure ridges (flanks and part of the axial region), and by banded travertine, which consists of veins filled by sparitic banded travertines injected within the axial region of the bedded travertine ridge. As previously proposed (e.g., Bargar, 1978; De Filippis et al., in press; Hancock et al., 1999; Uysal et al., 2007, 2009), the growth of fissure ridges (i.e., both bedded and banded travertines) mostly occurs through a crack-and-seal mechanism (Ramsay, 1980), by which the bulk of the ridge (i.e., the bedded travertine) grows when the central conduit is open and fluids can upflow (crack stage), whereas when

the conduit becomes progressively sealed (seal stage), the growth of bedded travertine is substantially absent and, within the ridge, in a substantially closed system, vein growth (banded travertine) occurs. The subsequent cracking and opening of this system to feed again the external bedded travertine and to provide the inner space necessary to form the banded veins may be obtained through two main alternatives, namely active and passive extensional tectonic processes. A combination of these two end members is also possible. In this paper, with active extension, we refer to fissures kept open (and therefore pervious to fluids) by a remote or regional mechanism of tectonic nature (e.g., basin formation), whereas with passive extension, we refer to fissures kept open by local non-tectonic mechanisms such as fluid pressure, crystallization stress (i.e., exerted by the banded travertine formation), or lateral collapse of fissure ridge flanks as proposed by De Filippis et al. (in press) for some fissure ridges of the Denizli basin. Diagrams of Fig. 8 show a rather dispersed azimuthal distribution of fissure ridges (i.e., moderate or poor parallelism among them) and a moderate parallelism between fissure ridges and the related normal faults. Moreover, the size of fissure ridges is not particularly influenced by their angular relationship with the master normal fault (Δα; Fig. 8d; see also Figs. 3e and 5c). The angular relationship between the fissure ridges and the present-day extension determined from GPS data shows different cases, with the GPS-determined extension being (1) nearly perpendicular to several (but not all) fissure ridges in the Denizli basin (Fig. 8c) and (2) oblique or even parallel to several fissure ridges at BRP (Fig. 8b) and MHS (Fig. 8a), particularly in this latter locality. The apparent misorientation (i.e., obliquity or parallelism) between the GPS-determined extension and some fissure ridges analyzed in Fig. 8 may simply be explained

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L. De Filippis, A. Billi / Tectonophysics 548–549 (2012) 34–48

(a)

(b)

(c)

Fig. 9. On the left, aspect ratio diagrams for the fissure ridges studied at Mammoth Hot Springs and Bridgeport and for other known fissure ridges on the Earth (Table 1). On the right, frequency histograms of aspect ratios. (a) Fissure ridge length versus width; (b) fissure ridge length versus height; (c) fissure ridge width versus height.

with a differently-oriented extensional regime at the time of fissure ridge nucleation and development. It is also true, however, that, both at MHS and BRP, there are presently active fissure ridges, which are nonoptimally oriented with respect to the GPS-determined extension. Whereas the location of the fissure ridges on the hangingwall of normal faults (both in the studied cases and elsewhere; Altunel and Hancock, 1996; Çakir, 1999; Hancock et al., 1999) is a rather clear indicator of the (active) tectonic control on fissure ridges (the hangingwall of normal faults is, in fact, usually the most damaged fault block; e.g., Nelson et al., 2009; Rossetti et al., 2011), the poor parallelism among fissure ridges

and between them and the master normal faults may be interpreted as the evidence that fissure ridges have not grown, at least in part, perpendicular to the active regional extensional tectonics and that, therefore, passive tectonics may have played an important role. This notion is also supported by the occurrence of curved or bifurcated fissure ridges (Figs. 3e and 5c). If the active extensional tectonics would have been the exclusive driving mechanism of fissure ridge development, then tectonically-controlled fissure ridges would have likely maintained the same axis orientation (perpendicular to extension) instead of curving or bifurcating.

