Crustal Thickness and Vp/Vs Ratio of Yellowstone, Eastern Snake River

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Crustal thickness and Vp/Vs ratio of Yellowstone, Eastern Snake River Plain, Wyoming Province, and the northern Basin and Range Province through receiver function analysis

Avikant Dayma

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CRUSTAL THICKNESS AND 푉푝/푉푠 RATIO OF YELLOWSTONE, EASTERN

SNAKE RIVER PLAIN, WYOMING PROVINCE, AND THE NORTHERN BASIN

AND RANGE PROVINCE THROUGH RECEIVER FUNCTION ANALYSIS

AVIKANT DAYMA

A THESIS

Presented to the Faculty of the Graduate School of the

MISSOURI UNIVERSITY OF SCIENCE AND TECHNOLOGY

In Partial Fulfillment of the Requirements for the Degree

MASTER OF SCIENCE

GEOLOGY & GEOPHYSICS

2017

Approved by

Stephen Gao, Advisor

Kelly Liu

Maochen Ge ii

 2017

Avikant Dayma

ABSTRACT

For decades, numerous studies have been conducted and contradictory results were achieved about the origin and evolution of the Yellowstone supervolcano and the

Eastern Snake River Plain. Whether the 640 km long time-progressive chain of rhyolitic calderas was formed due to mantle-plume or a result of lithospheric extension allowing the emergence of melt on the surface is still a debate. Using receiver function analyses, this study determines the crustal thickness and the Vp/Vs ratio below the Yellowstone

Plateau, Wyoming Province (WP), Idaho Batholith (IB), Eastern Snake River Plain

(ESRP), and the northern Basin and Range Province (BRP). The thickness is highest below the Wyoming Province (46 to 54 km) and reduces to as low as 30 km towards the east in the Batholith Province. The Yellowstone Caldera sits above the Moho not deeper than 44 km and the entire extension of the Eastern Snake River Plain varies in Moho depth from 44 km below the Island Park Caldera, 40 km below the Heise Caldera and shallows up to 37 km below the Picabo and Twin Fall Calderas. The thickness of the crust below ESRP does not show a noticeable variation from the BRP lying to the south and north of it. The receiver functions below the Yellowstone Plateau show a low- velocity zone at a shallow depth probably signifying the presence of a shallow magma chamber. Compressional velocity variations in the mantle show low-velocity anomaly of

-9% till the depth of 280 km NW of the Yellowstone Plateau and a high-velocity region beneath it starting from 320 km. This study acknowledges both the controversial theories proposed for the explanation of the magmatic volcanic calderas. iv

ACKNOWLEDGMENTS

I would like to express my sincere gratitude towards my advisor, Dr. Stephen

Gao, who guided me through every step and without whose motivation this thesis would not have been possible. I appreciate his calm and patient behavior in response to the numerous questions I asked him.

I would also like to sincerely thank the other members of my committee, Dr.

Kelly Liu, and Dr. Maochen Ge for their assistance and support. This thesis was possible because of the intensive FORTRAN programs written by Dr. Gao and Dr. Liu.

I would like to thank my mother, Anita Dayma and my grandfather Radheshyam

Dayma for believing in me and making my education possible.

IRIS (Incorporated Research Institutions for Seismology) DMC (Data

Management Center) provided the data used for this study and I thank them for their huge contribution to the geophysics community.

TABLE OF CONTENTS Page

ABSTRACT ...... iii ACKNOWLEDGMENTS ...... iv LIST OF ILLUSTRATIONS ...... vi LIST OF TABLES ...... vii SECTION

1. INTRODUCTION ...... 1 2. PREVIOUS STUDIES ...... 4 3. TECTONIC SETTING ...... 7 4. DATA AND METHODS ...... 9 5. RESULTS ...... 15 5.1 AREA A ...... 15 5.2 AREA B ...... 17 5.3 AREA C ...... 18 5.4 AREA D ...... 18 5.5 AREA E ...... 19 6. DISCUSSIONS ...... 32 7. CONCLUSIONS ...... 40 APPENDIX ...... 41 BIBLIOGRAPHY ...... 65 VITA ...... 74

LIST OF ILLUSTRATIONS

Page Figure 4.1. Seismic stations and major geological provinces of the study area...... 10

Figure 4.2. Stacked receiver functions for the station D39 which lies in the Idaho Batholith region...... 13

Figure 5.1. Resulting crustal thickness from category A and B stations. The bold dashed line represents major tectonic provinces...... 16

Figure 5.2. Stations showing a phase reversal at 0 s indicating direct P-wave arrival from the edge of the magma chamber (Chu et al.,2010)...... 17

Figure 5.3. Resulting Vp/Vs derived from all the category A stations used in this study 21

Figure 5.4. Resulting R value map derived from all the stations used in the study...... 21

Figure 5.5. Bouguer gravity map of the entire region plotted using the data provided by the United States Geological Survey (USGS)...... 22

Figure 5.6. Elevation, crustal thickness, Bouguer gravity anomaly, Vp/Vs and R value for seismic stations along the profile A-A1 in Figure (5.1)...... 23

Figure 5.7. Same as the previous Figure 5.6 but for profile B-B1...... 24

Figure 6.1. P-wave velocity perturbations relative to the AK 135 earth model (Kennett et al., 1995) at the depths of 20 km, 40 km, 60 km, 80 km, 200 km, 240 km, 280 km, 320 km, 380 km, 400 km, 420 km, and 440 km obtained using the MITP_USA_2016MAY model for tomographic inversion by Burdick et al. (2017)...... 35

Figure 6.2. (top left) Comparison of crustal thicknesses obtained from 69 common stations between this study and (Yuan et al., 2011); (top right) comparison of thickness from 30 common stations from Eager et al. (2010) and this study; (bottom left) comparison of thickness from 24 stations in common with Yeck et al. (2014); (bottom right) comparison of Vp/Vs...... 37 vii

LIST OF TABLES Page Table 5.1. Summary of all the stations in the five areas showing the average crustal thickness (H) and Vp/Vs (K), along with their minimum and maximum values...... 20

Table 5.2 Resulting crustal thickness (H, Hn), Vp/Vs and R values...... 25

1. INTRODUCTION

The study area is composed of major parts of Wyoming, Idaho, and the southeastern Montana state. It includes one of the largest volcanoes in the world – Yellowstone

(YS) volcano, the Archean craton of Wyoming Province and the Eastern Snake River

Plain which is considered by many to be an active volcanic hotspot trail (Yuan, Dueker and Stachnik (2010); Smith and Braile (1993); Smith et al., 2009). There are two major models which have been widely proposed to break down the complex perplexity of the formation and arrangement of the calderas of the ESRP trending NE all the way to

Yellowstone. The one involving a unique case of continental mantle plume originating from the Core Mantle Boundary or formed due to upper-mantle convection starting from a much shallower depth and extending all the way to the crust [Humphreys et al., 2000].

The presence of mantle plume explains the clearly observable features such as the chain of volcanoes similar to Hawaii, Samoa or Tahiti which are caused due to mantle plumes lying below them, increased seismicity around the Yellowstone, anomalously high heat flow signatures, a large geoid anomaly often associated with a mantle plume ( Smith et al., 2009), 푉푝 and 푉푠 anomalies as deep as 900 km below the Yellowstone dipping northwest ( Obrebski et al., 2010), high 3퐻푒⁄ 4퐻푒 ratio of the isotopes observed along the ESRP which are believed to be formed at the core mantle boundary (Huang et al.,

2014; Welhan et al., 1981). The plume head has been hypothesized to impact below the present-day Oregon, forming the Columbia River Basalt Group due to decompression melt and was squeezed by the cratonic roots (Shervais et al., 2008). The plume tail detaches and is guided along SE direction by the thinned lithosphere. The proposed plume tail then resulted in formation of a sequence of calderas trending towards the 2

North-east starting with the McDermitt Caldera (16.5 푀푎) followed by Bruneau-

Jarbidge (~12.7 푀푎), Twin Falls (~10.5 푀푎) and Picabo Caldera (~10 푀푎). The calderas formed over the last 6.6 Ma mostly overlap each other due to multiple eruptions

(Christiansen, 1984, 2001), like the Heise Caldera (6.6-4.4 Ma), Island park (~4-2 푀푎) and the most recent Yellowstone Caldera (~0.64 푀푎) (Smith and Braile, 1994; Pierce and Morgan, 1992; Pierce et al., 2002). The calderas have formed in the opposite direction of the movement of the North American Plate since the last 12 Ma. Prior to that the alignment was mostly EW and may have been affected by the sinking of the Farallon slab (Geist and Richards, 1993; Pierce et al., 2002) causing mantle counterflow, deflecting the plume eastward (Smith et al., 2002; Steinberger and O’Connell, 2002).

