Tectonostratigraphic Record of Late Miocene–Early Pliocene Transtensional Faulting in the Eastern California Shear Zone, South

Research Paper

GEOSPHERE Tectonostratigraphic record of late Miocene–early Pliocene transtensional faulting in the Eastern California shear zone, GEOSPHERE, v. 17, no. 4 southwestern USA https://doi.org/10.1130/GES02337.1 Rebecca J. Dorsey1, Brennan O’Connell1,*, Kevin K. Gardner1, Mindy B. Homan1,†, Scott E.K. Bennett2, Jacob O. Thacker3, and Michael H. Darin1,4 15 figures; 1 table 1Department of Earth Sciences, University of Oregon, Eugene, Oregon 97403, USA 2Geology, Minerals, Energy, and Geophysics Science Center, U.S. Geological Survey, 2130 SW 5th Avenue, Portland, Oregon 97201, USA CORRESPONDENCE: [email protected] 3New Mexico Bureau of Geology and Mineral Resources, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801, USA 4Nevada Bureau of Mines and Geology, University of Nevada, Virginia Street, Reno, Nevada 89557, USA

CITATION: Dorsey, R.J., O’Connell, B., Gardner, K.K., Homan, M.B., Bennett, S.E.K., Thacker, J.O., and Darin, M.H., 2021, Tectonostratigraphic record of late ABSTRACT shows that the southern Blythe Basin was part 2015, 2016a, 2016b, 2017; Darin et al., 2016; Umhoefer Miocene–early Pliocene transtensional faulting in of a diffuse regional network of linked right-step- et al., 2018). The ECSZ can be divided into a western the Eastern California shear zone, southwestern USA: Geosphere, v. 17, no. 4, p. 1101–1125, https://doi.org The Eastern California shear zone (ECSZ; ping dextral, normal, and oblique-slip faults belt of active deformation (modern ECSZ) defined /10.1130/GES02337.1. southwestern USA) accommodates ~20%–25% of related to Pacific–North America plate boundary by GPS motions, modern seismicity, and Quater- Pacific–North America relative plate motion east dextral shear. Diffuse transtensional strain linked nary-active faults (e.g., Oskin et al., 2008; Spinler et Science Editor: Andrea Hampel of the San Andreas fault, yet little is known about northward to the Stateline fault system, eastern al., 2010; Parsons et al., 2013; Zeng and Shen, 2014, Associate Editor: Andrea Fildani its early tectonic evolution. This paper presents Garlock fault, and Walker Lane, and southward to 2016; U.S. Geological Survey and California Geolog- a detailed stratigraphic and structural analysis of the Gulf of California shear zone, which initiated ical Survey, 2020) and an eastern belt (paleo-ECSZ) Received 19 August 2020 Revision received 14 January 2021 the uppermost Miocene to lower Pliocene Bouse ca. 7–9 Ma, implying a similar age of inception for that displays slow to negligible modern strain and Accepted 19 March 2021 Formation in the southern Blythe Basin, lower the paleo-ECSZ. is defined by structures that were active in Miocene Colorado River valley, where gently dipping and time but are now mostly inactive (e.g., Guest et al., Published online 14 May 2021 faulted strata provide a record of deformation 2007; Mahan et al., 2009) (Fig. 1). The onset of fault- in the paleo-ECSZ. In the western Trigo Moun- ■■ INTRODUCTION ing in the ECSZ is poorly known, with proposed ages tains, splaying strands of the Lost Trigo fault zone of initiation ranging from ca. 10–12 Ma (Dokka and include a west-dipping normal fault that cuts the The eastern California shear zone (ECSZ; south- Travis, 1990; Schermer et al., 1996; Reheis and Saw- Bouse Formation and a steeply NE-dipping oblique western USA; Fig. 1) is a wide zone of diffuse yer, 1997; McQuarrie and Wernicke, 2005; Nuriel et dextral-normal fault where an anomalously thick strike-slip deformation that currently accommodates al., 2019) to ca. 5–7 Ma (Gan et al., 2003; Langenheim (~140 m) section of Bouse Formation siliciclastic ~20%–25% of relative Pacific–North America plate and Powell, 2009) to ca. 2–4 Ma (Du and Aydin, 1996; deposits filled a local fault-controlled depocenter. motion in the Mojave Desert east of the San Andreas Rubin and Sieh, 1997). While it is generally agreed Systematic basinward thickening and stratal wedge fault (Dokka and Travis, 1990; Miller et al., 2001; that the width of the deformation zone has narrowed geometries in the western Trigo and southeastern Meade and Hager, 2005; Oskin et al., 2007, 2008). and become more localized through time into the Palo Verde Mountains, on opposite sides of the Col- Since late Miocene time, the ECSZ has been kine- western (active) ECSZ belt (Dokka and Travis, 1990; orado River valley, record basinward tilting during matically linked to the Gulf of California shear zone Dixon and Xie, 2018), few constraints exist on the deposition of the Bouse Formation. We conclude (Fig. 1; Bennett and Oskin, 2014; Bennett et al., 2017), timing, distribution, and structural style of strain in that the southern Blythe Basin formed as a broad where major strike-slip and normal faults related the paleo-ECSZ. Documenting the geologic evolu- transtensional sag basin in a diffuse releasing ste- to oblique rifting across the Pacific–North America tion of the older, eastern belt of the ECSZ is needed pover between the dextral Laguna fault system in plate boundary developed ca. 7–9 Ma in the northern to understand how late Miocene dextral strain in the south and the Cibola and Big Maria fault zones Gulf of California and Salton Trough region (Seiler et the Gulf of California shear zone was kinematically in the north. A palinspastic reconstruction at 5 Ma al., 2010, 2011; Dorsey et al., 2011; Bennett et al., 2013, linked with paleo-ECSZ faults in the Mojave Desert east of the San Andreas fault and farther north in the Walker Lane. Rebecca Dorsey https://orcid.org/0000-0001-8390-052X This paper is published under the terms of the *Current address: School of Earth Sciences, University of Melbourne, Parkville, Victoria 3010, Australia Southern exposures of the uppermost Mio- CC‑BY-NC license. †Current address: Devon Energy Corp, 333 West Sheridan Avenue, Oklahoma City, Oklahoma 73102, USA cene to lower Pliocene Bouse Formation provide an

