Structural Analysis of Silicic Lava Flow Margins at Obsidian Dome, California

Structural Analysis of Silicic Lava Flow Margins at

Obsidian Dome, California

Abigail Martens, B.S.

A Thesis Submitted to the Department of Geological Sciences California State University Bakersfield In Partial Fulfillment for the Degree of Masters of Geology

Spring 2017

Abigail Elizabeth Martens

2017

i Structural Analysis of Silicic Lava Flow Margins at

Obsidian Dome, California

Abigail Martens, B.S.

This thesis or project has been accepted on behalf of the Department of Geological Sciences by their supervisory committee:

Dr. William C. Krugh Assistant Professor of Geology, California State University, Bakersfield Committee Chair

Dr. Anthony Rathburn Professor of Geology, California State University, Bakersfield Committee Member

Dr. Graham Douglas Michael Andrews Assistant Professor of Geology, West Virginia University Committee Member

111 Contents List of Tables ...... v List of Figures ...... vi Abstract ...... 1 1 Introduction ...... 2 1.1 Models of Silicic Lava Emplacement ...... 2

1.1.1 Competing models of silicic lava growth and advance ...... 2

1.1.2 Structural components in silicic lavas...... 4

1.1.3 The 2011-2013 eruption of Cordn Caulle and prolonged silicic lava emplacement ... 5

1.1.4 Hypothesis...... 6

1.2 Obsidian Dome, eastern California ...... 7

1.2.1 Long Valley caldera and Mono-Inyo volcanic chain ...... 7

1.2.2 Obsidian Dome ...... 10

2 Methodology ...... 13

2.1 Data Collection ...... 13

2.2 Analysis of Structural Data ...... 15

3 Results ...... 16

3.1 Lithofacies and fabrics ...... 16

3.1.1 Unit 1A...... 16

3.1.2 Unit 1B ...... 16

3.1.3 Unit 1C ...... 16

3.1.4 Unit 2A...... 16

3.1.5 Unit 2B ...... 17

3.2 Structural Data ...... 24

3.2.1 Gully 1 ...... 24

iv 3.2.2 Gully 2 ...... 24

3.2.3 Gully 3 ...... 25

3.2.4 Gully 4 ...... 25

3.2.5 Gully 5 ...... 26

3.2.6 Gully 6 ...... 26

3.2.7 Gully 7 ...... 27

3.2.8 Gully 8 ...... 28

3.2.9 Gully 9 ...... 28

3.2.10 Gully 10 ...... 29

3.2.11 Gully 11 ...... 29

3.2.12 Gully 12 ...... 30

4 Discussion ...... 54

4.1 Lithological Architecture ...... 54

4.2 Structural Analysis ...... 54

4.2.1 Structural Comparisons Between Gullies ...... 54

4.2.2 Structural Similarities Between Different Sides of the Same Gully ...... 57

4.2.3 Structural Evidence of Break-Out Lobes ...... 57

4.3 The Origins of Gullies Incising the Flow Margins at Obsidian Dome ...... 57

5 Summary ...... 59

6 References ...... 60

List of Tables Table 1 - GPS coordinates of study sites at Obsidian Dome 22 Table 2 - Summary of foliation data at erosional and localized gullies 65

v List of Figures Figure 1 - Conceptual diagrams of exogenous and endogenous growth for a silicic lava 3 Figure 2 - Foliation patterns typical of silicic lavas 5 Figure 3 - Map of the Long Valley region 8 Figure 4 -V olcanic activity in the Mono-Inyo Craters volcanic chain over the last 5,000 years 10 Figure 5 - An aerial photo and surface illustration of Obsidian Dome, North and South Glass Creek Dome 11 Figure 6 - Schematic representation of the lithological layering within Obsidian Dome as discerned from fieldwork and drilling 12 Figure 7 - The idealized lithostratigraphy of textural units within an obsidian lava based on studies of outcrops and scientific drilling 12 Figure 8 - Google Earth image of each measured gully 15 Figure 9 - Examples of unit 1A lithofacies and ductile deformation textures 17 Figure 9 (continued) - Examples of unit 1A lithofacies and brittle deformation textures 18 Figure 10 - Examples of unit 1B lithofacies deformation textures 19 Figure 11 - Examples of unit 1C lithofacies textures and brittle-ductile deformation 20 Figure 12 - Examples of unit 2A lithofacies textures and ductile deformation 21 Figure 12 (continued) - Examples of unit 2A lithofacies textures and brittle-ductile deformation 22 Figure 13 - Examples of unit 2B lithofacies textures and brittle-ductile deformation 23 Figure 14 - Gully 1 (E) 31 Figure 15 - Gully 1 (W) 32 Figure 16 - Gully 2 (E) 33 Figure 17 - Gully 2 (W) 34 Figure 18 - Gully 3 (E) 35 Figure 19 - Gully 3 (W) 36 Figure 20 - Gully 4 (S) 37 Figure 21 - Gully 4 (NE) 38 Figure 22 - Gully 5 (S) 39

vi Figure 23 - Gully 5 (NE) 40 Figure 24 - Gully 6 (N) 41 Figure 25 - Gully 6 (S) 42 Figure 26 - Gully 7 (N) 43 Figure 27 - Gully 7 (S) 44 Figure 28 - Gully 8 (E) 45 Figure 29 - Gully 8 (W) 46 Figure 30 - Gully 9 (NW) 47 Figure 31 - Gully 9 (SE) 48 Figure 32 - Gully 10 (NW) 49 Figure 33 - Gully 10 (SE) 50 Figure 34 - Gully 11 (SE) 51 Figure 35 - Gully 12 (NE) 52 Figure 36 - Gully 12 (SW) 53 Figure 37 - Conceptual diagram based on observations at Obsidian Dome. 58

vii

Abstract The emplacement of silicic lavas is a rarely observed phenomenon and most of what is known about the processes is interpreted from ancient lavas. Recent lava emplacements in Chile have challenged existing models of lava emplacement and suggest that silicic lavas are emplaced via internal flow, similar to basaltic lavas, and that the margin advances by breeches know as break- out lobes. Obsidian Dome in eastern California is an ideal natural laboratory to test emplacement models by looking for break-out lobes and the characteristic structures and lithological architectures that they produce. Structural and lithological data from the northwest margin of Obsidian Dome constrain the margins of discreet break-out lobes that flowed independently in slightly different directions. The combination of lithology, flow-banding and stretching vesicle directions support simple models of break-out lobe structure, and disagree with the predicted structure of non-lobate margins. This interpretation challenges common assumptions about silicic lava emplacement, and supports observations made at Cordon Caulle, Chile.

1 1 Introduction Silicic lavas are outpourings of dacitic and rhyolitic magma on the Earth’s surface and are a common feature of large-scale volcanism in numerous tectonic regimes. Even though silicic lavas are common, very few have ever been observed forming, and models of their emplacement are based on studies of ancient examples and analog experiments. To better understand the emplacement of silicic lavas, this study focuses on the structural geology of the margins of Obsidian Dome, California. The goal of this study is to predict the occurrence of break-out lobes around the margins of Obsidian Dome, and to document their location, shape, size, and structural architecture. This study will help to put observations from recent eruptions in Chile in context, and to test between current models of silicic lava emplacement. At Cordn Caulle a silicic lava continued to flow for eight months after the magma stopped being fed from the vent (Tuffen, et al., 2013). The lavas advanced by the budding of lava (“break-out lobes”) at the distal margin. These were unexpected observations and require a reevaluation of flow processes that are typically thought of as being short in duration and incidental to explosive volcanism. 1.1 Models of Silicic Lava Emplacement 1.1.1 Competing models of silicic lava growth and advance Two end-member models (Figs 1A and B) have been described for the emplacement of silicic lavas: exogenous and endogenous. Exogenous growth occurs by the external flow of fresh lava, usually over the upper surface, to the advancing flow margin (Eichelberger et al. 1986; Fink & Griffiths 1998; Merle 1998; Ventura 2004; Shields et al. 2015). Endogenous growth occurs by internal flow of fresh lava to the flow margin via enclosed lava channels (e.g., lava tubes in basaltic lavas), with no flow of fresh lava on the upper surface. Exogenous flow models are the most commonly proposed and widely analyzed in field studies and analog and numerical simulations. In contrast, endogenous flow models, although very widely applied to basaltic lavas, have received very little attention in silicic lavas. This is largely due to the dominance of studies of silicic lava domes in volcanic craters (e.g., Mount Saint Helens, WA (Fink et al., 1990); Chaiten, Chile (Bernstein et al., 2013); and Soufriere Hills, Montserrat (Couch et al., 2003)), where endogenous growth is obvious and indisputable. However, in all these cases, the lava domes are small (<0.1 km3) and they are topographically confined to the crater until they spill over an edge and decrepitate as a pyroclastic density current. Despite this, many large volume silicic lavas (>0.1 km3) are not confined and have instead flowed considerable distances (≤5 km) away from their vents. In cases like these, for example, Cordon Caulle (Chile), Newberry Crater (OR), and Obsidian Dome, Medicine Lake, and Glass Mountain (CA), it is unclear whether or not endogenous growth was an important factor. Exogenous flow advances like a ‘tank track’ where the uppermost lava flows over and beyond older, lower lava, thus forming a radially spreading disk that is tongue-shaped in radial cross-

2 Fig.1A and 1B are illustrative examples of the two-end member models for a silicic lava flow emplacement (Andrews et al., 2016). section (e.g., Fink and Griffiths 1998, Merle 1998; Ventura 2004). In this regard, it is similar to an ice or salt glacier (e.g., Talbot & Pohjola 2009). Key structures to look for to support this model include: autobreccia and basal breccia at flow fronts, steep anticlines near the vent and more recumbent isoclinal folds at the front verging away from the vent, sub horizontal foliations and lineations from gravity-driven flattening causing plane strain (Castro, 2002), internal deformations are less oblate than surface and marginal deformations (Smith & Houston 1994, Merle 1998; Ventura 2001), and, crucially, more or less equal advance distances and total strain around the entire margin. A modification of this process, the ‘permeable foam model’ was first proposed at Obsidian Dome (Eichelberger et al., 1986), where a highly-inflated foam of pumiceous rhyolite is inferred to have flowed away from the vent and collapsed and densified to obsidian as it spread away (e.g., Castro et al., 1999). For this model to work, internal shearing would have to be present in the permeable foam to breakdown the vesicles to form obsidian. Internal shearing would be the dominant process followed by a compressional stress influenced by the weight of the lava.

