Final Technical Report - United States Geological Survey Earthquake Hazards Program

Project title: Recency and size of young displacements along the Mead Slope fault, Lake Mead Area, Arizona Project Award No. G18AP00003 – Intermountain West Award Dates: December 2017-November 2018

Submission Date: August 30th, 2019

P.I.s – Jeri Young Ben-Horin and Phil Pearthree, Arizona Geological Survey – Univ. Of Arizona

Additional Coauthors: Brian F. Gootee, Arizona Geological Survey – Univ. of Arizona

Acknowledgement of Support and Disclaimer This study is based on work supported by the U.S. Geological Survey under Grant Number G18AP00003. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the opinions or policies of the U.S. Geological Survey. Mention of the trade names or commercial products does not constitute the endorsement of the names or products by the U.S. Geological Survey.

Abstract The Mead Slope fault (MSF) has been considered an active late Quaternary fault for several decades; however, until this study, there have been weak constraints on slip rates, and the age and size of surface-rupturing earthquakes. We used high-resolution DEMs generated from multiple drone flight and ground-control points and aerial imagery to map the fault in detail. We determined that the fault consists of two main strands, both offsetting Quaternary alluvial fan remnants. The northwestern strand offsets late to latest Pleistocene fan deposits, as well as relatively young tributary gravel deposits exposed below a wave-cut bench associated with past high levels of Lake Mead. Examination of this exposure revealed 3 identifiable surface ruptures, with the latest two events occurring within the last ~20,000 yrs to 50,000yrs. We collected 3 OSL samples to date the sediments that will help to constrain the earthquake ages, as well as 18 3He cosmogenic surface rock samples to date various Quaternary landforms displaced by the fault. Sampling efforts for the cosmogenic dates were focused on early to middle Pleistocene landforms due to the high age inheritance of the younger fan surfaces. We tentatively estimate the slip rate to range from 0.06mm/yr to 0.15mm/yr based on amount of offset of Qi3-4 channels and the correlation of the soil ages from the wave cut exposure. If the samples collected for OSL dating yield reasonable ages, then the wave cut exposure could be examined in more detail as a potential paleoseismic site.

Introduction The Mead Slope fault (MSF) poses a significant seismic hazard for the growing population of the

Las Vegas area and Hoover Dam. The Las Vegas Valley metropolitan area is home to over 2 million people, has been one of the fastest growing areas in the United States over the past 50 years, and draws over 40 million visitors per year. The MSF is one of several fault zones in the Las Vegas area that have evidence of late Quaternary activity, but the close proximity of the MSF to Hoover Dam is particularly concerning as it impounds the largest reservoir by volume in the U.S. and supplies water to millions of people in Arizona, Nevada, and Southern California. The MSF is a NE-SW-trending fault that clearly displaces Quaternary alluvial fan and terrace deposits, ranging in age from early Pleistocene to as young as latest Pleistocene-early Holocene (Anderson and O’Connell, 1993, Beard et al., 2010). Previous mapping and reconnaissance investigations of the fault indicate that it extends some distance beneath Lake Mead to the SW and NE, and it may link up with other fault zones (Longwell, 1936; 1963; Anderson and O’Connell, 1993; Beard et al., 2007). Age constraints on the most recent fault event are fairly general and limited information exists on the displacement of this event (Anderson and O’Connell, 1993; Taylor and dePolo, 2005). There is obvious evidence for recurrent fault movement in the form of higher fault scarps on older alluvial surfaces, but until this study there had been essentially no quantitative constraints on the long-term fault slip rate or the timing of recurrent movement on the fault zone.

Using detailed mapping techniques and high-resolution Digital Elevation Models (DEMs), along with Optically Stimulated Luminescence (OSL) and 3He cosmogenic dating methods, we will estimate the long-term slip rate, as well as constrain the age of the last two earthquakes. The MSF has proven to be a complex series of predominantly left-lateral faults with a significant vertical component of offset. Using time-dependent characteristics of soil development and change (e.g. Machette, 1985), we preliminarily estimate the age of offset sediments in an exposure along the fault to be approximately 20,000 years old. These relatively young sediments have recorded 2 earthquakes in since their deposition. Upon completion of the OSL and cosmogenic dating within the next 12 months or so, we anticipate age estimates that will bracket the timing of the last two earthquakes and will provide age constraints for slip rate calculations. This information will provide the first ever well-constrained estimate of fault activity and associated seismic hazard to the area.

