Elsevier

Tectonophysics

Volumes 548–549, 15 June 2012, Pages 34-48
Tectonophysics

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

https://doi.org/10.1016/j.tecto.2012.04.017Get rights and content

Abstract

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.

Highlights

► Understanding the relationships between surface degassing, fluid flow, and active tectonics ► Comparison between morphotectonic features from several active geothermal areas on the Earth ► Active tectonics plays a moderate role in making fractures pervious to geothermal fluids.

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 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 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, Altunel and Hancock, 1996; Hancock et al., 1999; Haluk Selim and Yanik, 2009; Temiz et al., 2009, Uysal et al., 2007, Uysal et al., 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, Shipton et al., 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, we attempt to understand the causes of travertine deposition at Mammoth Hot Springs and Bridgeport.

Section snippets

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 (Fig. 1, Fig. 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

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.

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, Uysal et al., 2009). In synthesis, these structures are composed by bedded travertine, which constitutes the bulk of fissure ridges (flanks and part of the axial

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

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.

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.

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