The second hypothesis that counters the most to the plume model is the pure plate model, based on lithospheric extension (Foulger and Anderson, 2015). This hypothesis highlights the fact that there is melt available ubiquitously in the upper mantle and is allowed to emerge on the surface by the extension of the lithosphere. In this case, the

Basin and Range extension initiated at 17 Ma resulted in the eruption at the Columbia

River Basalt. This was caused by the tearing Farallon slab, decompressing and allowing the extant melt reservoir to move along the crevices all the way to the surface from the asthenosphere (Silver et al., 2006). At the same time period, there was extension happening at the Basin and Range Province to the south of the Idaho Batholith resulting in the initiation of the ESRP. Opposite to the idea of volcanism along the ESRP causing the migration of fault activities towards northeast (Pierce and Morgan, 2009), plate theory suggests the alignment of the volcanoes is due to the northeast trend of the normal-fault movement. The ESRP lies between zones of extension on its north and south and is suggested to be formed because of similar fault extensions allowing the pre- 3 existing melt to escape the mantle to the surface (Silver et al., 2006). The initial widening of normal faults is supposed to cause silicic volcanism, followed by basaltic volcanism moving away from the axis of initiation in opposite directions. This means all the rhyolitic eruptions causing the initial caldera formation are followed by basalt eruptions later in time, that fill in those calderas and last for a longer duration (Thatcher et al.,

1999; Foulger et al., 2015). And for the reason of alignment of the ESRP towards the northeast, it is believed to be caused due to the lithospheric structure. The Great Falls tectonic zone lying in the north of ESRP (O’Neill and Lopez,1985) and St. George and

Valles zone (Smith and Luedke, 1984) are all observed to have an alignment parallel to the ESRP towards the northeast and are controlled by the regional lithospheric structure.

Three massive eruptions shaped the Yellowstone Caldera, occurring about 2.1

Ma, 1.3 Ma, and 0.64 Ma ago with a total eruption volume of ~8,500 푘푚3 (Christiansen and Blank, 1972; Christiansen, 1984, 1993). The period for these devastating eruptions is almost cyclic with an interval of approximately 0.7 Ma. Thus, the hazardous potential of the dormant volcano demands detailed study of its structure and composition.

2. PREVIOUS STUDIES

Often referred to as the “window” of the inner Earth, there have been countless studies at Yellowstone and its surrounding areas starting from the second half of the nineteenth century. Geoscientists have divided into two groups, either supporting the deep-rooted plume model or a plume originating at a shallower depth, and the ones who completely discard the existence of the plume. The two competing groups have presented numerous facts through several types of research done which have enhanced the knowledge of Yellowstone and ESRP regardless of the fact that they differ on the fundamental beliefs about the region. The plume hypothesis first introduced by Morgan et al. (1972) was endorsed by various other authors (e.g., Armstrong et al., 1975 ; Camp et al., 1995; Graham et al., 2009; Parsons et al., 1994 ; Anders et al., 1989 ; Xue and

Allen, 2010 ; Yuan and Dueker, 2005 ; Shervais and Hanan, 2008 ; Smith and Braile,

1994 ; Pierce et al., 2002 ; Richards et al., 1989 ; Hill et al., 1992 ; Pierce and Morgan,

1992; Duncan et al., 1982; Draper et al., 1991; Geist and Richards, 1993 and Huang et al., 2015). Foulger, Christiansen, and Anderson (2015) proposed the pure plate model also mentioned in other papers (e.g., Christiansen et al., 2002; Anderson et al., 1998;

Christiansen and Lageson, 2003; Tikoff and Benford, 2008). Humphreys et al. (2000) proposed an upper-mantle convection model which explains the northeast trending ESRP and the northwest trending Newberry magmatism. Existence of an ancestral Yellowstone plume below the Kula plate around 50 Ma was proposed by Murphy et al. (2003).

According to this theory, the plume has existed below the oceanic crust, close to the convergent boundary for much earlier period than the formation of the first caldera. It has resulted in generation of oceanic mafic complexes similar to current Yellowstone in 5 terms of paleomagnetic and geochemical characteristics. The plume then migrated from sub-oceanic lithosphere to sub-continental lithosphere with erratic cessation of magmatism.

Several individual researchers have contradicted the features presented by the pro plume group by presenting different results. For example, the presence of mantle plume affects the mantle transition zone (MTZ). The high temperature associated with the plume should cause the discontinuity at 410 km to go deeper and makes the 660 km discontinuity shallower (Reed et al., 2016). But a recent study by Gao and Liu (2014), does not show the anticipated depression and uplift of the d410 and d660 discontinuities respectively. Others like Leeman et al. (2009) have shown that melting is not possible in the mantle deeper than 100 km below the Snake River Plain and suggests that the

Yellowstone plume is incapable of eroding the thick lithosphere. Similarly, the topographic maps for the depths of 410 km and 660 km show no uplift or depression near

Yellowstone in the study by Cao and Levander (2010). Montelli et al. (2004), through their velocity perturbation model, suggested that Yellowstone is not even a shallow plume.

Whether a plume exists or not, Yellowstone has attracted plentiful researchers because of its active volcanism, hydrothermal pageant, high seismicity, immense heat flow and a recently accelerated uplift (Chang et al., 2007) due to the expanding magma chamber underlying the Yellowstone caldera. Various receiver functions analyses have been conducted to determine the crustal thickness and 푉푝/푉푠 values of Yellowstone and

ESRP. Yuan et al. (2010) calculated the thickness under the Wyoming Province to be around 48-54 km, and 47-52 km below the Yellowstone caldera. For the ESRP, their thickness reduces from 47 km in the NE to 40 km in the SW. Smith and Braile (1993) 6 used crustal thickness of 42-45 km below Yellowstone for their plume-plate hypothesized interaction model with thickness reducing to 40 km below the Twin Falls caldera. A lower P-Wave velocity of around 6.0 km/s for the upper 10 km and the Moho depth of 44 km were proposed by Schilly et al. (1982). The Deep Probe experiment (Henstock et al.,

1998; Snelson et al., 1998; Gorman et al., 2002) running along a 2400 km long array was aimed for crustal and upper mantle study of the Wyoming Province, Southern Rocky

Mountains and, the Colorado Plateau. They obtained a crustal thickness of 50 km beneath the Wyoming Province accounted to the 10-15 km thick, high velocity 7.x layer in the lower crust. Various studies using tomography and velocity modelling have suggested a shallow (5-16 km) magma reservoir, responsible for the observed low velocities and high hydrothermal activities (Huang et al., 2015) and a 4.6 times larger basaltic melt reservoir at the depth of 20-50 km. Chu et al. (2010) has shown that the upper magma reservoir is about 3.6 km thick and lies between the range of 5.5 to 9 km. They proposed low P and

S-wave velocities of 3.30 km/s and 1.10 km/s respectively inside this Low Velocity Zone.

Tomography results obtained below Yellowstone were visible at 400 km depth by

Christiansen et al. (2002). The observations had shown both low and high 푉푝 anomalies below Yellowstone. Yuan and Dueker, (2005) had observed deeper 푉푝 anomalies, both low and high ranging as low as ~3.2%, tilting northwest and away from Yellowstone.