3 Endogenous growth requires the internal channeling and supply of magma to the lava margin beneath an insulating carapace of older, cold lava (e.g., Merle 1988). Changes in the volume of channelized lava can lead to inflation and deflation of the carapace, where deflation follows from a marginal ‘break-out’ and lateral draining. Such break-outs are lobate in shape and localized at the ends of endogenous channels, and therefore advance of the lava is not steady around the entre margin; instead, advances are episodic or unpredictable. This process is best demonstrated during the advance and inflation of pahoehoe basaltic lavas (e.g., Hon et al. 1994; Self et al. 1998). Key structures to look for to support this model include: characteristic breakout flow lobes from the margins (e.g., Domagall-Semple 2008; Tuffen et al, 2013), prolate fabrics instead of a flattened structures, for example, sheath folds (Branney et al., 2004; Andrews & Branney, 2011), and doubly- imbricated folds; that is, folds at the base will verge towards the margin and those at the top will verge back towards the vent due to the undertow that they have experienced from the central channel. Break-out lobes are relatively easy to identify in active and some very young (Holocene) lavas, but not documented in any ancient (i.e. pre-Holocene) nor the majority of Holocene lavas. 1.1.2 Structural components in silicic lavas Foliations Flow banding is a primary foliation that can develop within extrusive volcanic rocks. It forms during emplacement and typically mirrors the surface over which the lava flowed. Flow banding is often sub horizontal at the base of the flow and often folded and faulted in the upper part of the flow. In general, foliation measurements are horizontal in the lower parts of lavas and grade upwards to steeper dips near the surface (Fig. 2). This is because of load where the principle strain axis is vertical and perpendicular to the underlying surface. The uppermost parts of lavas do not have the same amount of load acting upon them and instead accommodate vertical motion (folds or thrust faults) due to horizontal strain during flow. Folds Folds in lava form during ductile deformation (i.e. flow) before the lava was completely cooled. Because there is minimal difference in strength between flow bands, and the obsidian is generally very weak, folds form easily and can rotate to any angle during transport. As more stress is applied the folds will tighten reducing the interlimb angle. A lava that experienced strong deformation will have short wavelength and high amplitude folds. A lava that experienced weaker deformation will have long wavelength, low amplitude folds (e.g., Fink and Griffiths, 1998). Folds typically vary from harmonic to disharmonic, cylindrical to non-cylindrical along the hinge line, upright to recumbent, and symmetrical and asymmetrical folds. Sheath folds and irregular shaped (disharmonic, inhomogeneous) folds occur sporadically in strongly deformed areas; sheath folds are easily identified by ‘eye’-shaped cross-sections and elongation and hinges parallel to the stretching lineation.

Fig. 2. Modification of foliation patterns typical of silicic lavas. The red box indicates the foliation patterns. (Christiansen and Lipman, 1966)

Lineations Stretching lineations are used to show the transport flow direction and movement during deformation. Lineations in lavas are typically formed by stretching or flattening of vesicles that grew during flow as volatile gases exsolved from the melt. Flattening under a vertical maximum principle stress will produce oblate (‘pancake-shaped’) vesicles (e.g., Lisle, 1979; Fossen, 2016). Pure horizontal stretching will produce horizontal prolate (‘cigar-shaped’) vesicles. Typically, a vertically imposed load (i.e. flattening) combined with sub-horizontal flow (i.e. stretching) will produce ‘plane-strain’ (‘surfboard-shaped’) vesicles. Boudins Boudins form during extension of a competent rock layer bounded by less competent layers (Fossen, 2016). In lavas, boudins are found in strongly foliated rocks where some degree of strength difference has developed, for example, between rhyolite flow bands of different vesicularity. Boudins can form by ductile, or brittle, or both types of deformation. 1.1.3 The 2011-2013 eruption of Cordn Caulle and prolonged silicic lava emplacement The 2011-2013 Cordn Caulle, Chile, is one of only two scientifically monitored silicic lava emplacement events, and exhibited exogeneous growth (Tuffen et al., 2013; Farquharson et al.,

5 2015). The lava field is 35 m-thick, with over 80 break-out lobes, and by 2013 the frontal margins extended about 3.6 km from the vent (Tuffen et al., 2013). The advancement of lava continued even though the lava fed from the vent ceased 6-8 months prior. Continuing advance of lava after the eruption ceased at the vent supports the endogenous model of an insulated core with a cooled outer surface. The initial eruption began with a 27-hour long Plinian explosive eruption on June 4th 2011, followed by effusion of a 50 m-wide and 150 m-long lava flow starting on June 20th 2011 (Tuffen et al., 2013). This lava continued to grow until August 2011 (Tuffen et al., 2013). Between August 18th and October 9th 2011 break-out lobes were observed at the frontal margins of the flow (Tuffen et al., 2013). Frequent sub-Plinian and Vulcanian explosions at the vent continued until June 2012. March 15th 2012 is roughly when the effusion of lava from the vent stopped, however, the lava flow continued to advance until January 2013 (Tuffen et al., 2013). Direct observations of break- outs were made in the southeastern region on January 11th 2013, and over 80 break-out lobes were observed by January 13th 2013. Vertical changes in thickness, inferred to result from inflation, were recorded by photointerferometry (Farquharson et al., 2015). Between January 4th – 10th 2012 thickness changes of +5 to -5 m were observed, and were contemporaneous with advance at 50% of the margin surveyed. Between January 10th 2012 and January 17th 2013, thickness changes of +5 - +40 m were associated with very localized advance (30% of the surveyed margin length) by up to 30 m. The inflation of the lava flow and continuation after cessation of vent activity were more like the behavior associated with basaltic lavas than anticipated by current theories, and is unprecedented. Observations of the break-out process show that they occur at the sheared margins of previous break-out lobes and their continued formation can create complex internal structures that can be seen in the field. Cordn Caulle demonstrated that the internal thermal efficiency of a silicic lava flow is better maintained than previously expected; and because of this increases in lava transportation distance and duration, the associated hazards become increasingly more serious. 1.1.4 Hypothesis Break-out lobes have only been described and observed occurring at Cordn Caulle, where they are a critical component of the endogenous growth of the silicic lava and its prolonged advance. If endogenous growth is an important or necessary stage in the advance of large-volume silicic lavas, then the formation of break-out lobes may be important. If break-out lobes are indeed important, then they should be present around, at least, the margins of other large volume silicic lavas, like Obsidian Dome. Break-outs lobes will be identifiable at Obsidian Dome as discreet structural lobes separated from each other by carapace breccia, and demonstrating the same internal structures dominated by sub-horizontal strain fabrics and doubly imbricated fold patterns. The identification of break-out lobes at Obsidian Dome would be a major breakthrough in the study of silicic lavas as it would be the first identification in a pre-historic lava, and challenge the models of emplacement at Obsidian Dome, the most thoroughly studied silicic lava.

6 1.2 Obsidian Dome, eastern California 1.2.1 Long Valley caldera and Mono-Inyo volcanic chain Long Valley caldera (LVC) is in eastern California, west of the Basin and Range Province, and at the base of Sierra Nevada escarpment (Fig.3). The LVC is the product of several distinct magmatic events through time (Fig.3; Hildreth, 2004), each triggered by extension of the crust and decompression melting of the mantle. At 0.76 Ma melting produced a rhyolitic magma chamber that erupted explosively as the 600 km3 Bishop Tuff that formed the LVC. After the Bishop Tuff eruption, several large rhyolite lavas erupted within LVC. Activity moved to the western margin of the caldera at about 160 ka with the formation of the Mammoth Mountain trachydacite- trachyrhyodacite dome complex and 35 associated mafic vents. After 50 ka activity spread northwards for 30 km from Mammoth Mountain along the Mono-Inyo volcanic chain (Hildreth, 2004). The Mono-Inyo volcanic chain

The Mono-Inyo volcanic chain is controlled by intrusion of dikes along a N-S trending fault zone that starts at the west moat of the Long Valley caldera extending north for 45 km to Mono Lake (Bailey, 1989). The Mono-Inyo chain consists of post-caldera basalt and quartz-latite lavas erupted from vents and fissures; the volcanism started at the Mono Craters and ended at the Inyo Craters (Bailey, 1989). The Mono Craters are the northern most section of the Mono-Inyo chain, extending 17 km as an arcuate ridge which ranges from Mono Lake to June Lake (Bailey, 1989). The Mono Craters include roughly 30 pit craters and volcanic domes. The highest ridge in the chain is 610 m taller than the surrounding topography reaching 2,800 m above sea level.

CSUB

Fig.3 Modified map of the Long Valley Caldera area emphasis on Obsidian Dome and an outline of California with stars showing the location of Obsidian Dome as well as California State University (Andrews et al 2016)

8 Inyo Domes The Inyo Domes are a 9 km long volcanic chain (Fink, 1985) that intersects the northwest rim of Long Valley caldera. Inyo Domes were created by a silicic dike that advanced northward that separated into three en echelon segments. The Inyo Domes includes silicic lava domes and craters; from oldest to youngest: a small dome north of Deadman Creek, Wilson Butte, South Glass Creek, North Glass Creek, Obsidian Dome, and pit craters around Deer Mountain (Fig. 4). Obsidian Dome is the largest volume lava in the Inyo volcanic chain. The main eruptions were four rhyolitic lavas around 600 years ago: the North and South Deadman Creek lavas, the Glass Creek lava, and the Obsidian Dome lava. Other domes and craters contain ash fall tephra and are older. The four major Inyo Domes lavas include both phenocryst- poor and phenocryst-rich rhyolite. Coarsely porphyritic lava with a white vesicular groundmass makes up the lava at the centers of Glass Creek and the Deadman domes. The eruptions started in the southern portion of the Inyo chain at the Deadman Creek vent. A major fault was intruded by a dike near the surface; and en route hit an aquifer causing a phreatic eruption that created a pyroclastic deposit to the northeast (Higgins et al., 2009). Immediately after the explosive eruption ceased a lava was erupted from the same vent. The next eruption occurred in the northern region of the Inyo volcanic chain at Obsidian Dome; this eruption also erupted a tephra deposit to the northeast. Finally, an eruption in the middle of the Inyo chain occurred at Glass Creek (Miller, 1985).

Fig. 4. Modification of volcanic activity in the Mono-Inyo Craters volcanic chain of the past 5,000 years, (Hill D et al. 2004). 1.2.2 Obsidian Dome Obsidian Dome is a 550 – 650 year – old silicic lava (Fig. 5). The eruption was initially an explosive eruption that deposited rhyolitic tephra that spanned over 25 km and blankets 140 km2 area. A pyroclastic density current deposit soon followed and covered the tephra. Obsidian Dome is 0.1 km3, elliptical (1.5 by 1.8 km-diameter), and 30 - 50 m thick at its margins and at the vent the thickness increases to 100 m (Eichelberger, et al., 1986). Obsidian Dome has sharp steep walls, with a talus slope of millimeter- to meter-diameter, angular clasts. Obsidian Dome characteristically consists of black obsidian, and light brown, gray, and orange rhyolite.

10 Phenocrysts are roughly 10% of the rock and the top and bottom portions of the dome are glassy whereas center portions are more crystalline. Scientific Drilling at Obsidian Dome Eichelberger et al. (1985) summarizes the results of the Inyo Drilling Program (1984) that was designed to better understand the pre-lava explosive eruption, lava effusion, and associated hydrothermal systems. Drill hole RDO-2A is in the southern distal edge of Obsidian Dome (Fig. 6) and cored through 50 m through the whole lava. The second drilling was from August 16th to September 9th, 1984. The drill hole (RDO-2B) was inclined and intersected the conduit beneath Obsidian Dome (Fig. 6). The drilling data from Obsidian Dome and observations from other silicic lavas are summarized in Figure 7, where the lithological layering within the lava is shown. A third drilling site (RDO-3) was between the Obsidian Dome and Glass Creek lavas to insect the underlying feeder dike from September 18th to October 28th, 1984.