Project Tasks Our project targeted 4 main tasks to accomplish the goals of mapping and identification of

potential paleoseismic study areas: 1. DEM generation from drone aerial photography – We generated multiple DEMs from several drone flight campaigns. 2. Detailed surficial geologic and geomorphic mapping - We completed detailed surficial and geomorphic mapping of a 1.3km X 6km long, using the DEMs generated during the study, aerial photographs, and field mapping techniques. During this task we identified only one site that had the potential to provide evidence of geologically recent ruptures of young sediments. 3. Collection of 3 OSL and 21 3He cosmogenic samples by Drs. Tammy Rittenour and Cassandra Fenton for the purposes of dating faulted sediments, and to estimate the ages of older fan surfaces – We collected 3 OSL samples from the young faulted sediments, and 21 3He cosmogenic samples from surface basalt boulders on Qo, Qi1, Qi2 and Qi3-4 aged fan deposits, and 4. Publication of maps and research generated by this study will be released as 2 open-file reports within the Arizona Geological Survey, and will be presented at the Geological Society of America annual meeting

September 22, 2019. A minimum of one paper will be submitted for publication in a peer-reviewed scientific journal, such as the Bulletin of the Seismological Society of America. This report will be sent to stakeholders such as the Bureau of Reclamation and the National Park Service on August 30th, 2019.

Figure 1:Bedrock compilation map of the Mead Slope fault area, on the east side of Boulder Basin (scale: 1” = 2.2km) (from Beard, et al., 2007). Small black box in inset in upper left shows the approximate study area in NW Arizona with known Quaternary faults (red lines). Area mapped in this study by the black outlined rectangle near the center of the image. Most of the area shown as “Tcm” by previous workers along the MSF is in fact Pleistocene tributary deposits.

Mead Slope fault

Methods Drone Aerial Photography and DEM-Generation – Task 1 Because the location of the study area lies within a National Recreation Area, we had to obtain a permit with the National Park Service to fly a small unmanned aerial vehicle (sUAV) along the Mead Slope fault. Given there is a general moratorium on flying drones in National Parks, the permitting process took approximately one year to navigate, with 5 different applications required by the NPS to finalize the process. This permitting process delayed the project by a minimum of 6 months, which resulted in a no-cost extension of the project.

During October and November of 2018, a sUAV, DJI Phantom 4 Pro, was used to capture high-resolution aerial imagery within a 6.4km by 1.3km wide zone along the Mead Slope fault system. The drone carried a 20 Megapixel camera/4K video recorder (Model WM331A, S/N 0AXCE7P0B30561); Brian Gootee was the remote pilot (FAA registration no. 4062679). Two main strands of the MSF were targeted in 5 overlapping survey areas (Figure 2) with ground-control points (GCP) placed and surveyed with a differential Trimble GPS ahead of each flight. GCPs consisted of 1-m square Tyvek sheets marked with a “X” to identify from aerial images in processing software. The October survey included 40 GCP targets over much of the MSF area and took 3 days to place, survey and retrieve across challenging foot terrain. The November survey used an additional 13 GCP targets to cover gaps and parts of the south and far northeast MSF strands. A combination of automated software and manual control were used to fly the drone during field surveys. Aerial surveys included the collection of JPG images for surveying and oblique imagery as raw TIF images. Videos were captured in 4K and were used to record general overviews of MSF and areas with more complicated geology and/or terrain (Table 1).

Figure 2: Location of drone flight areas (colored polygons) and ground control point targets (numbered yellow circles) along the MSF, Lake Mead National Recreation Area.

During October and November of 2018, a total of 7 UAV surveys captured 8,300 photos and 8 high-resolution videos, a total of 80.8 Gigabytes of data (Table 1a). Agisoft PhotoScan Professional (v1.4.4) was used to process all images into three-dimensional point clouds used to make ortho-images and digital elevation models (DEMs). Ground control points significantly improved accuracy of the surveys and resulted in orthoimagery between 2 and 3 cm/pixel; however, the size of the files was too large to utilize so imagery was down sampled to 10 cm/pixel. The DEM’s produced using Agisoft range from 5 to 12 cm/pixel (Table 1). Agisoft was also used to generate automated reports for each of the areas processed (Appendix A), which also includes detailed location data for all GCPs.