3. TECTONIC SETTING

The study area is geologically varied with a major portion of it lying in the ~2.5 퐺푎

Archean Wyoming Craton in the eastern and northern parts, and the Grouse Creek Craton in the southwest. These two cratons are separated by the Farmington Zone (Foster et al.,

2006). The Wyoming Craton is bound by the Great Falls tectonic zone (O’Neill and

Lopez, 1985; Hoffman et al., 1989] in the north and separated from the Superior Craton in the east by the Trans-Hudson orogeny (Sims et al., 1995). The study area is also comprised of the Rocky Mountain Range, which was raised during the Laramide orogeny in the late Cretaceous period, extending from western Montana and Idaho, through the

Yellowstone Plateau and Wyoming Province in the eastern region of the study area. The existence of Rocky Mountains 700-1500 km deep inside the foreland has resulted in various contesting opinions about its formation. One such mechanism is the flat-slab subduction model by Bird et al. (1988). The subducting Farallon slab was coupled with the overriding continental plate up till ~700 푘푚 from the convergent boundary (Coney and Reynolds, 1977; Dickinson and Snyder, 1978). The coupling may have been caused due to a buoyant oceanic plateau (Murphy et al., 2003) which subducted under the continental plate and remained at shallow depths till it sunk into the deep mantle. This shallow slab-subduction could be a possible explanation for the inboard Laramide strain zone, uplifting the Rocky Mountains (English and Johnston, 2004). Another possible cause for the Laramide orogen could be strike-slip movement of the collided Baja BC terrane (Maxson and Tikoff, 1996).

Before the plume was below the Yellowstone Plateau, the region had similar terranes as the rest of the Rocky Mountains. The current physiography of the 8

Yellowstone Plateau has been shaped by the most recent volcanic explosion in the early

Ionian age of the Pleistocene epoch (640 ky) causing the caldera to collapse. The recurring eruptions destroyed the previous topography of the region and molded it along through the volcanic magmatism to what we see today. The plateau is a bimodal rhyolite- basalt igneous field with 95 percent of the rhyolitic composition by volume (Christiansen et al., 1984). The Island Park caldera was overlaid by basalt, which extends further along the ESRP as a mid-crustal basaltic sill (Yuan et al., 2010). To the south and north of the

ESRP lies the Basin and Range Province which has undergone massive extension of 50-

200 % (Faulds and Varga, 1998) and faulting which began in the Miocene Epoch, about

17 Ma ago. The crustal thickening caused by the subduction of the Farallon Plate during the Cordilleran Orogeny was followed by crustal thinning caused by extension. To the north of the ESRP lies the Idaho Batholith, formed by the rising basaltic magma chambers from the upper lithosphere. During the late stage of subduction, dikes were generated which injected mafic magmas into the continental crust causing melting to form the batholith (Foster et al., 1986). The melting of the subducting oceanic plate led to the formation of these magma chambers which rose to the surface and crystallized into granites and granodiorites.

4. DATA AND METHODS

For this study, teleseismic data from three-component broadband seismic stations were requested from the IRIS (Incorporated Research Institutions for Seismology) DMC

(Data Management Center). A total of 193 stations ( Figure 4.1) had the data within the study area of 42.0° to 47.0° N and -117.0° to -106.0° E. Most of the stations belong to TA

(EarthScope Transportable Array) with 75 stations. The other networks are - XT

(Western Idaho Shear Zone- University of Florida) with 25 stations, XC (Yellowstone

Intermountain Seismic Array) with 23 stations, XV (Bighorn Arch Seismic Array) with

13 stations, YH network (LaBarge- EarthScope Flex Array) with 13 stations , 8 WY stations (Yellowstone Wyoming Seismic Network), 7 IW stations (Intermountain West

Seismic Network), 6 stations from ZH (Big Horn/ UC Boulder), 5 stations from US

(United States Seismic Network), 5 stations from XS (Montana BB Array), 5 stations from ZG ( Wallowa Earthscope Flex Array) and 2 stations from both IM (International

Miscellaneous Stations and Z2 (NOISY), have one station each in XK (CDROM), XL

(DeepProbe), UU (University of Utah Regional Network) and XJ ( Eastern Snake River plain Experiment) networks. The TA stations are spaced 70 km from each other and are the most prominent stations often used for large scale reliable studies.

The dataset requested from IRIS DMC had earthquakes with epicentral distance between 30° and 90° and had a cut-off magnitude (Mc) calculated using the formula,

Mc=5.2+(D-30.0)/ (180.0-30.0)-H/700.0, where D is the epicentral distance in degree and

H is the focal depth in km (Liu and Gao, 2010). For the range values of D and H, the value of Mc varies from 4.2 (for D = 30 and H = 700km) and 6.2 (for D = 180 and H = 0 km). 10

CRB IB

BRP

ESRP WP BRP

Figure 4.1. Seismic stations and major geological provinces of the study area. The red triangles represent the seismic stations and the various geological provinces are demarcated by the different colored lines.

Numerous tests have resulted in choosing the parameters used in the equation aimed at balancing the quality and quantity of the seismic data to be used (Liu and Gao,

2010). For this study, the seismograms have been bandpass filtered between the frequencies of 0.08 and 0.8 Hz which helps in enhancing the signal. The seismograms were also windowed 20 s before and 300 s after the expected P-wave arrival, recording a total of 320 s for each receiver function. Ammon et al. (1991) presented a procedure for deconvolving the vertical component from the radial component, to obtain radial receiver functions which are only selected if their SNR is above 4.0. The width of the Gaussian filter is chosen to be 5 s and the water level value to be 0.03. The SNR filter removes all low-quality receiver functions and improves the results. Manual checking of individual stations was done to get rid of RFs which have a weak Primary arrival and anomalously high 푃푚푆 arrivals. Also, the receiver functions having a P-wave peak considerably 11 deviated from 0 s were discarded as they are caused due to the presence of low velocity sedimentary layers below the receivers (Nair et al., 2006; Yu et al., 2015). After all the filtering a total of 27,653 receiver functions were used for this study.

The three component seismograms record the teleseismic waves that travel thousands of kilometers within the Earth and are a product of the source function, the

Earth function which is the travel path going through geometrical spreading and absorption due to the local structure and geological properties of the media (Burdick and

Langston, 1977; Langston et al., 1979). When the incident wave field encounters a contrasting interface causing a sharp variation in the acoustic impedance, in our case the

Moho discontinuity, it goes through seismic phase change generating several phase conversions according to the Zoeppritz equations. We focus on the 푃푠 phases converted at the Moho and travelling through the crust carrying the information about its thickness and composition. The primary conversion is called 푃푚푆 in this study and the multiple phases are called 푃푃푚푆 푎푛푑 푃푆푚푆. The primary conversion is the one produced due to conversion of direct P-wave into S-wave, and the 푃푃푚푆 is the phase which passes through the Moho unchanged and is reflected back from the surface to be changed into S- wave at the Moho again. The slowest of these three phases is the 푃푆푚푆 phase which is visible as a negative phase on the receiver function. It is converted into an S-wave while being reflected downward from the surface and is reflected upward at the Moho as an S- wave before being recorded by the station. The converted 푃푠 phases are recorded on the radial component of the seismogram whereas the vertical component mostly comprises of the compressional wave. The near vertical incidence of these waves from below the surface helps in separating the compressional and converted shear waves. Receiver 12 functions are mainly used to study the near Earth’s structure close to the receivers, hence the name is derived [Langston, 1979].