L. De Filippis, A. Billi / Tectonophysics 548–549 (2012) 34–48

An alternative and valid explanation for the apparently odd orientations of some fissure ridges and the local master normal faults is provided by Hancock et al. (1999) and Brogi and Capezzuoli (2009), who invoke local deformation mechanisms (rather than the regional ones) such as those ones occurring in or close to step-over zones between normal fault segments, where the regional direction of maximum extension may locally rotate for the interference between displacements of adjacent faults. In synthesis, evidence shown in this paper and in previous ones suggests that fluid pathways feeding the travertine deposits are not necessarily kept open exclusively by the regional active tectonics. If this is true, then other influencing factors, such as a pre-existing fracture pattern (not necessarily parallel to the master normal fault), fluid pressure, or rotational collapse of ridge flanks (passive tectonics), may play a role in the outflow of mineralizing geothermal fluids and, ultimately, in the growth of fissure ridges. For instance, Altunel and Karabacak (2005) measured post-extinction non-null dilation (c. 10 − 2 mm/a) across fissure ridges from the Denizli basin, thereby showing that, regardless of whether this dilation was induced by active or passive tectonics, the extinction itself is not much correlated with the absence of dilation across the fissures, rather with other factors, perhaps an inefficient artesian pressure (e.g., Bargar, 1978). This and previous notions are conceptually explained in Fig. 10, where fissure ridges growing under an active tectonic extension hypothetically assumed as perpendicular to the master normal fault, become progressively extinct simply for an insufficient artesian pressure and an excessive height of fissure ridges inhibiting their further feeding. As a consequence, one or more adjacent fissures, perhaps non-optimally oriented with respect to the active extension, may become a preferential pathway for the mineralizing fluids, thereby giving birth to a new fissure ridge travertine possibly oblique to the master normal fault. As above explained, in this model, the active extensional tectonics is only one of the possible influencing factors. The model shown in Fig. 10 is also supported by the morphometric data (Fig. 9), which suggest that a progressive growth in length of fissure ridges is accompanied by their progressive growth in height and width, although the growth in height (and also in width) is inhibited compared with the growth in length. This is particularly evident at BRP, where a c. 6 m threshold for the fissure ridge height apparently

45

occurs (Fig. 9b). This morphometric evidence suggests that the growth in height is very difficult, at least compared with the growth in length and also in width, thereby involving the capability of groundwater to rise up through the fissure ridge. Such capability is obviously connected with the artesian pore pressure (Bargar, 1978). The height of fissure ridges (maximum known height is around 25–30 m for the Akköy and Kizilseki fissure ridges in Turkey; Altunel and Hancock, 1996) and their average life duration (c. 10 3– 10 4 years; Uysal et al., 2007, 2009) support the hypothesis that artesian pressure and influencing factors other than active tectonics are important in fissure ridge growth. Tectonic regimes and geothermal anomalies in extensional settings may, in fact, last for a time much longer than 10 4 years and, in such a case, it would be unclear why a fissure ridge should become extinct in only 10 4 years if solely driven by active tectonics and geothermalism. To this latter end, Faccenna et al. (2008) proposed that the bedded geothermal travertine of Tivoli (central Italy) mostly grew during warm and humid periods of Quaternary time, when the underground water table was high and the fluid discharge particularly abundant. Such a fluid pressure must have contributed to activate faults (below the travertine deposit) that were tectonically stressed, thereby creating pathways (fault-related open fractures) for the upflow of bicarbonate-rich geothermal fluids. Uysal et al. (2007, 2009) ascribed the growth of banded travertine within fissure ridges of the Denizli basin during cold periods to the CO2 oversaturation of deep reservoirs in connection with the general reduction in surface discharge of CO2 by spring or geothermal waters during these periods. Host rock fracturing in response to seismic shaking and fluid overpressure must have resulted in rapid exsolution and expansion of the dissolved gas, leading to hydrothermal outflow and precipitation of the banded travertine within the fissure ridges. In both the above mentioned cases (Tivoli and Denizli), the formation of geothermal travertines is driven by the interplay between active and passive tectonic processes, with the fluid pressure likely playing the dominant role. To better understand this latter concept, it is relevant to report that, in the Yellowstone area, Aly and Cochran (2011) observed, with GPS and InSAR methods, significant deformation with no measurable displacements along any fault zone during the entire period of observation (1992–2009), thereby concluding that magmatic and hydrothermal