Surveys flown and DEMs Date shot Size (mi2)

Resolution (cm)

Area 1 (Fishfinder-Castle Cove) 10/22/18 1.17 12 Area 2 (Joes Hill to Megafan) 10/22/18 0.47 5.5

Table 1. UAV flight and data statistics along MSF zone.

UAV Collection Date

Photos Videos

Agisoft PhotoScan Pro

files Orthoimagery

Digital Elevation

Model

No. files Size (Mb)

No. files

Size (Mb)

No. files

Size (Mb)

No. files

Size (Mb)

No. files

Size (Mb)

Oct-18 4,295 30,500 4 6,200 390 53,200 2 4,300 3 3,700 27-Nov-18 1,875 13,500 0 0 903 109,200 1 563 4 3,800 28-Nov-18 2,068 18,300 1 1,000 245 32,200 2 854 4 6,300 23-Apr-19 68 2,300 3 9,000 0 0 0 0 0 0

Totals 8,306 64,600 8 16,200 1,538 194,600 5 5,717 11 13,800

We focused our mapping efforts on areas with relatively young landforms and adjusted the resolution of the DEMs to aid in identifying offset measurements of deflected channels, offset debris lobes on fan surfaces, and offset landform edges. DEMs and hillshades were useful in analyzing surface texture or roughness of landforms due to different levels of channel dissection, desert pavement development, and vegetation density. All these variables are related primarily to the landform’s age, and secondarily to the distance away from the fan apices and bedrock. For example, in Figure 3 below, the surface of an early Pleistocene fan labeled Qi1, appears to have less surface roughness compared to a younger fan surface such as Qi3-4, largely due to the more well-developed desert pavement on Qi1. The older surfaces have had time to reorganize their surfaces and have more uniformity in clast distribution. The source material for the fans does influence the surface roughness as can be seen in the field when traversing the Qi3-4 fan surface. This fan surface is armored with boulder-sized debris lobes, and with desert pavement developing at a slower rate when compared to fan surfaces with smaller average clast size.

Area 3 (Burro ridge patch) 11/27/18 0.03 5 Area 4 (south MSF strand) 11/27/18 0.51 5

Area 5 (Qi4 patch) 11/27/18 0.05 5 Area 6 (Fishfinder-Joes Hill) 11/28/18 0.49 5

Area 7 (Fishfinder-Fortification) 11/28/18 0.34 5

Figure 3: Example hillshade overlain aerial imagery with the MSF shown as red lines in right image (left image is plain to show the surface trace of the fault). The hillshade was generated from a 12cm DEM created from drone imagery and GPS ground-control points. The older fan surfaces (labeled QTa, Qo) are lighter in color, have rounded margins, and a smooth surface. Qi2 to Qi3-4 surfaces appear dark to very dark in color and have rougher surface texture compared to the QTa and Qo surfaces.

For field and laboratory mapping, we mapped at a variable scale depending on the location of the fault. For areas surrounding both main fault strands, we mapped at a scale of 1:3,000 or better. In the rest of the mapping area, we mapped at a scale of 1:6,000. A compilation map can be accessed through our GIS interactive link where we have made the map available: https://uagis.maps.arcgis.com/apps/webappviewer/index.html?id=3a52e869f32b41c2a0849b47d9c51645

While mapping along the northwestern-most strand, we discovered an exposure that had been created by the highest lake level as a “wave-cut” bench on faulted tributary gravel and sand deposits (Figure 4). The location of the bench cut is: 36.0855, -114.7011. We mapped the exposure and estimated the number of earthquakes that offset and fractured multiple sand to gravel-rich layers. In addition, we collected samples for OSL dating from three sand-rich layers that bracket the earthquake event horizons. Overall, the fault zone in the bench cut is approximately 5 m wide and shows evidence of relatively long-lived fault activity. On the east side of the exposure there is a brecciated zone that is about 1 m wide formed in late Tertiary gravels. Adjacent to the brecciated zone, there is a 0.5 meter-wide, dense shear zone with gypsum-filled veins and rotated grains and pebble to cobble-sized clasts. The total amount of offset and number of events are not discernable on the eastern side of the exposure, but given the amount of deformation of older gravels, the thickness of the dense shear zone, and mismatch of units across the fault zone, substantial displacement in multiple surface ruptures is likely recorded. Adjacent to the dense shear zone, is a 2.5-meter-wide faulted zone of relatively young, poorly to moderately sorted subangular sand and gravel deposits.