Using a grid search procedure (Chevrot and Van der Hilst, 2000; Zhu and

Kanamori, 2000; Nair et al., 2006; Bashir et al., 2011), the crustal thickness (퐻), 푉푝/푉푠

(퐾) and maximum stacking amplitude (푅) is determined for individual stations. The receiver functions are stacked by varying the H and 퐾 values. For a candidate H-K pair, the ray parameter is used to calculate the arrival times of all the three phases - 푃푚푆,

푃푃푚푆 and 푃푆푚푆. Then the amplitudes of the phases are stacked at the calculated arrival times using the equation (Zhu and Kanamori, 2000)

푛 (푖,푗) (푖,푗) (푖,푗) 퐴(퐻푖,휙푗)= ∑푘=1 휔1 × S푘( t1 ) + 휔2 × S푘(t2 ) − 휔3 × S푘(t3 ), (1)

Where 휙 = 푉푝/푉푠, 휔1, 휔2and 휔3 are the weighting factors and t1, t2 and t3 are the predicted arrival times for the 푃푚푆, 푃푃푚푆 푎푛푑 푃푆푚푆 phases respectively, and n is the number of receiver functions for this station. 퐴(퐻푖,휙푗) is the total stacking amplitude for the corresponding H-K pair. The values for 휔1, 휔2 and 휔3are chosen to be 0.5, 0.4 and

0.1 respectively for this study. The values of t1, t2 and t3 are calculated using the following formulae (Nair et al., 2006): –

(푖,푗) 0 −2 2 −2 2 푡1 = ∫ [ √(푉푝(푧)⁄휙푗) − 푝 − √푉푝(푧) − 푝 ]푑푧 (2) −퐻푖

(푖,푗) 0 −2 2 −2 2 푡2 = ∫ [ √(푉푝(푧)⁄휙푗) − 푝 + √푉푝(푧) − 푝 ]푑푧 (3) −퐻푖

(푖,푗) 0 −2 2 푡3 = ∫ 2√(푉푝(푧)⁄휙푗) − 푝 푑푧 (4) −퐻푖

푝 here stands for P-wave ray parameter, 퐻푖 and 휙푗 are the candidate thickness and 푉푝/푉푠 values respectively. A typical H-k plot used in this study, showing all three phases is shown in the Figure 4.2.

PmS PPm S

PSmS

Figure 4.2. Stacked receiver functions for the station D39 which lies in the Idaho Batholith region. Individual receiver functions (127) have been shown by black lines and are stacked against the back-azimuth. The resultant of all the receiver functions stacked is traced by the thick red line, clearly showing the 푃푚푆 arrival at about 5 s, and the two multiples including the positive 푃푃푚푆 and the negative 푃푆푚푆. The bottom plot shows the result of the H-K grid search with the black dot at the optimal pair of thickness (H) 푎푛푑 푉푝/푉푠 (K). 14

For producing the H-K plots, the value of Hi is initially assigned to 15 km and then incremented up to 65km with an increase of 0.1 km for every iteration. Similarly, the value of 휙푗 is varied from 1.65 to 1.95 with a raise of 0.0025. 퐴(퐻푖,휙푗) is obtained by stacking the RFs at every candidate H-K pair using Equation (1).

Many stations have been rechecked with the above ranges changed to accommodate the maximum stacking peak in the examination, correlating with the nearby stations. Also, a lot of stations had identified false peaks at 15 km and K = 1.65, so the range was modified from 20 km to 55 km for these stations. The average P-wave crustal velocity is chosen to be 6.1 km/s for the grid search or the H-k stacking. The H-k stacking also provides the R value which reflects the sharpness of the Moho and is the ratio of the stacking amplitude corresponding to the optimal pair of H-k and the direct P- wave. The R value also shows how consistent the crust is spatially (Nair et al., 2006).

To compare the H-K stacking resuts with mantle structure revealed by previous studies, P-wave velocity perturbations for various depths are plotted using the AK135 earth model as references to see how deep the low-velocity anomaly is visible. Such anomalies are possibly caused by temperature variation associated with a magma reservoir in the crust and a possible plume-like structure in the mantle.

5. RESULTS

Crustal thickness, 푉푝/푉푠 and R values were obtained for all the 193 stations. The stations, after being manually checked, are divided into two categories based on their quality. Category “A” stations have receiver functions showing a clear 푃푚푆 arrival between 3.5 and 7 s along with 푃푃푚푆 or 푃푆푚푆 or both arrivals. When 푃푚푆 is accompanied by any of the later converted phase arrivals, it results in a clearly defined peak on the H-K plot (Figure 4.2). The category “B” stations have a wider and less defined peak compared to the category “A” stations due to the absence of 푃푃푚푆 and

푃푆푚푆 arrivals. For these stations, we choose the thickness corresponding to the 푉푝/푉푠 value of 1.78, which is the global average value of crustal 푉푝/푉푠 (Christensen et al.,

1996). The composition of the crustal rocks determines the 푉푝/푉푠 values as it depends on the Poisson’s ratio (Tarkov and vavakin, 1982; Christensen et al., 1996; Chevrot and van der Hilst, 2000). The 푉푝/푉푠 values are affected primarily by the presence of SiO2 in the crust (Nair et al., 2006). The abundance of quartz in the upper crust results in essentially felsic composition and the increase in plagioclase content towards the lower crust leads to its more mafic composition. The crustal thickness map, Vp/Vs map, and the

R value for the entire region have been plotted in the Figure 5.1, Figure 5.3, and Figure

5.4 respectively.

5.1 AREA A

Area A comprises of 86 stations, in the Wyoming Province comprised of Archean thick and strong lithosphere (Thomas et al., 1987; Henstock et al., 1998; Dueker et al.,

2001). It has major parts of the Wyoming Basin and the Rocky Mountains in western 16

Wyoming. Area A has the thickest crust of around 54 km in the Wyoming Basin beneath the Laramie Range. The average crustal thickness, using 10 seismic stations, beneath the

Figure 5.1. Resulting crustal thickness from category A and B stations. The bold dashed line represents major tectonic provinces. Two profiles have been plotted parallel (A-A1) and perpendicular (B-B1) to the Eastern Snake River Plain (A-A1).

Wind River Range is 45.4 km. The Wyoming Range and the Salt River Range in the eastern edge of Wyoming State, have a high crustal thickness around 49 km. Both Wind

River and the Wyoming Range are an extension of the Rocky Mountains. Three stations beneath the Teton Range to the north of the Wyoming Range have an average Moho depth of 37 km. The Beartooth Mountains to the northeast of Yellowstone Plateau have an average crustal thickness of 43 km and the 12 stations at the Bighorn Mountains give an average Moho depth of 38.5 km. The 푉푝/푉푠 value ranges from 1.67 to 1.85 for the entire region. The R value is the least below the Wyoming Province 0.07 to 0.28 with an 17 average of 0.14. The average crustal thickness and 푉푝/푉푠 values in area A are 45 km and

1.80 respectively.

5.2 AREA B

The Yellowstone Plateau region is defined by the geologic map of Wyoming compiled by Love and Christiansen (1985) and lies in area B. It has a thickness ranging from 38 to 48 km, a region shaped by the rhyolite-basalt eruptions and having a 푉푝/푉푠 value ranging from 1.75-1.88, derived from the analysis of 11 stations. The crustal thickness agrees with many previous studies conducted in this region (e.g., Smith and

Braile, 1993; Schilly et al., 1982). The average crustal thickness of the Yellowstone

Plateau is ~ 43 푘푚 and average 푉푝/푉푠 value is 1.80. The stations at the Yellowstone caldera show high amplitude reverberations, possibly caused because of the presence of a low velocity magma layer (Yu et al., 2015). Two of the stations, YML and Y02 show phase reversal before the primary arrival (Figure 5.2).

Figure 5.2. Stations showing a phase reversal at 0 s indicating direct P-wave arrival from the edge of the magma chamber (Chu et al.,2010). 18

This was also observed by Chu et al. (2010) referring it as the arrival of direct P- wave from the edge of the low velocity layer. The R value ranges from 0.08 to 0.25 and the average R value is 0.16, which is higher than that of the stable Wyoming Craton.