Fig. 5. LiDAR imaging of the Inyo volcanic chain to the right, and to the left is a zoomed section of Obsidian Dome. Courtesy of the National Center of Airborne Laser Mapping (NCALM).

Fig. 6. Schematic representations of the lithological layering within Obsidian Dome as discerned from fieldwork and drilling. Adapted from Fink & Manley (1987), Manley & Fink (1987), Fink et al. (1992). Right – Structural fabrics within core RDO-2A (adapted from Fink & Manley 1987). FVP – fine vesicular pumice, CVP – coarse vesicular pumice, OBS – obsidian, RHY – microcrystalline rhyolite.

Fig. 7. The idealized lithostratigraphy of textural units within an obsidian lava based on studies of outcrops and scientific drilling (Stevenson et al. 1994). BR – autobreccia, OBS – obsidian, CVP – coarse vesicular pumice, FVP – fine vesicular pumice.

12 2 Methodology 2.1 Data Collection Various methods were used at Obsidian Dome to determine the structure, strain, and continuity of silicic lava units. The first was to find gullies with adjacent cliff exposures of in situ rhyolite, and then to describe the talus to help determine what might be in the gully above. The term gullies in this research is describing the two adjacent side walls of rhyolitic lava that have been separated by a channel of talus. The gullies are approximately 130 m apart; and 19 meters wide. A GPS coordinate (in decimal degrees, DD) was taken at each side of a gully with an elevation measurement in meters (Table 1; Figure 8). Each GPS coordinate was labeled (AM00#) where # corresponds with each side of a gully in the order they are measured. The vertical measurements within the column were recorded by Abigail Martens’ ‘eye height’ (5’0”). ‘Eye height is determined by a measuring tape place on to the floor to the person’s eye level and the distance from the floor to eye level determines the eye height. This only used a rough estimate of vertical change. A Jacobs staff was not used to measure the vertical change throughout the column, because the steep walls of Obsidian Dome would have been too dangerous to safely take measurements this way. A lithostratigraphic column was constructed with additional space reserved for foliation and lineation measurements. The recording vertical change using ‘eye height’ and structural measurementprocess was repeated for each meter or two, with contacts between different lithostratigraphic units identified and recorded as necessary. Areas that were highly fractured or covered by talus were recorded as necessary. Each gully has two stratigraphic columns and associated measurements, one on either side (Figures 14-36). Twelve gullies around the northwestern margin of Obsidian Dome were explored.

13 Name Location (°) Location (°) Elevation (m) AM001 N 37.75789 W 119.02529 2541 AM002 N 37.75785 W 119.02541 2552 AM003 N 37.75776 W 119.02519 2550 AM004 N 37.75773 W 119.02513 2546 AM005 N 37.75794 W 119.02637 2527 AM006 N 37.75788 W 119.02630 2563 AM007 N 37.76025 W 119.02721 2538 AM008 N 37.75985 W 119.02744 2546 AM009 N 37.76130 W 119.02662 2495 AM010 N 37.76162 W 119.02657 2535 AM011 N 37.76271 W 119.02578 2519 AM012 N 37.76293 W 119.02546 2530 AM013 N 37.76431 W 119.02391 2520 AM014 N 37.76422 W 119.02392 2514 AM015 N 37.76598 W 119.02014 2488 AM016 N 37.76613 W 119.02015 2485 AM017 N 37.76553 W 119.01642 2491 AM018 N 37.76546 W 119.01623 2476 AM019 N 37. 76535 W 119.01500 2467 AM020 N 37.76514 W 119.01476 2450 AM021 N 37.76437 W 119.01352 2433 AM022 N 37.76128 W 119.02677 2539 AM023 N 37.76151 W 119.02677 2533

Table 1 are the decimal degree and elevation recorded at each side of a single gully. The base of the outcrop was described and multiple pictures were taken of various structures. Each picture has a scale bar, orientation of the picture, brief rock description, and the height that the photograph was taken. Foliation of flow-banding and lineation of stretched vesicles of fold hinge measurements were taken if available to measure. Typically, five representative foliations and five representative lineations were measured every one or two meters moving upwards through the unit on each side of the gully.

14 Gully 9Gully Gully 8 Gully Gully 7

Gully 6 Gully 5 Gully Gully 4

Gully 3 Gully 1 Gully 2

Figure 8 shows an aerial view of all the gullies measured at Obsidian Dome

2.2 Analysis of Structural Data Structural data collected at Obsidian Dome in the twelve gullies explored were analyzed in Stereonet 9© (version 9.9.5 Windows 64 bit; Allmendinger et al., 2012; Cardozo and Allmendinger, 2013). Data were first entered into Microsoft Excel™ 2016 by hand in batches representing 10 m increments, and as either foliation (i.e. planar) data or lineations (i.e. linear) data. Data from a single side of each gulley were loaded separately. The Excel datasheets were then formatted to be read by Stereonet 9 and saved as text files (.txt). Stereonet 9 requires data to be entered as either planar or linear, although data can be projected together after loading. Foliation data are presented as poles to planes. Lineations are presented as points on stereonets (trend and plunge) and in superimposed rose diagrams (histograms of trend only). Stereonet 9 was used to contour the foliation data using the Kamb method. The Kamb method identifies the mean of the foliation data and draws two sigma (95% confidence) contours around the remaining data (Kamb, 1959).

15 3 Results Data from Obsidian Dome is divided into lithological and structural types. Lithological units are distinctive rhyolitic lithofacies identified in multiple locations. Structural data are presented for each location (i.e. each gully-side). 3.1 Lithofacies and fabrics Two distinctive lithological facies are identified at Obsidian Dome. The first (Unit 1) is massive rhyolite of varying crystallinity and vesicularity. The second (Unit 2) is texturally heterogeneous rhyolite of intercalated flow bands of highly variable crystallinity and vesicularity. Figures 9 to 13 show lithofacies 1A, 1B, 1C, 2A, and 2B. 3.1.1 Unit 1A Unit 1A occurs in gullies1–3, 5-7, and 9-12. Unit 1A is massive black obsidian composed of ≥80% glassy groundmass, ≤20% 1 mm-diameter white spherulites (Fig. 9A). Weathered surfaces range in color from orange to gray to chalky white (Fig. 9B). Unit 1A has faint black and gray subparallel flow bands spaced 1 mm apart, where the black glassy flow bands are separated by variable pumiceous bands. Surface textures include hackly fractured obsidian (Fig. 9 .1G), blocks of black obsidian with gray weathering surfaces, parallel bulbous to ropey textures developed along flow bands exposed by fractures, and zones of brecciated black obsidian (Figs 9.1D and 10.1H). The higher up a gully the more fractured and difficult to measure Unit 1A becomes. 3.1.2 Unit 1B Unit 1B is consistently stratigraphically and structurally above Unit 1A, and occurs in gullies1-4, 6, and 7. Unit 1B is pumiceous rhyolite composed of ≥75% light gray, glassy groundmass, ≤20% 1 mm-diameter blocky to sub-rounded spherulites, and ≤5% 0.5 mm-long elongated obsidian clasts (Fig. 10A). Flow bands are up to 20 cm-thick. Like in Unit 1A fractures and brecciation become more intense upwards through Unit 1B. 3.1.3 Unit 1C Unit 1C occurs in gullies 7-9, 11, and 12. Unit 1C is transitional between 1A and 1B, and is composed of approximately equal proportions of black obsidian and grey pumiceous rhyolite (Fig.12 A). Surface textures include ropey texture (Fig. 12B) and brecciated zones. One diorite xenolith was observed (Fig. 12D). 3.1.4 Unit 2A Unit 2A occurs in gullies 3,4, and 6. Unit 2A is composed of three distinct layers: 1. Black, glassy, obsidian with sub-rounded white glassy to smoky spherulites up to 3 mm- diameter and subparallel 0.5 – 1 mm-thick, gray flow bands; 2. medium brown to gray, pumiceous zones; and 3. light gray, with brown weathering surfaces, glassy groundmass with abundant euhedral to subhedral, 3 mm-sized white spherulites (Fig. 12A).

16 3.1.5 Unit 2B Unit 2B occurs in gully 6. Unit 2B is composed of 50 % black obsidian, 30 % light gray pumice, and 20 % light gray, crystalline rhyolite. Unit 2B is more vesicular than unit 2A, and is above unit 2A (Fig. 13A). Both units 2A and 2B feature crystalline and pumiceous rhyolite layers being stretched and boudinaged within a larger mass of obsidian (Fig. 13B), and many stretched pumiceous layers exhibit string-snot structures (Figs 13D and F). Pumice vesicles are strongly deformed (Fig. 13C).

Fig. 9. Examples of unit 1A and ductile deformation textures. Image A. P 515 (AM020) facing SW. Unit 1A at 37 m a.g. Image B. P 304 (AM006) facing north. Unit 1A parallel flow banding with a tight parallel fold (upper center of picture) at 35 m a.g. Image C. P 184 (AM002) facing north. Tight fold with tension gashes on edges of unit 1A at 10 m a.g. Image D. P 465 (AM013) facing SE. Brecciated section on the left and tight fold on the right, unit 1A 0 m a.g.

17 Fig. 9 (continued). Examples of unit 1A and brittle deformation textures. Image E. P 471 (AM013) facing SE, 5 reverse faults in unit 1A at 28 m a.g. Image F. P 149 (AM002) facing north. Unit 1A with tension gashes ~ 12 cm long at 20m a.g. Image G. P 137 (AM001) facing east. Unit 1A with hackly texture (possible cooling texture) at 20 m a.g. Image H. P 484 (AM014) facing south. Weathered brecciated zone in Unit 1A at 38 m a.g.

Image C. 263 (AM P 001)Image facing

ecumbent folding in unit 1B at 25 m a.g. unitfolding at in 1B ecumbent 25 m

w banding in unit 1B at 2 m a.g. bandingw at in unit2 m 1B B. 379P Image facing SE. (AM008) sized Nearlywith tension verticalcm boudin Fig. 10. Examples of unitFig. 10. Examplestextures. of 1B A. 155 (AM003)P Image facing north. obsidiangray, vesicular, with black Light flo a.g. m at 35 1B in unit gashes tension with texture ropey vertical clasts, circular gashes, east. r Isoclinal

Fig.11. Examples of unit 1C textures and brittle-ductile deformations. Image A. P 490 (AM015) facing south. Obsidian and pumice flow banding with pumice boudins at 0 m a.g. Image C. P 491 (AM016) facing south. Ropey surface texture on unit 1C at 32 m a.g. Image C. P 494 (AM016) facing east. Banding of obsidian and pumice, S-shaped folds, pumice boudins in unit 1C at 43 m a.g. Image D. P 498 (AM016) facing east. Diorite xenocryst in unit 1C at 48 m a.g. Image E. P 478 (AM014) facing SE. Cuspate fold in unit 1C at 20 m a.g.