b

c

d

Fig. 10. Evolutionary model of a fissure ridge area, resulting from the interplay between active extensional tectonics (hypothetically assumed as perpendicular to the normal fault) and thermohydromorphological control in the hangingwall of a normal fault. (a) Fractures in a normal fault hangingwall under active tectonic extension and geothermal circulation. (b) Along the fractures, hot waters rise up to the surface depositing travertine. (c) The travertine area is growing in the hangingwall of the normal fault, with formation of travertine terraces, fissure ridges, and mounds. (d) Travertine area with extinct and active fissure ridges. (e) The lateral migration and extinction of fissure ridges is mainly caused by the inability of hot waters (i.e., insufficient artesian pressure) to rise up to the crest of the higher ridges. It follows that a new adjacent fissure ridge may start to form. This latter structure may be non-optimally oriented with comparison to the regional extension and be driven mainly by fluid pressure along an inherited fracture oblique to the master normal fault.

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L. De Filippis, A. Billi / Tectonophysics 548–549 (2012) 34–48

processes beneath the Yellowstone caldera were the main sources of deformation. An approximate estimate of the overpressure (p0) necessary to form veins of banded travertine within the studied fissure ridges can be obtained through the equations proposed by Gudmundsson (2011) (i.e., see example 9.1 in this textbook). If we consider a fissure 70 m long (L) (i.e., approximate mean length of fissure ridges studied in this paper at MHS and BRP) and 0.175 m thick (ΔuI) (maximum thickness = 0.25% of length as proposed by Gudmundsson, 2011), a travertine Young's modulus (E) and a Poisson's ratio (ν) of 20 GPa and 0.2 (García-del-Cura et al., 2012), respectively, we obtain:   2 p0 ¼ ΔI E=2L 1−ν ¼ 26 MPa

Assumptions and limitations to the above-estimated overpressure are extensively explained by Gudmundsson (2011) (see also Gudmundsson et al., 2002). It should also be considered that, for very porous travertines (porosity up to 25%), the Young's modulus may be as low as 2–3 GPa (García-del-Cura et al., 2012), thus providing a significantly lower value of p0 in the above equation (i.e., p0 ≈ 2–3 MPa). 5. Limitations and open questions To better understand the limitations of our results and hypotheses, below, we report a set of open questions concerning the studied fissure ridges: (1) Fissure ridges of MHS and BRP are not dated and therefore their temporal evolution is unknown. This undetermined factor may bias our hypothesized relationships between fissure ridge growth and active tectonics. An alternative explanation may be, in fact, that the studied fissure ridges have grown in different times under differently-oriented active tectonic extensions. No temporal constraints are, at present, available to support or refute this alternative explanation. (2) Geochemical data (including pH, temperature, or salinity) about the fissure ridges (at the time of their formation) of MHS and BRP and the sourcing fluids are not available. It is not therefore possible to understand whether these factors may have exerted a role in the fissure ridge growth and perhaps in their morphotectonic features (e.g., Fouke, 2011; Veysey et al., 2008). (3) The water supplies at MHS and BRP at the time of fissure ridge development are unknown as well as their possible variation with time. Also in this case, it is not therefore possible to understand whether these factors (water supplies and their variation with time) may have exerted a role in the fissure ridge growth and perhaps in their morphotectonic features. (4) No data are available concerning the fissure ridge erosion. This undetermined factor may therefore bias our morphometric analyses of Fig. 9. 6. Conclusions We conclude that, although the studied fissure ridges have developed and are still developing in geothermally and tectonically active areas, their genesis is only in part connected with active tectonics. Other factors such as fluid artesian pressure or the presence of an inherited fracture network (in the normal fault hangingwall) not necessarily oriented perpendicularly to the maximum active extension may have played a significant role in the development of the studied fissure ridges. Obviously, our conclusion about the non-exclusive role of active tectonics in the formation of fissure ridges at MHS and BRP may not be valid in other sites where active extension has been