5.3 AREA C

This area covers Eastern Snake River Plain from Island Park caldera in the NE to

Bruneau-Jarbidge volcanic field in the SW. It also covers the Owyhee- Humboldt volcanic field in the west of ESRP and most part of the Western Snake River Plain. The resulting crustal thicknesses range from 45 km to 48 km to the north of the Island Park caldera and averages out to 43.5 km beneath the Island Park, it further reduces towards the SW along the ESRP. The thickness is about 40 km below the Heise caldera and reduces to 37 km below Picabo caldera and 36.6 km beneath the Twin Falls and is 39 km below the Bruneau- Jarbidge caldera. The average 푉푝/푉푠 value for the ESRP is 1.82, with a maximum of 1.90 below the Heise caldera. The average crustal thickness below

Owyhee-Humboldt volcanic field is 39 km and 푉푝/푉푠 value is 1.79, which is comparable with the Moho depth of 40 km obtained by Eager et al. (2011). The WSRP is 36 km above the Moho in the southern region and 30 km in the north. The average R value of

0.20 for the ESRP crust is higher than the rest of the area, and is consistent till the Picabo complex and then reduces to 0.16 below the Twin Falls complex. The average thickness of the crust for the entire Eastern Snake River Plain is 39 km.

5.4 AREA D

The area covered by the Basin and Range has a crustal thickness from 27 km to

42 km with an average of 35.7 km calculated from 37 stations. The 푉푝/푉푠 value is lower than the ESRP in the north, and lies between 1.69-1.92 with an average of 1.80. The 19

Rocky Mountain Basin and Range in the vicinity of the ESRP has an average thickness of

36 km and an average 푉푝/푉푠 value of 1.77. The R value is distinctively lower than the

ESRP, ranging from 0.08 to 0.31 with an average of 0.16. Additionally, this area is separated from areas A and B which lie in the relatively stable Wyoming Province, while the Basin and Range Province has undergone crustal thinning and extension.

5.5 AREA E

Area E has 35 stations within the Idaho batholith and the Columbia River Basalt.

The resulting crustal thicknesses range from 29 to 40 km, with an average of 35 km. The

푉푝/푉푠 and R value range from 1.68-1.89 and 0.09-0.23 respectively. Their corresponding averages for 푉푝/푉푠 and R measurements are 1.78 and 0.16 respectively.

Eager et al. (2011) used receiver functions and Gaussian-weighted common conversion point stacking for calculating the crustal thickness and Poisson’s ratio distribution in

Idaho batholith and Columbia River Basalt, getting a thickness of 35 km below southern

Idaho Batholith and 40 km in the north.

To study the variation and relation of crustal thickness, Vp/Vs and R values with each other and with the elevation, and gravity anomaly, two profiles have been plotted crossing across the major tectonic provinces (Figure 5.6 and Figure 5.7). The gravity map of the entire region was plotted using data provided by USGS and was used for the study of crustal thickness and density change in the study area. Figure (5.5) shows the velocity perturbation at various depths below the Yellowstone region. It is observed that negative velocity anomalies are observed only to the depth of 240-260 km with a negative anomaly of -1.2%, which reduces to -1.0% at 280 km depth. Further below at the depth of

380-440 km beneath the Yellowstone, there is rather a positive velocity anomaly of about

+1%, indicating the presence of a colder body compared to the surrounding media in the 20 same depth range. The low-velocity layer which indicates higher temperature or presence of melt, is not seen at this depth. Table 5.1 summarizes the average crustal thicknesses and Vp/Vs ratios for all the areas in the study area and Table 5.2 lists all the data for all the stations used for the study.

Table 5.1. Summary of all the stations in the five areas showing the average crustal thickness (H) and Vp/Vs (K), along with their minimum and maximum values.

Area H K Range-H Range-K

A 45.00 1.80 38-55 1.67-1.85

B 42.65 1.80 38-48 1.75-1.88

C 38.60 1.82 35-42 1.76-1.94

D 35.70 1.77 27-42 1.69-1.92

E 34.42 1.78 29-40 1.68-1.89

Figure 5.3. Resulting 푉푝/푉푠 derived from all the category A stations used in this study.

Figure 5.4. Resulting R value map derived from all the stations used in the study.

Figure 5.5. Bouguer gravity map of the entire region plotted using the data provided by the United States Geological Survey (USGS). 23

Longitude

Figure 5.6. Elevation, crustal thickness, Bouguer gravity anomaly, 푉푝/푉푠 and R value for seismic stations along the profile A-A1 in Figure (5.1). Stations within the area of 50 km from either side of that line have been used to plot this figure.

Longitude

Figure 5.7. Same as the previous Figure 5.6 but for profile B-B1.

Table 5.2 Resulting crustal thickness (H, Hn), 푉푝/푉푠 and R values.

Name Area Network Lat. Lon. H (Km) 푉푝/푉푠 Hn (Km) R N Category

A10 A YH 42.815 -109.585 38.50 1.733 36.97 0.15 17 A

AHID A US 42.765 -111.100 51.91 1.669 37.64 0.07 337 A

BB1 A XV 44.545 -107.628 36.07 1.810 36.67 0.20 24 A

BH1C A XV 44.933 -107.766 35.36 1.772 35.20 0.11 51 A

BH1E A XV 44.200 -106.961 - - 41.08 0.13 70 B

BH2A A XV 44.686 -108.609 49.33 1.785 49.50 0.09 38 A

BH2D A XV 44.658 -107.540 36.05 1.811 36.79 0.16 64 A

BH2E A XV 43.894 -106.881 - - 44.76 0.08 38 B

BH3D A XV 44.417 -107.390 37.35 1.808 38.08 0.17 72 A

BH3E A XV 43.593 -106.893 - - 51.56 0.12 115 B

BH4A A XV 43.705 -108.712 - - 49.15 0.07 114 B

BH4D A XV 44.055 -107.272 - - 44.98 0.07 81 B

BHM2 A XV 44.617 -107.120 35.42 1.902 37.63 0.20 64 A

BHM4 A XV 44.575 -107.295 35.61 1.834 36.64 0.11 60 A

BHN1 A XV 44.750 -107.505 - - 35.31 0.12 42 B

BW06 A US 42.767 -109.558 40.55 1.725 0.00 0.10 516 A

DCID1 A IW 43.595 -111.185 38.60 1.830 39.73 0.21 46 A

E17A A TA 46.462 -110.858 38.62 1.760 39.28 0.10 156 A

E18A A TA 46.566 -109.914 45.32 1.834 52.23 0.18 110 A

E19A A TA 46.461 -108.786 43.41 1.809 48.13 0.22 64 A

E20A A TA 46.504 -108.130 44.40 1.831 50.06 0.14 102 A

F18A A TA 45.905 -109.716 49.59 1.765 50.34 0.16 133 A

F19A A TA 45.854 -108.944 45.47 1.821 49.54 0.10 103 A

FXWY A IW 43.638 -111.027 37.08 1.856 38.50 0.14 397 A

G17A A TA 45.321 -110.740 40.08 1.742 40.22 0.28 62 A

G18A A TA 45.317 -109.563 47.81 1.759 48.59 0.13 78 A

H20A A TA 44.487 -107.999 39.74 1.844 46.08 0.07 126 A

H21A A TA 44.628 -107.042 40.80 1.776 42.04 0.20 45 A 26

Table 5.2 Resulting crustal thickness (H, Hn), 푉푝/푉푠 and R values. (cont.)