Fig.12. Unit 2A textures and ductile deformations. Image A. P 325 (AM007) facing south. Rhyolite boudins that are angular, circular, tight folds, and flow banding in rhyolite in unit 2A at 24 m a.g. Image B. P 284 (AM006) facing east. Rhyolite boudins elongated vertically, s-shaped rhyolite folds, elongated pumice horizontally, S-shaped flow banding all orientated NW-SE in unit 2A at 19 m a.g. Image C. P 368 (AM008) facing SE. Cuspate fold in two planes in unit 2A at 19 m a.g. Image D. P 297 (AM006) facing south. Obsidian and rhyolite sheath folding in two planes in unit 2A at 24 m a.g. Image E. P 292 (AM006) facing west. Fold hinge of obsidian and tight folding in unit 2A at 21 m a.g.

F G

H I

Fig.12 (continued) shows examples of unit 2A textures and brittle-ductile deformations. Image F. P 281 (AM006) facing south. Nearly vertical flow banding with a deformed rhyolite clast that has a circular rhyolite clast enveloped in cm sized flow banding, and to the left, westerly expanding pumice in unit 2A at 18 m a.g. Image G. P 374 (AM008) facing SW. Ropey surface texture on unit 2A at 31 m a.g. Image H. P 372 (AM008) facing SE. Weathered ropey texture in unit 2A at 26 m a.g. Image I.P. 318 (AM007) facing SW, rhyolite boudins with 2 cm gaps in unit 2A at 12 m a.g.

B. P 437 (AM011) P rhyolite B. facingbandingunit SE, and clastspumiceat boudinsflow in Image 2B in

Fig. 13. Examples of unit of Fig. and 13. Examples textures brittle-ductile deformations. 2B flow rich 435 P pumice A. facing SE, (AM011) Image a.g. bandingm at in 2B unit 0 a.g. 19 m a.g. D. at C. Image 44515 m P surrounded Image by obsidian facingin east.Pumice vesicles2B (AM012) unit deformed 298 (AM006)P facing SW. Expanded withata.g. in snot2B unit 26 m pumice and surfaces stringy structure ropey 449 P E. Image (AM012) facing east. a.g. at 2B 15 m cuspate unit fold Possible F.north. 283 (AM006) Image P facing sized layer, Cm obsidian a.g. capturing surroundedsnot by pumice,unit boudins, andpresent rhyolite stringy structureat in 2B pumice 18 m P Image G. 458 facing SE. a.g. at Possible21 m reverse in faulting 2B unit H. 448(AM012)cuspatein P plane fold Image facing east. Two a.g. unit at 2B 15 m

23 3.2 Structural Data 3.2.1 Gully 1 Gully 1 east-side (E) Gully 1 (E) is the east side of gully 1. The east side of gully 1 is 25 meters tall; 23 meters is composed of rhyolite and 2 meters of talus (Fig.14). Unit 1A makes up the lower most 20 meters and Unit 1B makes up the upper 3 meters. The base of unit 1A is buried by talus and the upper margin of unit 1B is eroded. The majority of foliations are close to horizontal, and some dip moderately or steeply southeast. Interval 0 - 9 m foliations are mostly horizontal and some dip moderately to steeply southeast. Interval 10 - 19 m foliations are close to horizontal. Interval 20 - 29 m foliations are dipping gently to the east. Interval 0 - 9 m Lineations are plunging northeast; interval 10 - 19 m Lineations are plunging west and east, see figure 14. Gully 1 west-side (W) Gully 1 (W) is the west side of gully 1. The west side of gully 1 is 33 meters tall, 28 meters is composed of rhyolite and 5 meters of talus (Fig.15). Unit 1A makes up the lower most 25 meters of lava. Unit 1B makes up the upper 3 meters. The base of unit 1A is buried by talus, and the upper margin of unit 1B is eroded. The majority of foliations are close to horizontal. Interval 0 - 9 m foliations are mostly horizontal, some dip moderately or steeply southeast, northeast, and northwest, and some gently dipping north to south. Interval 10 - 19 m foliations are close to horizontal, one steeply dipping northeast, one steeply dipping northwest, and two gently dipping north. Interval 20 - 29 m foliations are horizontal. Interval 0 - 9 m Lineations plunge from west through northwest to north. Interval 10 - 19 m Lineations plunge northwest, north east, and one plunges to the southwest, see figure 15. 3.2.2 Gully 2 Gully 2 (E) Gully 2 (E) is the east side of gully 2. The east side of gully 2 is 27 meters tall, 22 meters is composed of rhyolite and 5 meters of talus (Fig.16). Unit 1A makes up the lowermost 19 meters of lava; Unit 1B makes up the upper 3 meters. The base of unit 1A is buried by talus and the upper margin of unit 1B is eroded. The majority of foliations are moderately or steeply dipping southeast, some are horizontal and some dip steeply to the northeast. Interval 0 - 9 m foliations are mostly horizontal, some dip moderately southeast. Interval 10 - 19 m foliations are mostly moderately or steeply dipping southeast, and some are horizontal. Interval 20 - 29 m foliations are steeply dipping northeast. Lineations are plunging mostly northeast to southwest see figure 16. Gully 2 (W) Gully 2 (W) is the west side of gully 2. The west side of gully 2 is 25 meters tall; 23 meters is composed of rhyolite and 2 meters of talus (Fig.17). Unit 1A makes up the lower most 20 meters of lava. Unit 1B makes up the upper 3 meters. The base of unit 1A is buried by talus and the upper margin of unit 1B is eroded. Interval 0 - 9 m foliations are mostly horizontal, some dip

24 moderately northeast, northwest, and southeast, and three steeply dipping northeast, one steeply dipping southeast and one steeply dipping southwest. Interval 10 - 19 m foliations are steeply dipping northeast. Interval 20 - 29 m foliations are mostly steeply dipping west and one steeply dipping east. No lineations were measured, see figure 17. 3.2.3 Gully 3 Gully 3 (E) Gully 3 (E) is the east side of gully 3. The east side of gully 3 is 42 meters tall; 24 meters is composed of rhyolite and 18 meters of talus (Fig.18). Unit 2A makes up the lowermost 19 meters of lava and the uppermost 5 meters. The base of unit 2A is buried by talus, and the upper margin of unit 2A is eroded. In between, are alternating layers of unit 2A and unit 1B. Interval 10 - 19 m foliations are mostly horizontal, some dip moderately northeast, and some gently dipping south. Interval 20 - 29 m foliations are moderately or steeply dipping south, some are moderately dipping north, and one moderately dipping southwest. No foliations are measured in interval 30 - 39 m. Interval 10 - 19 m lineations are plunging mostly west and east, and two plunging southwest. Interval 20 - 29 m lineations are plunging mostly southeast and west with a few plunging north, southwest, northwest, and northeast. Interval 30 - 39 m lineations are plunging southeast and southwest and one plunging south, see figure 18. Gully 3 (W) Gully 3 (W) is the west side of gully 3. The west side of gully 3 is 46 meters tall, 34 meters is composed of rhyolite and 10 meters of talus (Fig.19). Unit 1A makes up the lowermost 19 meters of lava and unit 2A makes up the upper 17 meters. The base of unit 1A is buried by talus, and the upper margin of unit 2A is eroded. The majority of foliations are horizontal or steeply dipping north, one steeply dipping northeast and one moderately dipping northwest. Interval 10 - 19 m foliations are mostly horizontal, some dip moderately northwest, northeast, and south, and one steeply dipping northeast. Interval 20 - 29 m foliations are either horizontal or steeply dipping north and northeast. Interval 30 - 39 m are either horizontal or steeply dipping north. Lineations are plunging mostly northwest and southeast, and some northeast and southwest, see figure 19. 3.2.4 Gully 4 Gully 4 (S) Gully 4 (S) is the south side of gully 4. The south side of gully 4 is 38 meters tall, 30 meters is composed of rhyolite and 8 meters of talus (Fig.20). Unit 2A makes up the lowermost 27 meters of lava and unit 1B makes up the upper 3 meters. The base of unit 2A is buried by talus and the upper margin of unit 1B is eroded. Interval 10 - 19 m foliations are mostly horizontal, some moderately dipping south, southeast and southwest. Interval 20 - 29 m foliations are mostly steeping south, and gently dipping north, southeast and southwest. Interval 30 - 39 m are moderately dipping northeast and west, and steeply dipping south and southwest. Interval 10 - 19 m lineations are plunging near north, northeast, west, west and one south. Interval 20 - 29 m

25 lineations are plunging southeast, southwest, west, northwest and one plunging northeast. No lineations were measured in interval 30 - 39 m, see figure 20. Gully 4 (NE) Gully 4 (NE) is the northeast side of gully 4. The northeast side of gully 4 is 40 meters tall; 28 meters is composed of rhyolite and 12 meters of talus (Fig.21). Unit 2A composes all 28 meters of the lava. The base of unit 2A is buried by talus, and the upper margin of unit 2A is eroded. Interval 10 - 19 m foliations are horizontal to moderately or steeply dipping south, north northeast, and southwest. Interval 20 - 29 m foliations are gently to moderately dipping south, southeast, and northeast. Interval 30 - 39 m are mostly steeply dipping south, and south east with one horizontal. No lineations were measured in Interval 10 - 19 m. Interval 20 - 29 m lineation is plunging southeast. Interval 30 - 39 m are plunging southwest and northeast, see figure 21. 3.2.5 Gully 5 Gully 5 (S) Gully 5 (S) is the south side of gully 5. The south side of gully 5 is 38 meters tall; 23 meters is composed of rhyolite and 15 meters of talus (Fig.22). Unit 1A composes all 23 meters of lava. The base of unit 1A is buried by talus and the upper margin is eroded. Interval 10 - 19 m foliations are horizontal to gently or moderately dipping northeast and southeast. Interval 20 - 29 m foliations are horizontal to gently dipping northeast and northwest. Interval 30 - 39 m are horizontal and gently dipping southeast. No lineations were measured in snterval 10 - 19 mand 30 – 39 m. Interval 20 - 29 m has lineations plunging to the north, south, northwest, and southwest, see figure 22. Gully 5 (NE) Gully 5 (NE) is the northeast side of gully 5. The northeast side of gully 5 is 48 meters tall; 33 meters is composed of rhyolite and 15 meters of talus (Fig.23). Unit 1A composes all 33 meters of lava. The base of unit 1A is buried by talus and the upper margin is eroded. Interval 10 - 19 m foliations are moderately or steeply dipping northwest. Interval 20 - 29 m foliations are horizontal to gently or moderately dipping northeast and northwest. Interval 30 - 39 m foliations are gently or moderately dipping northeast and gently dipping northwest. Interval 40 - 49 m foliations are gently dipping to the east. Interval 10 - 19 m lineations are all plunging northwest. Interval 20 - 29 m lineations are mostly plunging northeast, three are plunging west, and one plunging southwest. Interval 30 - 39 m lineations are mostly plunging northwest, one plunging northeast, and one plunging southeast, see figure 23. 3.2.6 Gully 6 Gully 6 (N) Gully 6 (N) is the north side of gully 6. The north side of gully 6 is 38 meters tall; 23 meters is composed of rhyolite and 15 meters of talus (Fig.24). Unit 2B makes 24 meters of lava and unit 1A is the uppermost 5 meters. The base of unit 2B is buried by talus and the upper margin of unit