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excellent opportunity to address these questions because of their location within a transtensional Walker zone of right-stepping, NW-striking dextral faults Lane and north-striking normal faults related to the Gulf DV LV Colorado Stateline River of California shear zone and ECSZ (Fig. 1). Previous fault system 36°N studies found that faults in this area were active GF during late Miocene time (Sherrod and Tosdal, SoAvFZ 1991; Richard, 1993) and that fault-related defor- NV Fig. 15 CA BC mation continued during deposition of the Bouse CMF 35°N Formation (Buising, 1990; Dorsey et al., 2017; Gard- CRF N ner and Dorsey, 2021; Thacker et al., 2020). The A paleo- modern ECSZ age of the Bouse Formation is bracketed between ECSZ ca. 6.0 and 4.6 Ma (House et al., 2008; Sarna-Wo- SAF P BWRFZ 34°N jcicki et al., 2011; McDougall, 2008; McDougall and ETR Miranda Martínez, 2014; Dorsey et al., 2018; Crow B Blythe CFZ Basin AZ et al., 2019a), which thus constrains the age of syn-depositional structures. Post–4.5 Ma broad sag- Fig. 2 ST 33°N ging is recognized along the lower Colorado River LFS . Y R (Howard et al., 2015; Crow et al., 2018; Cohen et Gila al., 2019), including possible isostatic responses to sedimentation and erosion (Karlstrom et al., 2017), but the influence of ECSZ faults on regional sub- Pacific Ocean U.S.A.32°N sidence patterns during deposition of the Bouse Pi Mexico Formation remains poorly understood. Detailed GCSZ studies are needed to test kinematic models for the N GoC ECSZ and its links to the northern Gulf of California, 118°W 31°N San Andreas fault system, and Walker Lane (e.g., 0 km 100 SON Dolan et al., 2007; Oskin et al., 2008; Liu et al., 2010; 116°W 114°W Dixon and Xie, 2018). Figure 1. Map of the southern San Andreas fault system, Eastern California shear zone Stratigraphic analysis offers a powerful method (ECSZ), and Gulf of California shear zone (GCSZ), showing faults (black lines), surface expo- for documenting fault-related tilting and defor- sures of the Bouse Formation (purple), modern dry lakes (yellow), and inferred distribution mation of the Earth’s surface in areas of crustal of Bouse sedimentary basins in the lower Colorado River region (light blue). Pink shading extension, subsidence, and sedimentation (Gaw- highlights the modern ECSZ as defined by geodesy, active seismicity, and Quaternary-active faults; green shading shows the older, late Miocene to early Pliocene paleo-ECSZ. Red lines thorpe and Leeder, 2000; Gawthorpe et al., 1997, are faults in the modern ECSZ with historical surface-rupturing earthquakes. Abbreviations: 2018; Sharp et al., 2000; Withjack et al., 2002; Serck A—Amboy; AZ—Arizona; B—Blythe; BC—Bullhead City; BWRFZ—Bill Williams River fault and Braathen, 2019). This approach is especially zone; CA—California; CFZ—Cibola fault zone; CMF—Cave Mountain fault; CRF—Camp Rock useful in areas of slow or diffuse deformation, fault; DV—Death Valley; ETR—eastern Transverse Ranges; GF—Garlock fault; GoC—Gulf of California; LFS—Laguna fault system; LSBM—Little San Bernardino Mountains; LV—Las where low strain rates produce gentle bedding dips Vegas; N—Needles; NV—Nevada; P—Parker; Pi—Pinacate volcano; SAF—San Andreas that may be difficult to quantify with standard struc- fault; SoAvFZ—Soda-Avawatz fault zone; ST—Salton Trough; SON—Sonora; Y—Yuma. tural analysis or where structures are concealed or poorly exposed. Tilting related to syn-depositional normal and oblique-slip faults produces systematic use this tectonostratigraphic approach to interpret 2020). In addition, these methods offer a powerful thickness variations and distinctive stratal architec- structural controls on stratigraphic architecture and approach that could be used to reconstruct the late tures that can be used to reconstruct the timing, fault-related syn-depositional tilting dynamics that Cenozoic kinematic evolution of other important geometries, and kinematics of the causal fault were not identified in previous published studies in strike-slip fault zones such as the North Anatolian systems (e.g., Gawthorpe et al., 1997; Young et al., the paleo-ECSZ (e.g., Miller and McKee, 1971; Sher- fault (northern Turkey; Şengör et al., 2005), Dead 2003; Lewis et al., 2017; Muravchik et al., 2018). We rod and Tosdal, 1991; Richard, 1993; Thacker et al., Sea transform (Garfunkel, 2014), and regional