Figure 4: Generalized oblique ground view (looking southwest) of young sediments faulted and exposed in a wavecut generated by Lake Mead's highest water level in 1983. Sedimentary units are labeled 1-5; Earthquake event horizons are labeled Events 1-3; OSL sample locations shown as blue stars.

OSL and Cosmogenic Dating – Task 3 We used two different dating methods to tackle two major questions: 1. What are the ages of the young, faulted sediments and the events they bracket, and 2. What is the overall slip rate for the fault? We collected three sand-rich samples for Optically-Stimulated Luminescence (OSL) dating to determine the ages of sediments that bracket Event 2 and would help to constrain Event 3(Figure 4). This required three additional samples at the same locations to estimate the natural luminescence signal. The three main samples are being analyzed as single aliquot regenerative samples to estimate the burial luminescence dose, and once compared to the natural signal, an age estimate will be derived from a dose-response curve (personal communication, Tammy Rittenour, 2019). Results from Dr. Tammy Rittenour’s laboratory at Utah State University are expected to be completed by late Fall of 2019.

Table 2. List of OSL sample ID's and description of sampled units.

OSL Sample # Unit Description

MSF-042219-1 3 Alternating coarse, subangular sand and gravel; overall, clast-supported; some thin layering; little carbonate coating on bottom of clasts;

MSF-042219-2 2 Slightly reddened, coarse, subangular pebble to small cobble-sized gravel dominated by volcanic clasts; little carbonate coating on bottom of some clasts;

MSF-042219-3 4 Clast-supported subangular, small-cobble sized gravel; little carbonate coating on undersides of clasts;

With Dr. Cassandra Fenton, we collected 18 samples for 3He cosmogenic dating of surface rocks exposed on Qo, Qi1, Qi2 and Qi3-4 alluvial fans (Table below) to help constrain the overall slip rate. We targeted basaltic rocks that were medium to large boulder-size and didn’t appear to have moved much from their initial site of deposition. We anticipate better age constraints on the Qo, Qi1 and perhaps the Qi2 fan

surfaces, but dates are problematic for younger fans due to high age inheritance of the system (Fenton and Pelletier, 2013). We will estimate longer-term slip rates and attempt to correlate the offsets estimated for Qi3-4 surfaces with events we identified in the bench cut. Completion of the cosmogenic dating is estimated for Spring of 2020.

Date LAT LONG Unit Sample No. General Description 3/18/2019 36.0852 -114.69724 Qo Mead 1 A and B horizon gone; some exhumation;

carbonate litter abundant; sample on basalt with pyroxene visible.

3/18/2019 36.08505 -114.69728 Qo Mead 2 Same surface as Mead1 sample; basalt boulder sampled; 2cm Av depth; weathering rind on smaller cracked and separated rock about 2m thick;

3/18/2019 36.08534 -114.69743 Qo Mead 3 Same Qo surface as Mead 1; sub-rounded basalt boulder; desert varnish; well developed; not cracked but vesicular.

3/18/2019 36.08536 -114.69773 Qo Mead 4a Basalt boulder; well-developed varnish; subangular; with partner but separated and partner is rotated and lower in height; vesicular basalt;

3/18/2019 36.0822 -114.70425 Qi3-4 Mead 5 Well-developed desert pavement; little carbonate litter; clasts have 1-2mm of carbonate coatings, mostly on the bottom end; sample taken on vesicular to massive, sub-rounded cracked basalt boulder;

3/18/2019 36.08211 -114.70425 Qi3-4 Mead 6 Cracked vertically all the way through; still in place; partner not separate; same surface as Mead 5; massive to vesicular; good varnish (weak brown); taken from top 5cm of boulder;

3/18/2019 36.0751 -114.71205 Qi2 Mead 7B Sample from accumulated soil in fault zone; desert pavement sample, using basalt.

3/18/2019 36.0751 -114.71205 Qi2 Mead 7q Desert pavement sample, using granite, targeting quartz; developed in fault zone and younger than Qi2.