I17A A TA 43.920 -110.576 40.91 1.853 47.26 0.30 19 A

I18A A TA 43.701 -109.817 49.72 1.788 49.90 0.13 81 A

I19A A TA 44.036 -108.994 49.50 1.814 50.36 0.15 20 A

I20A A TA 43.950 -108.128 50.03 1.896 59.84 0.14 54 A

I21A A TA 43.812 -107.292 - - 54.90 0.08 246 B

IMW A IW 43.897 -110.939 37.35 1.850 38.80 0.11 634 A

J18A A TA 43.211 -110.020 46.76 1.805 51.94 0.17 117 A

J19A A TA 43.265 -109.053 54.68 1.754 54.82 0.18 92 A

J21A A TA 43.346 -107.446 - - 54.73 0.08 156 B

K17A A TA 42.751 -110.920 52.54 1.749 52.95 0.22 80 A

K19A A TA 42.825 -108.847 45.18 1.796 49.10 0.05 207 A

K20A A TA 42.658 -108.342 - - 45.75 0.12 137 B

K21A A TA 42.633 -107.251 - - 51.90 0.09 187 B

K22A A TA 42.651 -106.524 - - 52.24 0.07 139 B

K23A A TA 42.755 -105.625 38.01 1.831 42.45 0.08 164 A

L17A A TA 42.099 -110.873 41.94 1.744 42.20 0.26 76 A

L18A A TA 41.924 -110.036 38.66 1.883 44.15 0.20 86 A

L21A A TA 41.964 -107.369 - - 51.33 0.13 172 B

L22A A TA 42.031 -106.434 53.14 1.763 53.20 0.10 157 A

L23A A TA 42.114 -105.701 - - 52.40 0.07 158 B

L32 A YH 42.413 -110.284 42.63 1.790 42.90 0.24 11 A

L33 A YH 42.413 -110.283 42.85 1.769 42.68 0.18 11 A

L34 A YH 42.414 -110.280 42.97 1.770 42.77 0.17 11 A

L35 A YH 42.414 -110.277 43.38 1.758 42.89 0.22 11 A

L36 A YH 42.415 -110.273 43.08 1.753 42.62 0.22 12 A

L37 A YH 42.415 -110.271 43.78 1.766 43.36 0.22 12 A

L38 A YH 42.415 -110.268 43.44 1.760 43.00 0.22 12 A

L39 A YH 42.416 -110.265 43.86 1.750 43.20 0.21 13 A

L40 A YH 42.417 -110.262 44.59 1.753 44.04 0.23 14 A

Table 5.2 Resulting crustal thickness (H, Hn), 푉푝/푉푠 and R values. (cont.)

L41 A YH 42.417 -110.259 44.34 1.759 43.97 0.23 13 A

L50 A YH 42.395 -110.295 43.99 1.770 43.83 0.11 18 A

L51 A YH 42.394 -110.293 44.40 1.778 44.35 0.12 19 A

LOHW A IW 43.612 -110.604 34.16 1.864 35.60 0.12 766 A

MOOW A IW 43.749 -110.745 36.74 1.794 37.12 0.19 512 A

N00 A XK 42.461 -107.699 49.88 1.852 38.35 0.12 31 A

N02 A XS 46.251 -109.205 42.66 1.853 44.36 0.13 36 A

PCWY A Z2 43.845 -110.451 44.11 1.814 46.03 0.17 16 A

PD31 A IM 42.767 -109.558 35.90 1.732 0.00 0.15 992 A

PD32 A IM 42.767 -109.558 35.96 1.731 0.00 0.18 1124 A

REDW A IW 43.362 -110.852 39.03 1.813 39.76 0.11 325 A

RLMT A US 45.122 -109.267 54.82 1.754 54.33 0.11 235 A

RRI2 A IW 43.347 -111.320 34.90 1.877 38.61 0.23 192 A

S05 A XS 45.414 -108.940 51.43 1.817 52.43 0.10 34 A

S101 A XL 42.446 -108.894 45.23 1.767 45.10 0.14 29 A

S13 A XS 45.745 -109.434 50.34 1.847 51.97 0.21 18 A

S18 A XS 45.954 -109.498 46.36 1.856 47.91 0.15 37 A

S20 A XS 46.062 -108.940 45.20 1.865 46.80 0.09 30 A

SM22 A ZH 44.524 -108.040 39.26 1.792 39.48 0.19 17 A

SM30 A ZH 44.584 -107.625 35.41 1.838 36.60 0.23 29 A

SM31 A ZH 44.587 -107.584 36.38 1.813 37.17 0.22 26 A

SM32 A ZH 44.585 -107.536 37.81 1.773 37.74 0.19 16 A

SM36 A ZH 44.630 -107.366 35.14 1.846 36.77 0.28 29 A

SM38 A ZH 44.615 -107.277 35.44 1.840 36.92 0.21 35 A

Y07 A XC 42.559 -110.886 - - 47.86 0.16 68 B

Y19 A XC 43.619 -110.604 34.08 1.860 35.44 0.12 60 A

Y22 A XC 42.590 -109.252 - - 37.95 0.19 28 B

Y47 A XC 45.993 -110.043 42.98 1.854 46.34 0.16 27 A

Y50 A XC 45.153 -108.966 54.04 1.763 53.51 0.15 24 A

EHMT B Z2 45.067 -111.191 40.48 1.762 40.01 0.19 23 A 28

Table 5.2 Resulting crustal thickness (H, Hn), 푉푝/푉푠 and R values. (cont.)

H16A B TA 44.704 -111.248 42.13 1.880 46.90 0.16 121 A

Y17 B XC 44.098 -111.186 41.32 1.811 41.94 0.19 37 A

YHB B WY 44.751 -111.196 48.24 1.763 47.86 0.11 503 A

YHH B WY 44.788 -110.851 38.13 1.838 39.80 0.15 1242 A

YHL B WY 44.851 -111.183 45.32 1.794 45.60 0.08 285 A

YMR B WY 44.669 -110.965 46.63 1.791 46.92 0.24 690 A

YNE B WY 45.008 -110.008 40.95 1.754 40.35 0.14 206 A

YPK B WY 44.732 -109.922 39.45 1.833 40.69 0.12 960 A

YPP B WY 44.271 -110.804 45.09 1.796 45.54 0.25 227 A

YTP B WY 44.392 -110.285 41.42 1.859 43.67 0.15 204 A

BRK C XJ 42.548 -114.965 - - 38.82 0.15 75 B

I15A C TA 44.000 -112.485 41.25 1.840 49.30 0.14 79 A

I16A C TA 43.876 -111.487 41.81 1.844 44.11 0.14 157 A

ID004 C XC 42.917 -116.627 36.63 1.813 37.18 0.08 83 A

ID008 C XC 42.547 -115.980 37.65 1.800 38.40 0.19 135 A

ID009 C XC 42.602 -116.980 36.34 1.828 37.31 0.12 140 A

ID010 C XC 42.545 -116.783 38.53 1.789 38.74 0.17 129 A

ID014 C XC 42.340 -116.288 40.24 1.764 39.97 0.18 131 A

ID015 C XC 42.309 -116.170 38.40 1.817 39.22 0.14 125 A

ID016 C XC 42.294 -116.690 39.30 1.776 39.28 0.16 146 A

ID017 C XC 42.282 -115.986 39.30 1.776 39.26 0.15 134 A

J14A C TA 43.323 -113.518 37.16 1.825 42.34 0.26 104 A

J15A C TA 43.400 -112.433 38.14 1.904 45.03 0.15 111 A

K11A C TA 42.771 -116.032 36.79 1.821 38.43 0.19 131 A

K12A C TA 42.636 -114.903 37.48 1.939 40.03 0.18 119 A

K13A C TA 42.649 -114.084 36.66 1.802 37.18 0.13 100 A

L10A C TA 42.077 -116.471 38.73 1.759 39.40 0.20 212 A

L11A C TA 42.167 -115.754 38.72 1.828 41.74 0.18 168 A

L12A C TA 42.146 -115.016 38.37 1.807 39.84 0.22 160 A

Table 5.2 Resulting crustal thickness (H, Hn), 푉푝/푉푠 and R values. (cont.)