26 1A is eroded. Interval 10 - 19 m foliations are gentle or moderately dipping northeast, and northwest. Interval 20 - 29 m foliations are horizontal to moderately or steeply dipping east, northeast, southeast, and three steeply dipping west. Interval 30 - 39 m foliations are moderately or steeply dipping southeast, and one horizontal. Interval 10 - 19 m lineations are plunging west, northwest, and northeast. Interval 20 - 29 m lineations are mostly plunging north and south, some southwest, southeast, northeast and one is plunging west. No lineations are measured in Interval 30 - 39 m, see figure 24. Gully 6 (S) Gully 6 (S) is the south side of gully 6. The south side of gully 6 is 44 meters tall; 29 meters is composed of rhyolite and 15 meters of talus (Fig.25). The base of unit 2B is buried by talus and the upper margin of unit 1B is eroded. Unit 2A makes all 3 meters of lava. Unit 2A is the uppermost 9 meters. On the top of unit 2A is unit 1B is eleven meters. Interval 10 - 19 m foliations are horizontal to gentle or moderately dipping northeast, east, and southeast. Interval 20 - 29 m foliations are moderately to steeply dipping northeast, northwest, and southeast. Interval 30 - 39 m foliations are moderately or steeply dipping northwest, northeast, and southeast. Interval 10 - 19 m lineations are plunging northeast, and southwest. Interval 20 - 29 m lineations are mostly plunging northeast, some are plunging north, and one is plunging west. No lineations were measured in interval 30 - 39 m, see figure 25. 3.2.7 Gully 7 Gully 7 (N) Gully 7 (N) is the north side of gully 7. The north side of gully 7 is 44 meters tall; 24 meters is composed of rhyolite and 20 meters of talus (Fig.26). The base of unit 1C is buried by talus, and the upper margin of unit 1B is eroded. Unit 1C makes up the lower 11 meters of exposed lava, interlayered with 3 meters of unit 2A between meters 22 to 25. Unit 1A, 7 m-thick, overlies unit 1C and, in turn, capped by 3 meters of Unit 1B. No foliations were measured in Interval 10 - 19 m. Interval 20 - 29 m foliations are gently or moderately dipping south, and southeast. Interval 30 - 39 m foliations are moderately or steeply dipping southeast, east, and northwest. No lineations were measured in Interval 10 - 19 m. Interval 20 - 29 m Lineations are mostly plunging mostly to the southwest, two plunging southeast, two northeast, and one east. Interval 30 - 39 m lineations are all plunging southwest, see figure 26. Gully 7 (S) Gully 7 (S) is the south side of gully 7. The south side of gully 7 is 49 meters tall; 21 meters is composed of rhyolite and 28 meters of talus (Fig.27). Unit 1A makes up 16 meters of lava, unit 1B the remaining 5 meters. The base of unit 1A is buried by talus and the upper margin of unit 1B is eroded. Foliations range from horizontal to gently or moderately dipping south and southeast. Interval 20 - 29 m foliations are horizontal to gently or moderately dipping south, and southeast. Interval 30 - 39 m foliations are horizontal, and some are gently dipping south, and southeast. Lineations are plunging mostly north, south, no plunging southwest, one plunging

27 northwest, and one plunging northeast. Interval 20 - 29 m Lineations are mostly plunging mostly to the southwest, two plunging southeast, two northeast, and one east, see figure 27. No lineations or foliations were measured in Interval 10 – 19 m; no lineations were recorded in Interval 30 – 39 m. 3.2.8 Gully 8 Gully 8 (E) Gully 8 (E) is the east side of gully 8. The east side of gully 8 is 60 meters tall, 30 meters is composed of rhyolite and 30 meters of talus (Fig.28). Unit 1C makes up all 30 meters of lava. The base of unit 1C is buried by talus, and the upper margin of unit 1C is eroded. Interval 30 - 39 m foliations are horizontal to gently or moderately or steeply dipping north, south, northeast, and one steeply dipping northwest. Interval 40 - 49 m foliations are mostly horizontal, and some are gently dipping northwest, northeast, and southwest. Interval 50 - 59 m foliations are horizontal and steeply dipping north. Lineations are plunging mostly plunging northwest, and southeast. No lineations were measured in intervals 30 – 39 m and 40 – 49 m. Interval 50 - 59 m lineations are plunging northwest, and southeast, see figure 28. Gully 8 (W) Gully 8 (W) is the west side of gully 8. The west side of gully 8 is 51 meters tall; 16 meters is composed of rhyolite and 35 meters of talus (Fig.29). Unit 1C makes up all 16 meters of lava. The base of unit 1C is buried by talus and the upper margin of unit 1C is eroded. Interval 30 - 39 m foliations are horizontal to gently or moderately dipping northeast, north, and northwest, and one is steeply dipping south. Interval 40 - 49 m foliations are mostly gently or moderately dipping north northwest, and northeast. No foliations are measured in Interval 50 - 59 m. No lineations were measured, see figure 29. 3.2.9 Gully 9 Gully 9 (NW) Gully 9 (NW) is the northwest side of gully 9. The northwest side of gully 9 is 46 meters tall; 19 meters is composed of rhyolite and 27 meters of talus (Fig.30). Unit 1C makes up 18 meters of lava and is capped by 1 meter of Unit 1A. The base of unit 1C is buried by talus and the upper margin of unit 1A is eroded. No foliations were measured in intervals 10 - 19 mand 30 – 39 m. Interval 20 - 29 m foliations are moderately or steeply dipping southeast, south, and southwest. Interval 40 - 49 m foliations are steeply dipping southwest. Interval 20 - 29 m Lineations are mostly plunging to the southwest, two plunging southeast, two northeast, and one east. Interval 40 – 49 m Lineations are plunging mostly northeast, one is plunging east, and one is plunging west, see figure 30. No lineations were measured in Interval 30 – 39 m. Gully 9 (SE) Gully 9 (SE) is the south side of gully 9. The southeast side of gully 9 is 53 meters tall; 24 meters is composed of rhyolite and 29 meters of talus (Fig.31). Unit 1C makes up 11 meters of lava and

28 intercalated with 13 meters of Unit 1A at meters 32-36 and 42-51. The base of unit 1C is buried by talus and the upper margin is eroded. Interval 20 - 29 m foliations are gently dipping northeast. Interval 30 - 39 m foliations are horizontal to gently or moderately dipping northeast, some gently dipping northeast, and south. Interval 40 - 49 m foliations are horizontal to gently or moderately, or steeply dipping northwest. Interval 20 - 29 m lineations are all plunging northeast. Interval 30 – 39 m lineations are plunging northwest, north, and northeast, and one plunging south, see figure 31. 3.2.10 Gully 10 Gully 10 (NW) Gully 10 (NW) is the northwest side of gully 10. The northwest side of gully 10 is 50 meters tall; 28 meters is composed of rhyolite and 22 meters of talus (Fig.32). Unit 1A makes up all 28 meters of lava. The base of unit 1A is buried by talus and the upper margin of unit 1A is eroded. Interval 20 - 29 m foliations are gently dipping northeast. Interval 30 - 39 m foliations are horizontal to moderately or steeply dipping, east, southeast, and south. Interval 40 - 49 m foliations are steeply dipping northwest, west, southwest, and southeast. Interval 20 - 29 m lineations are plunging northeast, southeast, southwest, and two plunging south. No lineations were measured in Interval 30 - 39 m. Interval 40 - 49 m lineations are plunging southwest, northwest, and northeast, see figure 32. Gully 10 (SE) Gully 10 (SE) is the southeast side of gully 10. The southeast side of gully 10 is 50 meters tall; 18 meters is composed of rhyolite and 33 meters of talus (Fig.34). Unit 1A makes up all 18 meters of lava. The base of unit 1A is buried by talus and the upper margin of unit 1A is eroded. No foliations or lineations were measured in intervals 20 - 29 m and 40 – 49 m. Interval 30 - 39 m foliations are gently or moderately or steeply dipping, northwest, northeast, east, southeast and southwest. Interval 30 - 39 m lineations are plunging mostly northeast, and some are north, southwest, and southeast, and one is steeply dipping northwest, see figure 33. 3.2.11 Gully 11 Gully 11 (SE) Gully 11 (SE) is the southeast side of gully 11. The southeast side of gully 11 is 50 meters tall; 18 meters is composed of rhyolite and 32 meters of talus (Fig.34). Unit 1A makes up the lower 15 meters and Unit 1C makes up the upper 3 meters. The base of unit 1A is buried by talus and the upper margin of unit 1C is eroded. Interval 30 - 39 m foliations moderately or steeply dipping mostly northwest, some steeply dipping west, two moderately dipping north, and one steeply dipping northeast. Interval 40 - 49 m foliations are mostly steeply dipping northwest, and southeast, and one is moderately dipping west. No lineations were measured, see figure 34.

29 3.2.12 Gully 12 Gully 12 (NE) Gully 12 (NE) is the northeast side of gully 12. The northeast side of gully 12 is 49 meters tall; 22 meters is composed of rhyolite and 27 meters of talus (Fig.35). Unit 1A makes up all 22 meters of lava. The base of unit 1A is buried by talus and the upper margin of unit 1A is eroded. Interval 20 - 29 m foliations are horizontal to gently dipping northwest, and northwest. Interval 30 - 39 m foliations are horizontal gently or moderately dipping northeast, and east. No foliations or lineations were measured in Interval 40 - 49 m. Interval 20 - 29 m lineations are plunging northeast, and southwest. Interval 30 – 39 m no lineations are measured, see figure 35. Gully 12 (SW) Gully #12 (SW) is the southwest side of gully 12. The southwest side of gully 12 is 43 meters tall; 25 meters is composed of rhyolite and 18 meters of talus (Fig.36). Unit 1C makes up the lower 15 meters of lava and Unit 1A makes up the upper most 10 meters. The base of unit 1C is buried by talus and the upper margin of unit 1A is eroded. Interval 20 - 29 m foliations are to gently dipping northwest. Interval 30 - 39 m foliations are horizontal to gently or moderately dipping northeast, southeast, one moderately dipping south, and one moderately dipping southwest. Interval 40 - 49 m foliations are horizontal to gently dipping east, southeast, and south. Interval 20 - 29 m lineations are plunging northwest, one plunging west, and one plunging south. Interval 30 – 39 m lineations are plunging mostly northeast, and southwest, one is plunging south, one is plunging southeast, one is plunging east, and two are plunging north, see figure 36.