3/19/2019 36.07464 -114.71175 Qi2 Mead 8 Vesicular, massive basalt; partly cracked; well-developed desert varnish; ;

3/19/2019 36.07475 -114.71194 Qi2 Mead 9 Basalt boulder, massive; sub-rounded; crack all of the way through with partners; well-developed desert varnish; flat surface

3/19/2019 36.07481 -114.711963 Qi2 Mead 10 Vesicular basalt boulder; cracked through with partners; not iridescent;

3/19/2019 36.0795 -114.70801 Qi1 Mead 11 Desert pavement; basalt clast with amydulation; some clasts iridescent, but majority are good;

3/19/2019 36.07804 -114.70605 Qi1 Mead 12 2mm weathering rind; cracked through with partners; sub-rounded vesicular basalt boulder with well-developed varnish; carbonate pendants on clast bottoms; AV depth about 4.5" ; calcite litter but not as extensive as on Qo surface

3/19/2019 36.07858 -114.70665 Qi1 Mead 13 Cracked all the way through; sub-rounded boulder; basalt; 2-3mm weathering rind

3/19/2019 36.07851 -114.70657 Qi1 Mead 14 Massive basalt boulder; not cracked; has well developed varnish; flat surface; no shielding as with the rest of the samples; 1-2mm weathering rind;

3/19/2019 36.07772 -114.7076 Qi2 Mead 15 Massive basalt boulder; well-developed varnish; cracked through and 1cm separated but partnered; southern part fell off but on ground near main rock; well-developed varnish; AV is 5.5";

3/19/2019 36.07776 -114.70774 Qi2 Mead 16 Massive basalt boulder with well-developed varnish; some spalling off, but that part not sampled; cracked all the way through;

3/19/2019 36.07812 -11470917 Qi2 Mead 17 Vesicular basalt boulder; well-developed varnish except where 29" above ground where the rock spalled off and not sampling there; cracked all the way through with the partner slipped down but still there;

Table 3: Location of cosmogenic dating samples (LAT/LONG), and their associated mapped unit.

Results Mapping Efforts Detailed mapping along MSF resulted in a better understanding of fault kinematics, and provided offset estimates, relative age constraints on faulted landforms and aided in locating a potential site to conduct a detailed paleoseismic study. Figure 5 below shows an overview of the mapped area, which can be accessed at as an interactive GIS map with this link: https://uagis.maps.arcgis.com/apps/webappviewer/index.html?id=3a52e869f32b41c2a0849b47d9c51645

Figure 5: Overview of completed mapping. Major faults shown as black lines; Lake Mead on the west and southwest side of the image. Mapping was performed at 1:3,000 scale near the fault zone and 1:6,000 in the remaining surrounding area. See active GIS link for mapping database: https://uagis.maps.arcgis.com/apps/webappviewer/index.html?id=3a52e869f32b41c2a0849b47d9c51645

Movement on the MSF is predominately left-lateral, with a west-dipping normal fault component along parts of the fault preserved on older Pleistocene landforms, such as Qo alluvial fans. A small reverse component accompanied the left-lateral motion and is preserved on younger landforms (Qi2, Qi3, Qi4) (Figure 5). These observations could be evidence for the linkage of the fault strands along the Mead Slope fault system to a regional detachment fault system observed to the west, Saddle Island fault (Kattenhorn et al., 2008, Lamb et al., 2005). An older, but active strand just east of the main active strand has cut Qi2 and Qi3 surfaces in some places and may have transferred much of the younger slip

to the youngest main strand in more recent events. The fault system could be part of the larger Lake Mead Fault System that includes the Pinto fault, the Hamblin Bay fault, and other faults to the north.

Figure 6: Shaded relief map of Qi alluvial fan surfaces and the main fault trace, artificially illuminated from bottom or south. On the labeled image to the right, the blue lines represent deflected north-flowing channels. The red lines represent active fault traces. Area of offset debris lobes marked with yellow rectangle (see next image for close up of lobes). An early to middle Pleistocene Qi1 surface is labeled on the bottom left side of the right panel. A late Pleistocene alluvial fan surface is labeled in the upper center of the same right panel. Field observations of the relatively younger age of the Qi3-4 include preserved bar and swale topography, and less rounding of the fan’s margins compared to the margins of older landforms.

Table 4 below shows the amount of offset estimated for alluvial fans and channels within the mapping area. There is consistency in the amount of offset on Qi3-4 landforms (Figure 6) and Qi2 surfaces, with the younger landform averaging 3m of left lateral offset, and the older Qi2 averaging 8m. A relatively older landform, Qo, is vertically offset up to 24m, with approximately 110m of left lateral offset. Offset measurements on Qo were taken from piercing points that included the centers and margins of each landform. The youngest alluvial fan surface with identifiable offset was located near the center of the mapping area. This area was targeted for more detailed mapping and included an additional drone flight to create a 5cm DEM used for offset measurements and mapping (Figure 6). Left laterally offset debris lobes on a Qi3-4 fan surface were 8m, 4m and 3m (Figure 7 below). Offset channel measurements on similar aged landforms were 3m and 8m (see Offset Measurement Table 4).