L13A C TA 42.089 -113.944 35.10 1.860 41.24 0.28 192 A

Y02 C XC 44.073 -112.637 40.37 1.804 41.07 0.27 31 A

Y03 C XC 43.805 -112.315 39.96 1.890 49.24 0.22 46 A

Y04 C XC 43.513 -111.900 - - 36.60 0.23 63 B

Y16 C XC 44.398 -111.601 40.48 1.878 42.20 0.22 22 A

ASI2 D UU 44.574 -114.258 - - 36.03 0.16 81 B

BOZ D US 45.597 -111.630 34.13 1.733 33.57 0.13 670 A

D08 D XT 43.938 -114.197 41.12 1.752 40.38 0.18 120 A

D11 D XT 44.072 -114.022 38.99 1.779 38.98 0.10 108 A

D32 D XT 44.533 -113.987 39.25 1.775 39.00 0.12 75 A

D33 D XT 44.505 -114.349 35.90 1.763 35.70 0.22 106 A

E13A D TA 46.442 -114.188 32.38 1.818 36.92 0.21 181 A

E14A D TA 46.416 -113.493 27.46 1.792 27.64 0.13 154 A

E15A D TA 46.425 -112.641 - - 31.40 0.08 164 B

E16A D TA 46.534 -111.676 32.15 1.794 33.40 0.17 142 A

F13A D TA 45.789 -114.332 36.99 1.760 36.70 0.14 153 A

F14A D TA 45.812 -113.370 32.72 1.790 33.02 0.08 31 A

F15A D TA 45.841 -112.493 34.59 1.755 34.96 0.07 203 A

F16A D TA 45.784 -111.626 34.09 1.775 34.66 0.16 155 A

G14A D TA 45.243 -113.460 34.10 1.800 35.30 0.21 129 A

G15A D TA 45.166 -112.489 35.45 1.737 34.72 0.14 122 A

G16A D TA 45.229 -111.805 34.07 1.715 33.75 0.30 113 A

H13A D TA 44.564 -114.255 - - 36.59 0.12 227 B

H14A D TA 44.617 -113.367 38.49 1.693 37.59 0.31 75 A

H15A D TA 44.617 -112.644 41.91 1.712 41.49 0.15 98 A

I13A D TA 43.915 -114.117 38.71 1.921 40.08 0.06 243 A

I14A D TA 43.929 -113.452 39.56 1.700 38.80 0.09 213 A

J16A D TA 43.274 -111.612 36.81 1.862 45.22 0.16 126 A

K14A D TA 42.545 -113.176 34.71 1.835 39.57 0.23 210 A

Table 5.2 Resulting crustal thickness (H, Hn), 푉푝/푉푠 and R values. (cont.)

K15A D TA 42.685 -112.531 37.42 1.732 37.60 0.11 136 A

L02 D XT 44.307 -113.603 37.80 1.818 38.25 0.08 83 A

L05 D XT 44.254 -114.324 36.54 1.806 37.40 0.16 110 A

L06 D XT 44.261 -114.462 34.27 1.797 34.78 0.16 128 A

L14A D TA 42.034 -113.240 36.08 1.783 37.23 0.14 159 A

L15A D TA 42.004 -112.386 - - 34.64 0.11 165 B

L16A D TA 42.015 -111.432 37.43 1.761 38.14 0.19 103 A

MSO D US 46.829 -113.941 35.58 1.752 35.09 0.11 378 A

Y05 D XC 43.188 -111.583 34.04 1.868 38.94 0.23 56 A

Y12 D XC 45.599 -113.112 - - 35.57 0.26 29 B

Y15 D XC 44.582 -112.064 40.68 1.768 40.52 0.17 19 A

Y24 D XC 45.982 -112.615 33.17 1.786 33.38 0.20 48 A

Y37 D XC 46.179 -112.030 33.28 1.766 33.04 0.19 53 A

D01 E XT 43.636 -113.740 40.31 1.729 40.29 0.13 155 A

D03 E XT 43.606 -114.644 39.26 1.774 39.24 0.09 141 A

D06 E XT 43.695 -115.849 33.33 1.833 33.95 0.09 66 A

D15 E XT 44.011 -116.266 29.98 1.731 29.20 0.14 131 A

D34 E XT 44.564 -115.090 37.98 1.808 38.58 0.15 106 A

D35 E XT 44.529 -115.576 32.74 1.834 33.64 0.13 114 A

D38 E XT 44.863 -114.454 35.67 1.827 37.01 0.26 59 A

D39 E XT 44.549 -114.855 37.78 1.759 37.39 0.19 127 A

D49 E XT 44.949 -115.168 36.63 1.823 38.00 0.21 57 A

D50 E XT 45.042 -115.745 34.13 1.850 35.53 0.17 115 A

D51 E XT 45.119 -116.020 33.99 1.860 35.86 0.13 35 A

D53 E XT 45.066 -116.767 31.23 1.835 32.11 0.20 104 A

E11A E TA 46.356 -116.209 31.08 1.784 32.06 0.20 131 A

E12A E TA 46.415 -115.571 32.35 1.799 33.55 0.22 150 A

F11A E TA 45.888 -116.155 - - 32.79 0.15 98 B

F12A E TA 45.757 -115.255 35.79 1.708 35.08 0.14 227 A

Table 5.2 Resulting crustal thickness (H, Hn), 푉푝/푉푠 and R values. (cont.)

G11A E TA 45.400 -116.268 44.37 1.720 44.06 0.15 147 A

G12A E TA 45.129 -115.326 39.19 1.688 38.08 0.19 133 A

H10A E TA 44.589 -116.747 31.71 1.707 31.25 0.15 119 A

H11A E TA 44.703 -116.013 30.94 1.802 31.93 0.22 111 A

H12A E TA 44.549 -114.855 37.69 1.744 37.19 0.11 39 A

I11A E TA 43.912 -115.958 31.28 1.755 31.67 0.05 151 A

I12A E TA 43.794 -115.133 32.30 1.655 31.25 0.06 194 A

J10A E TA 43.428 -116.767 - - 35.86 0.09 170 A

L07 E XT 44.318 -114.713 36.25 1.780 36.30 0.15 108 B

L08 E XT 44.267 -114.919 36.64 1.803 37.18 0.11 118 A

L09 E XT 44.333 -115.063 35.79 1.814 36.58 0.20 86 A

L11 E XT 44.418 -115.494 31.38 1.890 33.50 0.17 96 A

L12 E XT 44.318 -115.640 33.80 1.832 34.73 0.10 109 A

L17 E XT 44.459 -116.647 29.12 1.754 28.82 0.15 137 A

W01 E ZG 44.921 -115.950 32.98 1.781 33.03 0.16 98 A

W02 E ZG 44.932 -116.140 31.52 1.874 33.48 0.23 90 A

W03 E ZG 44.991 -116.399 30.22 1.849 31.70 0.20 106 A

W04 E ZG 45.045 -116.664 - - 33.25 0.18 119 B

W30 E ZG 45.802 -116.365 34.62 1.853 36.26 0.19 49 A

6. DISCUSSIONS

The crustal thickness map obtained by this study shows similar thickness below most of the stations used in previous studies such as Yuan et al. (2010). The comparison of the US Transportable Array stations from this study and the crustal map of Yuan et al.

(2010) shows quite a correlation, though the average crustal thickness obtained under the

Yellowstone caldera in this study is 43 km compared to 47-52 km by them. Figure 6.1 shows the extension of low velocity zone at greater depths. Other studies like Smith and

Braile (1993) have reported the thicknesses of 42- 45 km for their model which is quite comparable to results from this study. The thickness along the ESRP gradually decreases from Island Park (43 Km) Heise caldera (40 km) to the older Picabo and Twin Falls caldera (36-37 km). This is different from Yuan et al. (2010) in which they obtained an average thickness of 42 km underneath Snake River Plain, shallowing to 37 km on either side and abruptly getting thicker to about 47 km under Island Park to the northeast.

The biggest revelation of this study is that the thickness is quite comparable between the ESRP and the Basin and Range province which has a mean thickness of 37 km in the north of Eastern Snake River Plain and 36 km to the south of it. The overall average crustal thickness differs by 3 km between ESRP and BRP but the difference is because of stations with very low thickness away from the ESRP. The similarity in Moho depth between the ESRP and BRP could mean the crust beneath the ESRP might have evolved with the same extensional thinning and magmatism in both these regions.