30 Fig. 14 Lithogicallog and structural Gully #1 East Side Legend data for the east side of Gully # 1. - Talus Full Foliations All Lineations - Unit#1A (poles to planes) N N - Unit#18 n=6 Lineations 20 ·~ m • • .. & Foliations " _/

Kamb eontours in Standard DeviatiOM

Foliations Lineations (poles to planes) N 1=1m .. 25 m --.--.------. 20-29 m Kamb eontovrs ln Standard Oev~tiont

N n=2 ..

10-19m

Kamb c:ontou,.. ln Standard Deviations

N n=5 •

/ 0-9 m

Kamb contours in Standard Deviations

31 Fig. 15 Lithogical log and structural Gully #1 West Side Legend data for the west side of Gully # 1. - Talus All Foliations All Lineations (poles to planes) - Unit#1A

N N n = 11 - Unit#18

•• m Lineations Foliations

l=lm 33 m --r,....--...,

Foliations Lineations (poles to planes) N

Kamb contours In Stand.1rd Oe\o1atlons 20-29 m

N N n=6 " •

10-19m K.8.mb COfttOUI'$ l.n Standard O.vl.adons

N N n=5

• •

Kamb contou,.. in Sta~nd.1rd Oe\liations Sm • ~ 0-9 m

32 Fig. 16 Lithogicallog and structural Gully #2 East Side Legend data for the east side of Gully #2. - Talus

All Foliations All Lineations - Unit#1A (poles to planes) N N - Unit#1B n=3 .. m Lineations Foliations

Kamb eontours in StandardO.viationl

•

Foliations Lineations l=lm (poles to planes) 27m N

Ka.mb ccontol.lrs i n 24m Standard Oevi.ations 20-29 m

n = 11 N ..

Rubbly ( 10-19m Ka.mb ccontol.lrs i n Standard Deviations

N N n=3 ..

0-9 m 5m K.;mbcc.-.tourain Standard Oevi.ation.s

•

33 Fig. 17 Lithogical log and structural Gully #2 West Side Legend data for the west side of Gully #2. - Talus All Foliations (poles to planes) - Unit#1A N - Unit#1B m Lineations Foliations Kamb eontout'l In SUnd~rd O.Viatlons

Foliations (poles to planes) N

l=lm 25m Klmb contours in Standard Deviations 20-29 m

K.8mb COfttOUI'$ In SUinda.rd Deviations 10-19m

Kilmb eontou,. in Stu'l

34 Fig. 18 Lithogical log and structural Gully #3 East Side Legend data for the east side of Gully #3. - Talus All Foliations All Lineations - Unit#1B N (poles to planes) N n = 31 n = 24 - Unit#2A • m Lineations m Foliations

Kamb eontours il'l Standard O.viatlons •

N Lineations

Foliations n=7 l=lm (poles to planes)

30-39 m

• •

N N

Om 35 Fig. 19 Lithogical log and structural Gully #3 West Side Legend data for the west side of Gully #3. (Scale changed) - Talus All Foliations All Lineations - Unit#1A N (poles to planes) N n = 47 - Unit#2A III+JII Lineations ... . •t . . . . & Foliations I =lm 46m ......

Foliations (poles to planes) n = 11 N

30-39 m

29m

20-29m

10-19 m

10m Om

36 Fig. 20 Lithogical log and structural Gully #4 South Side Legend data for the south side of Gully #4. - Talus (Scaled changed) All Foliations - Unit#1A (poles to planes) All Lineations N N - Unit#18 n = 43 . .. . m Lineations Foliations

Foliations Lineations l=lm (poles to planes) 38 m ---r----,

N n=B 35m

~lllibCOIIII(M.II'S in S!otf'CIIf'

N n = 10

tt.llllbeonc~,.ll'l S&llncJ.rdO..Ions 20-29 m · ~

N n = 11

. . ,.f 10-19 m

8m Om

37 Fig. 21 Lithogical log and structural G ully #4 Northeast Side Legend data for the northeast side of Gully #4. - Talus (Scaled changed) N N All Lineations - Unit #1A n=6 m Lineations a Foliations

~mb ~ontou" in Standard Oe·tiations

• l=1m Foliations 40 m (poles to planes) N Lineations

Ka.m.b eonto.urs In Standard ~lations 30-39 m

• •

Ka.m.b eonto.urs In Sblndard O.V1atlont 20-29m

II: Ka.mb eontours In Standard Oevlatlont

10-19 m

12m

38 Fig. 22 Lithogicallog and structural Gully #5 South Side Legend data for the south side of Gully #5. - Talus (Scale changed) - Unit#1A Rei Lineations All Foliations All Lineations (poles to planes) a Foliations N n = 14

K.amb contours In SU~rd OtnttiOon•

Foliations Brecciated Lineations (poles to planes)

K1m1> coMo~otrs ln SUndatd Oovi1tlons 30-39 m

20-29m

K.amb contours In Standard O.Viriotls

K1mb coMoursln SundaM Oovlatlons 10-19 m

Fig. 23 Lithogicallog and structural data Gully #5 Northeast Side Legend for the northeast side of Gully #5. - Talus (Scale change) All Foliations All Lineations (poles to planes) - Unit#1B N N n = 33 n=;:19 .• . . ; m Lineations . - Foliations

l=lm N Foliations Lineations 48 (poles to planes)

IUmb contours In Standard Deviations 40-49 m

n=5 N

Klmb contours in Standard Deviations 30-39 m

n = 10 . . . ..

20-29 m Kllmb contoul'll in StJindard DevlaUons •

N n=4 ..

Kllmb contours In Standard Devli!Uons 10-19m

40 Fig. 24 Lithogical log and structural Gully #6 North Side data for the north side of Gully #6. Legend - Talus

All Foliations All Lineations Unit #28 (poles to planes) N - Unit#2A

•• - Unit#1 B . . • • Lineations Kamb c;ontCKII'$ in m SUndard O.vi.atio"- - Foliations • I

•

Foliations Lineations (poles to planes)

N l=lm 38m--.---...,

Kamb contours in Standard Devi.lltklns 30-39 m

N n=9 •• Rubbly ~ : !Umb eontou,. In • • . • Stan

• 18m Kamb e~tours in Standa.rd Deviations f 10-19 m 15m • •

41 Fig. 25 Lithogical log and structural Gully #6 South Side Legend data for the south side of Gully #6. - Talus

All Foliations All Lineations - Unit#2A (poles to planes)

N - Unit#1A N n = 11 m Lineations • • •• iations • • Kamb tontours in Standard De\liations t

l=lm • • •

Lineations

Kamb contours In Stand~td Oevlatlons 30-39 m

N n=6 • •• • • ~ i' • Kamb cont()urs in 20-29m Standa.rd Deviations

• •

N n=5 • • •• • •

Kamb contours In Stan

42 Legend Fig. 26 Lithogical log and structural Gully #7 North Side data for the north side of Gully #7. - Talus Unit #1C

All Foliations All Lineations - Unit#2A (poles to planes) - Unit#1A

N N - Unit#1B m Lineations ~ j' - Foliations Kamb eontoura In ,.,r Sland"d O.vlation \ • ••• • ... . •

l=l m 44m

Foliations Lineations (poles to planes) 41 m

N n =5

Kamb eontoul"8 In Standard Or.olatlons 30-39 m 34m ... .I •

N N n = 20 ,. Rubbly . ~.•" 2 20-29m Kamb eontoura In ~ Standard Oevlatlons . 25m ~ .. • • •

43 Fig. 27 Lithogicallog and structural Gully #7 South Side Legend data for the south side of Gully #7. - Talus

- Unit#1A

- Unit#18

•••Lineations All Foliations All Lineations (poles to planes) W•WFoliations

N N n = 10

Kamb contours in Standard Deviations

l=1m 49 m Foliations Lineations (poles to planes)

30-39m Kamb contours In Standard Deviations

N n = 10 •

20-29 m Kambcontoursin Standard Deviations

44 Fig. 28 Lithogical log and structural Gully #8 East Side Legend data for the east side of Gully #8. All Lineations - Talus N N All Foliations n=5 Unit #1C (poles to planes) " m Lineations .. liations

Ka mb contours in • Stand•rd Deviations l=l m 60m --.---, Foliations Lineations N (poles to planes) N n=5 •. K.amb contours in Standard O.Viations 50-59 m

50m -+---i N

,.,, 40-49m

Kamb contours In Standard O.Viatlone

Kamb contours In Stand.itd Oevlatlone 30-39 m

45 Fig. 29 Lithogicallog and structural Gully #8 West Side Legend data for the west side of Gully #8. - Talus

Unit #1C m Lineations Foliations

All Foliations (poles to planes)

N n =20

Foliations (poles to planes) 1=1m 51 m -.....------, N

Kolmb contours In Stlndard O.vl~tlon& 40-49m

N Rubbly ..,,

30-39 m

Kamb contours in Sundard Deviations 35m

46 Fig. 30 Lithogicallog and structural Gully #9 Northwest Legend data for the northwest side of Gully #9. - Talus

Unit #1C

- Unit#1A m Lineations All Foliations (poles to planes) All Lineations liations

N N n = 11 •• • •• • Kamb c;ontoul'$ in Standard Deviations

Foliations Lineations (poles to planes)

N N

46 m

45 m •

Kamb contours in Sta.nd8rd OtvtatiOM 40-49m

Rubbly

N N .. • •

~mb c;ontoure in 20-29 m SU!n~ rd Deviations

27m

47 Fig. 31 Lithogical log and structural Gully #9 Southeast Side Legend data for the southeast side of Gully #9. - Talus

Unit #1C

All Foliations All Lineations - Unit#1A (poles to planes) m Lineations N N n = 30 n = 20 EllFoliations •

Kamb~ eontout'l~ In Stand~rd Deviations

Foliations (poles to planes) 1=1m Lineations

Kamb contours in St.ondard Oovlodont 40-49 m

.. N 42 m .. •• • • ~ [ • Kamb contOUr$ i n 'r 30-39m Standard O.viations

•

Kamb contours In 20-29 m St.and.ard Oevlatlona r/

48 Fig. 32 Lithogical log and structural data Gully #10 Northwest Side Legend for the northwest side of Gully # 10. - Talus All Foliations All Lineations N (poles to planes) N - Unit#1A --~- n =15 II!JI Lineations - Foliations Kamb contours in SUndard Deviations

1=1m Foliations 50 m (poles to planes) Lineations

N n=5

Kamb contours In Standard Deviations 40-49 m

Kamb contours in Standard Deviations 30-39m

N n = 10

Kamb contou111 in Standard Deviations • 20-29 m

22m

49 Fig. 33 Lithogicallog and structural Gully #10 Southeast Side Legend data for the southeast side of Gully # 10 - Talus

- Unit#1A m Lineations ~~'a~~ Foliations All Foliations All Lineations (poles to planes)

N N n = 15 .. • • . .• K•mb contours In Swnd• rd Oevfotions •

•

l=lm 50 m

Foliations Lineations (poles to planes) N n = 15 • .. • .• K•mb conto!H's in • Standai'CI Devhtsons • 30-39 m

•

32m

50 Fig. 34 Lithogical log and structural data Legend for the southeast side of Gully # 11. Gully #11 Southeast - Talus

- Unit#1A Unit #1C All Foliations (poles to planes) m Lineations

N & Foliations

Kamb con tou,. In Sttnd"rd Devi.lliona

l=1m 55 m -r-r----.