Figure 7: Offset Qi3-4 debris lobes (bright green outlined oblong polygons) along main active fault zone (black lines); measurements from the centers and edges of the lobes averaged 3-4meters. See interactive map for more detail: https://uagis.maps.arcgis.com/apps/webappviewer/index.html?id=3a52e869f32b41c2a0849b47d9c51645

Table 4. Cumulative fault offset along MSF.

X = the distance from the northeastern-most QTa Landform (km) Landform type Relative Age

Horizontal offset (m and mm), LL=left lateral

Minimum Vertical

offset (m) 0 active channel up to present 54 meters (LL) /

0.24 smaller, inset channel on older Qi surface Qi2-Qy3 8 m, LL ~<1m

0.35 Offset margin on Qi3 inset terrace Qi3 8m, LL

0.72 Qi2 - northern edge or margin Qi2 19m, LL

0.81 Qi4 or Qy1 inset terrace along active wash Qy1 <3m

0.83 Qi1 - edge of landform/margin on southside Qi1 8m, LL ~<1 -2m

1 active channel up to present 6 - 8m, LL 1.26 Qi3-4 - lobes offset and rotated; Qi3-4 7.9m, LL 1.36 Qi3-4- lobes offset and rotated; Qi3-4 4m, LL 1.4 Qi3-4- lobes offset and rotated; Qi3-4 3m, LL

1.57 Qy1 - channel offset Qy1 to Qo 10-12m, 1.68 Qo - Early Pleistocene Qo 65-78m 24m

1.92

Tertiary Landform-northside edge offset with minor Qo on top Qo 109m 14m

2.01

active channel offset, deeply incised, but now filled with lake deposits up to present 14m

2.1 Margin or edge on southside of Qo landform Qo 57m

2.45 Qy1 - offset boulders from early Holocene debris or fan deposits Qy1 3-4m

One of the main goals of this study was to map and locate sites that could be studied in detail for paleoseismic histories of the MSF. Repeated filling and draining of Lake Mead since the mid-1930s have resulted in extensive erosion of late Pleistocene surficial deposits and their associating landforms, especially in areas where the fault crosses active channels and low-lying terraces. However, the young faulted sediments exposed in the bench cut mentioned above, has provided some information on the earthquake history of the MSF. So far, this site is the only place we have identified as having recorded individual earthquakes. If the OSL samples taken from this site generate reasonable age estimates, the site could be studied in greater detail.

Recent Earthquake History The MSF has been identified as an active fault in previous studies; however, well-constrained slip rates and the age of the latest events were unknown. We found one exposure of young sedimentary deposits contained evidence for at least 3 relatively recent surface ruptures. The oldest identifiable earthquake (labeled Event 1 in Figure 4) in the exposure can be found in the older sub-angular gravel units near the bottom, and the west and east ends of the exposure. There are likely several events that cannot be split out from one another in this older deposit, so we conservatively assigned a minimum of one event. The second earthquake, Event 2, cuts and offsets Unit 2 and has rotated and aligned clasts, and fractures. Event 2 also juxtaposes different thicknesses of Unit 2, as would likely occur with lateral fault offset. Event 3, the youngest identifiable earthquake recorded in this exposure, offsets Units 3 through 5 and with fracture terminations near the top of the exposures. Unit 5 is beige, silty, bioturbated and contains occasional subangular pebble to cobble clasts. Due to bioturbation, we could not confidently determine if the fractures in the exposure were closer than 0.5m to the surface. It is clear there are some fractures that cut Unit 5, therefore the event horizon was placed at the top of those fractures, within the middle of the unit.