The crustal thickness under the Archean Wyoming Province is expectedly high being an ancient craton with thick lithospheric roots and being stable compared to surrounding upper mantle (e.g., Pearson et al., 1995; Carlson et al., 1999). The chemical 33 buoyancy of Archean cratons is also high to balance out the cold temperatures (e.g., Boyd et al., 1989; Griffin et al., 1999; Sleep et al., 2005). The crust thickness below the Big

Horn Mountains is lower than the rest of the Wyoming Craton with an average value of

38.5 km. Yeck et al. (2014) used receiver functions and synthetic seismograms and obtained a thickness of 40 km beneath the northern Bighorn Mountains and 45 km below the southern region. The cross-correlation plot (XCC) shown in Figure 6.2 shows a high correlation between the thickness obtained by this study and their work. They have accounted this lower thickness compared to the Wyoming Province to an upwarp or

Moho arch under the Bighorn Mountains which occurred prior to Laramide shortening as a result of lithosphere buckling. More recent studies using analysis of P-wave velocity models from the Bighorn Arch Seismic Experiment (BASE), have observed upwarp of the Moho from ~50 푘푚 to ~37 푘푚 depth from the Bighorn Basin in the west towards the Bighorn Mountain Range in the east (Worthington et al., 2016). Their crustal thickness is comparable with that obtained in this study. The R value observed for the

Bighorn region is higher than the surrounding Wyoming Province, signifying the sharpness of the converted phase. This could be due to a weak or absence of the high velocity (7.x) lower crustal layer. Worthington et al. (2016) have also observed the absence of the high-velocity layer in their study. A possible explanation for the reduced thickness and high R value could be the delamination of the lower crust (Gao et al., 1998;

Jull and Kelemen, 2001; Frost et al., 2006). Frost et al. (2006) have suggested that

Archean crust can go through late stage delamination. Loss of lower mafic crust would increase the average felsic content of the crust, but the crustal composition is mostly mafic as observed by the high Vp/Vs values. Thus, delamination or eclogite sinking is not a possible explanation for the shallow Moho depth observed below the Bighorn region. 34

The Owyhee Plateau in the southwest of ESRP shows distinctive properties of continental plateaus like the Colorado Plateau having a thick crust and high 푉푝/푉푠 values. Bashir et al. (2011) obtained 42 km thickness beneath CP. Zandt and Ammon (1995) had shown that a thick Precambrian crust lies underneath the layer formed by Cenozoic volcanism.

The average 푉푝/푉푠 value for the entire region is 1.79 which is close to the global average value of 1.78, measured by Christensen et al. (1996) using laboratory experiments. The average 푉푝/푉푠 value for the upper crust is 1.74 and for the lower crust is 1.81, leading to the average continental crustal 푉푝/푉푠 of 1.78. 푉푝/푉푠 values less than or equal to 1.76 are because of rocks of felsic composition, likewise 1.78 to 1.81 correlate with intermediate rocks, while 푉푝/푉푠 values higher than 1.81 are associated with mafic rocks (Holbrook et al., 1992). Area A, B and C have an overall higher 푉푝/푉푠 than Area

D and E. The reasons for which the ratio is high for the three regions could be different.

The Wyoming Province is composed of very thick crust and predominantly Archean mafic rocks. The reason for higher 푉푝/푉푠 could also be the result of high velocity lower crustal underplating found by the Deep Probe experiment (Henstock et al., 1998; Gorman et al., 2002), which increases the velocity of the P-wave, consecutively increasing the

푉푝/푉푠 ratio. The intermediate 푉푝/푉푠 ratio observed below the Wind River Range is probably due to the higher S-wave velocity observed by Yuan et al. (2010) using ballistic surface wave dispersion measurements (Stachnik et al., 2008).

Area B has an upper crustal magma reservoir which reduces the S-wave velocity compared to the surrounding area, this reduction in shear wave velocity seen in the shear velocity model by Yuan et al., (2010) has resulted in an increase in the 푉푝/푉푠 ratio near

Yellowstone. The most interesting result is the high 푉푝/푉푠 ratio in area C along the

Eastern Snake River Plain. 35

20 km 40 km

60 km 80 km

200 km 240 km

280 km 320 km

380 km 400 km

420 km 440 km

This region has gone through copious magmatic eruptions resulting in the time- progressive volcanic chain and calderas. Basaltic magma intrusion is responsible for the increased 푉푝/푉푠 ratio in ESRP, which is distinctly higher than the Basin and Range 38

Province to the south and north. Assuming, prior to the formation of the volcanic track, the composition was homogeneous in the BRP and the ESRP. The basaltic eruptions modified the composition along the ESRP making it distinct from that of the BRP. The R value for ESRP is comparably high and could be because of the low velocity layer at the base of the lower crust caused by the presence of partial melt, as explained by Peng and

Humphreys (1998). This layer creates a larger-than-normal contrast between the subcrustal lithosphere and the lower crust, enlarging the 푃푚푆 arrival. The Owyhee-

Humboldt volcanic field has a 푉푝/푉푠 ratio of 1.79 which is close to the global average of crustal composition.

For area D and E, the composition is mainly felsic shown by the lower 푉푝/푉푠 ratios obtained by the receiver functions. The BRP has a lower ratio of 푉푝/푉푠 in the south and north of the ESRP. This is consistent with the 푉푝/푉푠 ratio obtained by

Frassetto et al. (2006) and Bashir et al. (2011). The felsic composition of BRP has been accounted to the delamination of the mafic lower crust also explaining the thinner crust

(Gao et al., 1998; Jull and Kelemen, 2001). But like Bashir et al. (2011), there is not an increase in R value due to delamination of lower crust or the presence of a low velocity layer. This concludes that the composition and the structure of BRP and ESRP are distinctively different although their crustal thickness is comparable. Area E comprised of the IB and CRB has a varying range of 푉푝/푉푠 values. The Cretaceous intrusive rocks forming the IB have a felsic to intermediate composition due to mixed mafic magmas and granites generating quartz diorite complexes (Foster et al., 1988). The intrusion of mafic magma as dikes into the host granite led to the metamorphized intermediate composition.

The dikes are composed of high alumina basaltic andesite to calc-alkaline dacite. The

CRB has a mafic composition as observed by the 1.80+ 푉푝/푉푠 values given by the 39 optimal H-k plots of the seismic stations. The solidified lava flow during the middle

Miocene, formed voluminous basalt in this region. The composition is similar to that in the ESRP and may have been formed by the same source for volcanism.

7. CONCLUSIONS

Several conclusions can be drawn from this study:

1. The composition of the Eastern Snake River Plain is different from that of the

Basin and Range Province. The ESRP has a higher 푉푝/푉푠 value of 1.82

compared to the lower value of 1.77 of the BRP. The higher 푉푝/푉푠 value of

ESRP is most likely due to the addition of basalt into the crust but this intrusion

does not cause an increase in the crustal thickness.

2. The crustal thickness of ESRP is comparable with that of BRP and the higher R

value of ESRP implies the presence of partial melt at the lower crustal base.

3. The crustal thickness below the Wyoming craton is the highest in the study area

and this area is characterized by high 푉푝/푉푠 values due to the presence of a high

velocity 7.x layer at the lower crust. The R value is the least compared to other

areas, probably due to stable cratonic roots.

4. The crust beneath the Yellowstone is 43 km thick and has a high 푉푝/푉푠 value of

1.80. This thickness is more accurate and is based on results obtained from better

quality and more number of receiver functions, and provides refined constraints

for various models proposed over the years for the depth of the lower magma

chamber, magmatic dikes, and the existence and depth extent of the perceived

mantle plume.

APPENDIX

EXAMPLES OF STACKED RECEIVER FUNCTIONS FROM ALL THE AREAS

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VITA

Avikant Dayma was born in Chennai, Nagpur. In May 2015, he received his

Bachelor’s in Technology (B.Tech.) in Mining Engineering from Visvesvaraya National

Institute of Technology in Nagpur, India. He completed three internships in mining industry during his undergraduate and an internship in Mercator Petroleum Ltd., during his Master’s at Missouri University of Science and Technology. Under the guidance of

Dr. Stephen Gao, he started working on Receiver Function, Seismic data interpretation,

Oil and Gas exploration and Seismic Anisotropy.

He received his Master’s degree in Geology and Geophysics from Missouri

University of Science and Technology in July 2017. He was the treasurer of the Society of Exploration Geophysicists student chapter in the University.