Foliations (poles to planes)

47 m

K.arnbcontcK.w't ln Stt.nda.rdOtwia~ 40-49m

Ko~unb c.ontour. in $Qncla.rcl O.vlatlon.s 30-39 m

32m

51 Fig. 35 Lithogical log and structural data Gully #12 Northeast Side Legend for the northeast side of Gully # 12.

All Foliations All Lineations (poles to planes) Foliations

N N n = 25 n=5 " ••

Kiiimb ~ontour& in S~oclilrd O.vbtioN

l=lm

Foliations Lineations (poles to planes)

N n = 15 ,, ,,

30-39 m

Ka.mb contoun l.n Sto~ndilrd O.vbtion•

N N n=5 " ..

Ka.mb contours in 20-29m Standard Deviations

52 Fig. 36 Lithogical log and structural data Gully #12 Southwest Side Legend for the southwest side of Gully # 12. - Talus All Foliations All Lineations Unit #1C (poles to planes)

N N n = 15 - Unit#1A :~ m Lineations ' . .1 B Foliations "" ..

Kamb c:ontoul'$ in 1=1m Standard O.viations •

43 m •

Foliations Lineations (poles to planes)

Rubbly

30-39 m Kamb <:ontours in Standard Deviations 33m

N N n = 10

,", :~ ! . .1 .:t . 20-29m Kamb cont oure In Stan

N N 18m

••

Kamb contours in 10-19 m Sta.ndard DhiatiMS

• Om

53 4 Discussion 4.1 Lithological Architecture Unit 1 lithologies are texturally uniform obsidian with differing degrees of vesicularity. Unit 1A is seen in gullies 1-3, 5-7, and 9-12. Unit 1B is seen in gullies 1-4, 6, and 7. Unit 1C is seen in gullies 7-9, 11, and 12. Unit 2 lithologies are heterogeneous and multi-layered with intercalated black obsidian, pumiceous, and crystalline rhyolite. Unit 2A is seen in 3, 4, and 6, and Unit 2B is seen in gully 6 only. Unit 1A is consistently below more pumiceous but still glassy Unit 1B. Because Unit 1A is below Unit 1B, and because Unit 1B is most often the top of the lava, the most likely explanation is that units 1A and 1B represent a continuum of upward-increasing vesicularity related to lithostatic pressure. At the uppermost surface of the lava where the load (i.e. lithostatic pressure) is minimal (i.e. atmospheric pressure), volatile gases like H2O and CO2 can readily exsolve from the glassy rhyolite to form vesicles and pumiceous texture during and shortly after flow (Houghton et al., 2015). This produces gray, pumiceous, glassy rhyolite Unit 1B. Deeper in the lava, the lithostatic pressure is higher due to more overburdening lava (i.e. load) and exsolution is inhibited; this produces black, non-pumiceous, obsidian (Unit 1A). Unit 1C is transitional between units 1A and 1B. The crystalline rhyolite intercalated within Unit 2 is not exposed as large-volume, homogeneous masses around the flow margins, unlike the dense (i.e. non-vesicular) and pumiceous obsidian. Similar dense, crystalline rhyolite is exposed over the inferred vent (Figs 5 and 6) and in the center of the vent-proximal cores drilled at RDO-2A and -2B (Fig. 6 Fink & Manley, 1987; Manley & Fink, 1987; Fink et al., 1992). Based on the drilling cores and observations of other lavas (Fig. 7), it is plausible that a dense, crystalline rhyolite center exists throughout the Obsidian Dome lava. If this is correct, then it is likely not exposed at the margins yet because erosion, both in gullies and along the flow margins, has not yet incised deep enough into the lava. The crystalline core, if indeed present, will be 10 – 30 m back from the original flow margin, similar to the thickness of the pumiceous and obsidian carapace elsewhere. This is because a crystalline core requires slow enough cooling to allow for pervasive crystallization when the cooling rate increases towards the flow margin. The well-exposed obsidian carapace (Unit 1A) and the glassy but pumiceous carapace (Unit 1B) are non-crystalline and cooled faster than the crystalline rhyolite in the flow interior. Unit 2 may represent a mixing zone between hot and fast moving rhyolite lava (going to quench as obsidian) and slightly older, colder, crystalline rhyolite from with the feeder dike or older parts of the lava’s interior. 4.2 Structural Analysis 4.2.1 Structural Comparisons Between Gullies Each gully was measured to represent two independent sides of a single gully. Combining the data from both sides of the gully allows for an examination of the three dimensional structure in a significant volume of lava (Table 2). Table 1 shows that the foliations recorded between gullies are highly variable. Generally, foliations are sub-horizontal towards the base and become steeper

54 dipping and more radially distributed higher in the lava. Foliations are most variable in dip direction and dip amount in the center. No two gullies are the same, either on either side of the same gully or between adjacent gullies (i.e. either side of a potential break-out lobe; e.g., Table 2). The heterogeneity of flow banding foliations is striking and indicates that even over only a few hundred meters of lava front, and at a relatively coarse scale, flow was inhomogeneous and complex.

se S h H h h se SE

h SE NE NE

h h n S h steep dip south to moderate dipmoderate south to gentle dip south to uniform direction one in mostly directions variable north west east south horizontal horizontal h S SW SW S = = = = n h h h H NE n = w = S e = S = s = s = h = NW NW E SE H NW NW se se

SE NW N h NW

ne h h W h NE S

is the collective foliation measurements of each withfoliation of gully, above. legend The table collectiveis the measurements is

NW W E W Table 2 Table describing the highest density trend of theseparate trendof datadescribing withat highestpositions the densitypoint each the stereonet a each single interval gully. of side for NW NW SE NE h h h 12SW 12NE 11SE 10NW 10SE 9NW 9SE 8W 8E 7N 7S 6N 6S 5NE 5S 4NE 4S 3W 3E 2W 2E 1W 1E 0-9 10-19 20-29 30-39 50-59 40-49

56 4.2.2 Structural Similarities Between Different Sides of the Same Gully No gullies exhibit flow banding that is mirror-imaged on either side (Table 2); rather, several gullies are very different across only very short distances (e.g., gullies 6 and 8). Gullies 1, 5, and 9 are moderately similar on either side in foliation, but gullies 5 and 9 differ on either side in their lineations and lithologies; only gully 1 seems to be reasonably similar on both sides. 4.2.3 Structural Evidence of Break-Out Lobes Given the significant differences between the gully sides it seems most likely that gullies 2 to 12 physically separate break-out lobes that flow, to some extent, independently of adjacent lobes (Table 1). Only gully #1 appears to be incised into a single mass; all others separate lobes such that, for example, the northern side of gully 6 and the southern side of gully 7 are the margins of a single lobe (“6-7” lobe). The simplest cross-sectional shape of a break-out lobe is elliptical, e.g., Table 2, and the flow-banding on either edge of a lobe should be mirror images of each other, typically facing away from each other (i.e. towards the adjacent lobes). Lobes “3-4”, “4-5”, and “6-7” fulfill the criteria of being lobes with mutually opposing margins on either side. Other pairs of gully sides do not show this pattern, either because the structural relationship is imperfect or because the erosion in the gullies is too complex. 4.3 The Origins of Gullies Incising the Flow Margins at Obsidian Dome Natural gullies incising the margins of Obsidian Dome expose the outermost 10s of meters of lava through a carapace of breccia. Based on identical structures and lithologies on either side of gully 1, it is likely that gully #1 is an erosional feature (type I; Fig. 37). Gully 1 is also the shortest gully surveyed. Gullies 2 to 12 show structural differences between either side of each gully. Gully 11 only has data for one side. These gullies are likely to be localized into clefts between adjacent break-out lobes (types II and III; Fig. 37). If this is correct, then the space between gullies, for example 2 and 3, or 9 and 10, constrains the maximum width of break-out lobes. The average width of lobes is approximately 130 m. The average distance across each gully is 19 m. The width between break-out lobes at Cordόn Caulle is 50 – 100 m (Tuffen 2013). The space between break- out lobes is 5 - 15 m (Tuffen 2013). Obsidian Dome’s average width is greater than Cordόn Caulle, and has a similar average width of 19 m. The widths of gullies seem to influence how deeply they incise into the break-out lobes. Narrow or shallow gullies (II) incise through the carapace of pumiceous lava and blocks (Unit 1B) into the dense obsidian beneath (Unit 1A). Wider or steeper gullies (III) incise through units 1A and 1B, and into Unit 2.

Fig. 37. Conceptual diagram, not to scale, based on observations at Obsidian Dome. Top - layer- cake, erosional gullies only incise laterally continuous layers of, from top to bottom, units 1B, 1A, 2 and a inferred wholly crystalline rhyolite (R), are intersected by erosional gullies (I). Paleo-flow direction is perpendicular to the page. Lithologies, sub-horizontal foliations, and stretching lineations are similar in all potential gullies. Middle - large lobes, erosional and localized gullies show large lobes (100ms) m across of, from outside to inside, units 1B, 1A, 2, and R, are intersected by erosional gullies (I) and separated by topographically localized gullies (II) that erode through 1B, 1A, progressively. Paleo-flow direction is perpendicular to the page. Lithologies, foliations, and stretching lineations will likely differ between localized gullies and across either side of gullies, as different lobes flowed independently. In contrast, erosional gullies may differ from each other, but each gully side will be mirror-image of the other. Gully #1 is an example I. Bottom - small lobes, localized gullies only show small lobes (10ms m across) are separated by topographically localized gullies that are narrow (II) or wide (III). Gullies erode through 1B, and 1A, progressively; wider gullies erode further into each adjacent lobe and expose deeper lithologies, foliations, and stretching lineations will likely differ between localized gullies and across either side of gullies, as different lobes flowed independently. This scenario best fits the data from gullies 2-12 at Obsidian Dome.

58 5 Summary The main objectives of this study were to determine the structural and lithogical properties present at Obsidian Dome, and to interpret the deformation history of how the silicic lavas were emplaced. The lava flow history was also tested against the two end member models of silicic lava emplacement. The findings show various structures are present throughout the margins of Obsidian Dome as well as two main texturally different rhyolite lavas with subunits. The deformational history found supports research in other large silicic lavas around work (i.e. Cordon Caulle, Chile). The first two hypothesizes scenarios of the occurrence of gully types (I, II, and III) at Obsidian Dome did not entirely explain all the marginal structural features that were seen. However, the third scenario could explain every gully type and formation occurrences. Scenerio 3, large lobes erosional and localized gullies support the endogenous break-out lobes marginal feature that researchers have found in Cordn Caulle. Both findings of Obsidian Dome, and Cordn Caulle add to the present knowledge of flow mechanics of silicic lava emplacement. The various styles of deformation seen at Obsidian Dome that support both end member models of silicic lava emplacement. The isoclinal recumbent folds, autobreccias, and basal breccias are key structures that support the exogeneous model. The break-out lobes identified using scenario 3 from the modified hypothesis, sheath folds, s-shaped folds (doubly-imbricated fold) are key structures that support the endogenous model. Overall, Structural and lithological data from the northwest margin of Obsidian Dome constrain the margins of discreet break-out lobes that flowed independently in slightly different directions. The combination of lithology, flow- banding and stretching vesicle directions support simple models of break-out lobe structure, and disagree with the predicted structure of non-lobate margins. This interpretation challenges common assumptions about silicic lava emplacement, specifically the duration of flow, and the thermal structure within an active lava, and supports observations made at Cordon Caulle, Chile.