considered predominant in the formation of fissure ridges (e.g., Terme San Giovanni fissure ridge, Italy; Brogi and Capezzuoli, 2009). The main inference of societal and industrial interest is that fluid escape from engineered subsurface storage of fluids (i.e., CO2 or CH4) may occur not only in strongly tectonic active areas but also in mildly active ones. Consequently, the injection of fluids into, not only faulted, but also simply fractured geologic reservoirs may lead to slow seepage. Future studies on fissure ridge travertines should, therefore, be focused on their age and origin of CO2 to identify the source of fluid seepage and therefore understand whether fissure ridges such as those studied in this paper may be reliably taken as analogs for post-emplacement seepage from artificial storages (e.g., Gilfillan et al., 2011; Shipton et al., 2004, 2005; Vrolijk et al., 2005). The life span of fissure ridges is, in fact, known to be on the order of 10 4 a (e.g., Uysal et al., 2007) and, as such, fissure ridges may help to understand the long term durability of subsurface disposals (Haszeldine et al., 2005). Acknowledgments We thank J. Fairley, A. Gudmundsson, and M. Liu for handling our manuscript and for their very constructive comments. We thank C. Faccenna, E. Anzalone, M. Brilli, M. Ozkul, M. Soligo, P. Tuccimei, and I. Villa for their cooperation and support in travertine studies. Appendix A. Supplementary data Supplementary data to this article can be found online at http:// dx.doi.org/10.1016/j.tecto.2012.04.017. References Allen, E.T., Day, A.L., 1935. Hot Springs of the Yellowstone National Park, 466. Carnegie Institution, Washington, DC. 525 pp. Allmendinger, R.W., Reilinger, R., Loveless, J., 2007. Strain and rotation rate from GPS in Tibet, Anatolia, and the Altiplano. Tectonics 26, TC3013, http://dx.doi.org/10.1029/ 2006TC002030. Altunel, E., Hancock, P.L., 1993. Morphology and structural setting of Quaternary travertines at Pamukkale, Turkey. Geological Journal 28, 335–346. Altunel, E., Hancock, P.L., 1996. Structural attributes of travertine-filled extensional fissures in the Pamukkale plateau, western Turkey. International Geology Review 38, 768–777. Altunel, E., Karabacak, V., 2005. Determination of horizontal extension from fissure-ridge travertines: a case study from the Denizli Basin, southwestern Turkey. Geodinamica Acta 18, 333–342. Aly, M.H., Cochran, E.S., 2011. Spatio-temporal evolution of Yellowstone deformation between 1992 and 2009 from InSAR and GPS observations. Bulletin of Volcanology 73, 1407–1419. Anderson, T.R., Fairley, J.P., 2008. Relating permeability to the structural setting of a fault-controlled hydrothermal system in southeast Oregon, USA. Journal of Geophysical Research 113, B05402, http://dx.doi.org/10.1029/2007JB004962. Atabey, E., 2002. The formation of fissure ridge type laminated travertine-tufa deposits microscopical characteristics and diagenesis, Kirsehir Central Anatolia. Bulletin of the Mineral Research and Exploration 123–124, 59–65. Bargar, K.E., 1978. Geology and thermal history of Mammoth Hot Springs, Yellowstone National Park, Wyoming. U. S. Geological Survey Bulletin 1444, 54. Barnes, I., Irwin, W.P., White, D.E., 1978. Global distribution of carbon-dioxide discharges and major zones of seismicity. United States Geological Survey, Water Resources Investigations 78-39, Open File report. Barton, C., Zoback, M.D., Moos, D., 1995. Fluid flow along potentially active faults in crystalline rocks. Geology 23, 683–686. Billi, A., Valle, A., Brilli, M., Faccenna, C., Funiciello, R., 2007. Fracture-controlled fluid circulation and dissolutional weathering in sinkhole-prone carbonate rocks from central-Italy. Journal of Structural Geology 29, 385–395. Bons, P.D., 2000. The formation of veins and their microstructures. Journal of the Virtual Explorer 2, http://dx.doi.org/10.3809/jvirtex.2000.00007 (paper 4). Brogi, A., 2004. Faults linkage, damage rocks and hydrothermal fluid circulation: tectonic interpretation of the Rapolano Terme travertines (southern Tuscany, Italy) in the context of the Northern Apennines Neogene–Quaternary extension. Eclogae Geologicae Helvetiae 97, 307–320, http://dx.doi.org/10.1007/s00015-004-1134-5. Brogi, A., 2008. Fault zone architecture and permeability features in siliceous sedimentary rocks: insights from the Rapolano geothermal area (Northern Apennines, Italy). Journal of Structural Geology 30, 237–256. Brogi, A., Capezzuoli, E., 2009. Travertine deposition and faulting: the fault-related travertine fissure-ridge at Terme S. Giovanni, Rapolano Terme (Italy). International

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