Until the OSL sample results have been completed, we are using soil and sediment characteristics to estimate a general age of the upper two units (Units 4 and 5, Figure 4) in the bench cut exposure. In arid regions in the southwestern United States, Stage 1 carbonate accumulation on gravel and sand deposits have been assigned to deposits that are latest Pleistocene to late Holocene (Machette, 1985; Monger et al., 2009). Unit 4 sediments are predominately subangular gravel to sand with Stage I or less carbonate development, little soil development, and are not well indurated. Unit 5 near the top of the exposure is a weakly developed light beige, bioturbated soil with little carbonate coating on the bottom of some pebble-sized clasts. Given the lack of soil development and the less than Stage I carbonate accumulation, we tentatively estimate the overall ages of units 4 and 5 to be less than 20,000yrs old. The uppermost Unit 5 could be early Holocene in age (Machette, 1985; Monger et. al., 2009). Although we do not have an accurate estimate of the age of Units 1 and 2, we can still constrain the earthquake history with the generalized age of Unit 4; therefore, there has been at least one earthquake before the

deposition of Unit 4 and one after. The most recent event likely occurred within or just before the Holocene, given that Unit 4 is estimated younger than 20,000yrs old.

Preliminary Slip Rate Estimates Detailed mapping and offset measurements using high-resolution imagery generated from drone flights, coupled with OSL and cosmogenic dating of sediments and landforms will result in the first constrained slip rate estimates for the MSF. We estimate the slip rate of the main fault strand by using the age of relatively young, faulted sand and gravel deposits exposed in a bench cut created by Lake Mead’s last high-water stand. The age of the uppermost deposits, Units 4 and 5 are approximately 20,000 years or younger. This equates to a slip rate of approximately 3m in the last 20,000yrs to 50,000 yrs (0.15mm/yr to 0.06mm/yr). This estimate is based on correlating left-laterally offset features on Qi3-4 landforms with faulted sediments that range in age from 20,000 to 50,000yrs with two events (Figure 4). Offset channels and debris lobes on Qi3-4 surfaces yields 3-4m, on average. If the young, faulted tributary sediments correlate to the Qi3-4 surface in age, then there could have been approximately 2 events to generate the 3-4m of left-lateral offset, yielding approximately 1-2m per event. If the Qi3-4 surfaces are significantly older than the sediments in the young exposure, then there could have been additional earthquakes to generate the 3m of left-lateral offset on the Qi3-4 surfaces, thus decreasing the amount of offset per event to less than 1m.

Publications and Presentations – Task 4 Upon completion of the OSL and cosmogenic dating, we plan to complete a manuscript on the earthquake history and slip rate of the Mead Slope fault and submit it to the Bulletin of the Seismological Society of America or similar publication. The Arizona Geological Survey will also release additional maps and accompany reports as open-file reports (OFRs), as well as make the data, such as the DEMs available to the Bureau of Reclamation (BoR) and the National Park Service (NPS). Given the large file size of the datasets, AZGS will send external hard drives to the BoR and NPS, and the USGS-NEHRP. To date, AZGS has an accepted abstract and will present the results of this study to the Geological Society of America Fall Meeting, Sept. 22, 2019. Reference citation for this abstract:

Ben-Horin, J.Y., Gootee, B.F., and Pearthree, P.A., 2019. Ground-rupturing earthquakes on the Mead Slope fault, Arizona; Geological Society of America, Fall Abstracts, Sept.2019.

The entire mapping database can be viewed in an active GIS link here: https://uagis.maps.arcgis.com/apps/webappviewer/index.html?id=3a52e869f32b41c2a0849b47d9c51645

Conclusions This study provides the first age constraints and amount of offset from repeated earthquakes on the Mead Slope fault. Using detailed DEMs generated from drone flights and ground-control points, we mapped the MSF in detail, and made multiple offset measurements on different aged landforms. An exposure of faulted, relatively young tributary sediments along the youngest northern fault strand shows evidence for at least 3 events, 2 of which occurred within the last 20,000-50,000 years. OSL samples from the young exposure will be completed during the winter of 2020 and will provide better age control for estimating the time since the last two events. Systematic observation of offsets of different aged surfaces, along with the young graben exposure reveal approximately 3 meters of offset

for the last two to three events. Based on mapping of Qi3 and older surfaces, we preliminarily estimate a slip rate of approximately 0.15mm/yr to 0.06mm/yr (cosmogenic dates still pending). Cosmogenic ages from surface boulders on Qo, Qi1, Qi2 and Qi3-4 fan surfaces will provide better age constraints on the overall slip rates. Dates still pending.

Acknowledgments We would like to acknowledge the logistical and financial support from the Bureau of Reclamation, especially Joanna Redwine and James Burke. The Bureau of Reclamation provided critical boat support for field access, and funds for OSL dating and Ar/Ar dating. Additional volunteers included Howard Peavey, Joe Cook and Chad Kwiatkowski.

References

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