59 6 References Allmendinger RW, Cardozo N, and Fisher D (2012) Structural geology algorithms: Vectors and tensors in structural geology: Cambridge University Press (book to be published in early 2012). Andrews GDM, Branney MJ (2001) Emplacement and rheomorphic deformation of a large, lava- like rhyolitic ignimbrite: Grey’s Landing, southern Idaho, Geol Soc Am Bull 123: 725- 743. Bailey RA (1989) Geological map of Long Valley Caldera, Mono-Inyo Craters Volcanic Chain and vicinity, eastern California, Department of the Interior U.S. Geological Survey. Bernstein M, Pavez A, Varley N, Whelley P, Calder ES (2013) Rhyolite lava dome growth styles at Chaitén Volcano, Chile (2008-2009): Interpretation of thermal imagery. Andean Geol 40: 295-309, 10.5027/andgeoV40n2-a07 Branney MJ, Barry TL, Godchaux MM (2004) Sheath folds in rheomorphic ignimbrites. Bull Volcanol 66: 485-491, doi: 10.1007/s00445-003-0332-8 Bursik M Kerry S (1989) Range Front Faulting and Volcanism in the mono Basin, Eastern California, J of Geophys Res vol.94, NO. B11, p. 15,587-15,609 Can-Tapia, E and Castro J (2004) AMS measurements on obsidian from the Inyo Domes, CA: a comparison of magnetic and mineral preferred orientation fabrics: J Volcanol Geotherm Res, v. 134: p. 169-182. Cardozo, N, and Allmendinger, RW (2013) Spherical projections with OSXStereonet: Computers & Geosciences 51: 193 – 205, doi:10.1016/j.cageo.2012.07.021. Carle SF (1988) Three dimensional modeling of geological structure of Long Valley Caldera, J Geophys Res 93: 13,237-13250. Castro J, Cashman K.V (1999) Constraints on rheology of obsidian lavas based on mesoscopic folds. J Struct Geol, 21: 807-819. Castro J, Manga M, Chashman KV (2002) Dynamics of obsidian flows inferred from microstructures: insights from microlites preferred orientation, Earth Planet Sci Lett 199: 211-226. Couch S, Sparks RSJ, Carroll MR (2003) The kinetics of degassing induced crystallization at Soufriere Hills Volcano, Montserrat. J Petrol 44: 1477-1502 Eichelberger JC, Miller CD, Younker LW (1985) 1984 drilling results at Inyo Domes, California: Eos 66: 384 Eichelberger JC, Carrigan CR, Westrich HR, Price RH (1986) Non-explosive silicic volcanism: Nature 323: 598-602

60 Fink JH (1985) Geometry silicic dikes beneath the Inyo Domes, California, J Geophys Res 90: 1127-11113 Fink JH, Griffiths RW (1998) Morphology, eruption rates, and rheology of lava domes: insights from laboratory models, J Geophsy Res 103: 527-545 Fink JH, Malin MC, Anderson SW (1990) Intrusive and extrusive growth of the Mount St Helens lava dome. Nature 348: 435-437 Higgins MD, Meilleur D (2009) Development and emplacement of the Inyo Domes Magmatic Suite, California: Evidence from geological, textural (CSD) and geochemical observations of ash and lava: J Volcanol and Geotherm Res186: 280-292. Hildreth W (2004) Volcanological perspectives on Long Valley, Mammoth Mountain and Mono Craters: Several Contiguous but Discrete Systems, J Volcanol Geotherm Res 136: 169- 198 Hill DP, Bailey RA, Miller CD, Hendley II JW, Stauffer PH (2004) Volcanic activity in the Mono- Inyo Craters volcanic chain of the past 5,000 years, http://pubs.usgs.gov/fs/fs073- 97/eruptions.html, accessed Aug 2016. Hon K, Kauahikaua J, Denlinger R, Mackay K (1994) Emplacement and inflation of pahoehoe sheet flows: Observations and measurements of active lava flows on Kilauea Volcano, Hawaii. Geol Soc Am Bull 106: 351-370 Houghton H, McNutt SR, Rymer H, Stix J (2015) The Encylopedia of Volcanoes, Elsevier Second edition: 163-183 Lisle RJ (1979) Strain analysis using deformed pebbles: the influence of initial pebble shape. Tectonophysics 60: 263-277. Kamb WB (1959) Petrofabric observations from Blue Glacier, Washington, in relation to theory and experiment: J Geophys Res 64: 1908-1909 Kingsbury CG (2012) Physical Volcanology of Obsidian Dome, California: A Complex Record of Emplacement of a Youthful Lava Dome, University of Ottawa. Merle O (1998) Internal strain within lava flows analogue modeling, J Volcanol Geotherm Res 81: 189-206 Miller CD (1985) Holocene eruptions at the Inyo Volcanic Chain, California: Implications for possible eruptions in Long Valley Caldera, Geology 13: 14-17 Nelson S. (2012) Metamorphic Rock Textures http://www.tulane.edu/~sanelson/eens212/metatexture.htm , accessed April 2017 Self S, Keszthelyi L, Thordarson Th (1998) The importance of pahoehoe. Ann Rev Earth Planet Sci 26: 81-110

61 Shields JK, Mader HM, Caricchi L, Tuffen H, Mueller S, Pistone M, Baumgartner L (2015) Unravelling textural heterogeneity in obsidian: shear-induced outgassing in the Rocche Rosse flow. J Volcanol Geotherm Res, doi:10.1016/J.JVOLGEORES.2015.12.003 Smith JV, Houston HC (1994) Folds produced by gravity spreading of a banded rhyolite flow, J Volcanol Geotherm Res 63: 89-94 Talbot CJ, Pohjola V (2009) Subaerial salt extrusion in Iran as analogues of ice sheets, streams and glaciers, Earth-Sci Rev 97: 155-183 Tuffen H, James MR, Castro JM, Schipper CI (2013) Exception mobility of an advancing obsidian flow at Cordn Caulle volcano in Chile, Nature Comm 2709: doi:10.1038/ncomms3709. Ventura G (2001) The strain path and emplacement mechanism of lava flows: a case study from Porri Volcano (Salina, Souther Tyrrhenian Sea). Bull Volcanol 59: 394-403

62 7 Acknowledgments

I’d to thank Dr. Andrews for seeing my potential at the GSA conference in Vancouver, Canada. Second, I would like to thank the CREST program for supporting me the last two years. I would also like to thank the professors at CSUB for being available to answer my questions and brainstorm ideas with me. My thesis would not be the thesis it is today without you. I would like to thank The National Center of Laser Mapper (NCALM) for the LiDAR imaging I used in my thesis. I would like to thank all of my field assistants: Maryanne Bobbitt, Stephen Anderson, and Blake Foreshee for being brave and or crazy enough to climb Obsidian Dome with me. Lastly, I would like to thank Wes Hildreth, and Judy Fierstien for taking the time to give me a personal tour of the Long Valley region and coming into the field with me at Obsidian Dome. Thank you all that were truly supportive of me the last two years, I’m forever grateful.

63 Abigail Martens 6912 N Halfmoon Dr. Apt. B. Bakersfield, CA 93309 Cell: (815) 618 -2690 Email: [email protected] Work Experience

California State University Bakersfield, CA Graduate Fellowship Researcher September 2015-Spring 2017

Structural and strain analysis on Obsidian Dome and surficial structure interpretations of Inyo Volcanic Chain.

Illinois State University Normal, IL Research As sistant April 2014-May 2015

High graded conodont samples from cores. The cores would be cut with various saw and acid tools/methods. This was to help establish first and last order datum of various conodont species.

Lamont Doherty Earth Observatory New York City, NY REU, NSF Summer Internship May 2014-August 2014

Determined whether Yellowstone-Snake River Plain volcanism is hotspot or not. Performed field and lab research to find the time and age progression of the Yellowstone hotspot track through the eastern Snake River plain using paleomagnetism and radiometric dating. Wrote a 20 page research paper and gave a talk and poster presentation at Columbia University Lamont Doherty Research Colloquium.

Illinois State University Normal IL Research As sistant January 2014-May 2014

Handled rock samples from Kaua’i, and sawed, ground, and polished samples to make thin sections.

Illinois State University Normal, IL Mineralogy TA August 2014-Decemeber 2014

Instructed all labs and material, gave personal tutoring, and graded labs.

Illinois State University Normal, IL Research As sistant August 2013-December 2013

High graded apatite grain samples from the Guadalupe and Sacramento mountains in southeast New Mexico. Looked for inclusions of apatite grains with an SEM. Wrote a detailed final report on research and processes throughout the semester.

Field Work

California State University Bakersfield, CA Thesis field work at Obsidian Dome, CA from April 24th – August 27th 2016.

Described units in field site, as well as photo cataloged the units, recorded structural data, and created stratigraphic columns with only a field assistant for a total of 15 days.

Lamont Doherty Earth Observatory New York City, NY REU, NSF Summer Internship 2 weeks of field work in June 2014 Collected rock samples from the caldera rim of the Picabo Volcanic Field, ID.

Awards

California State University Bakersfield, CA National Science Foundation (NSF) Grant: Centers for Research Excellence in Science and Technology (CREST) Grant . September 2015-May 2017.

California State University Bakersfield, CA The National Center for Airborne Laser Mapping (NSF). Awarded LiDAR data of the entire Inyo volcanic chain in Eastern California. September 2016-September 2017.

Conference A bstract

A. .Martens, V DiVenere, S.R. Hemming, (2014). Tracking Ca lderas of the Yellowstone-Snake River Plain Volcanoics: Paleomagenetic and 40AR/39AR Dating Results from the Picabo Volcanic Field. Poster presentation at GSA Vancouver, B.C. 2014.

Volunteer

California State University Bakersfield, CA April 2017

Guest speaker for CSUB paleoclimate class on the LiDAR.

California State University Bakersfield, CA June 2016

Guest speaker at local high school about Obsidian Dome, and led a half day field trip to Obsidian Dome.

Illinois State University Normal, IL Spring 2014

Lectured multiple groups of boy scouts of all ages on the basics of geology, to receive their geology badge.

Rock Valley College, IL Spring 2014

Shadow for geology professor in the geology 101 course, tutor for students enrolled in any geology related course.