strike-slip systems in southeast Asia (Sumatra, 0 km 5 114.8°W Tb 114.6°W Sagaing, and Red River fault zones; Morley, 2002). C o Qal,

l This paper presents a detailed stratigraphic Palo o Qt Tv r analysis of the Bouse Formation on opposing Verde a d o Cibola sides of the southern Blythe Basin, south of Bly- Mountains Fig. 4 Fig. 10 R

i the, California, integrated with geologic mapping Tb 36bbb v JTr 33.3°N e normal fault and structural data, to document late Miocene to Fig. 12 r PVF LTF early Pliocene deformation in the paleo-ECSZ in Qal, Qt Milpitas strike-slip fault the lower Colorado River corridor (Figs. 1, 2). We Wash Qal, Tb Qt Tb Tv document diagnostic stratal wedge geometries Qal, Qt Qal, Quaternary alluvium and that record a history of syn-depositional tilting and terrace deposits Qal, Qt subsidence in response to growth of normal and Qt Late Miocene to early oblique-slip faults around the margins of a transten- Tc Tb 33.2°N Pliocene Bouse Formation sional basin. These results provide new constraints Tertiary conglomerate JTr Tc on the time-space evolution of the ECSZ and sup- (Miocene) port prior suggestions that regional subsidence in Trigo Mountains Tb Tv Tertiary volcanic rocks a fault-bounded tectonic lowland controlled latest Tc Tv Tc Tv JTr Tv LRF Triassic-Jurassic metavolcanic Miocene shallow-marine inundation and subse- BP JTr N and metasedimentary rocks quent integration of the Colorado River into the JTr northern Gulf of California at ca. 5 Ma (e.g., Buis- Figure 2. Simplified geologic map of the southern Blythe Basin and surrounding ranges (compiled from Sherrod and ing, 1990; Bennett et al., 2016a; Dorsey et al., 2018). Tosdal, 1991). “36bbb” is a water well in which Bouse Formation was recorded to a depth of 92 m and did not penetrate the base of the Bouse Formation (Metzger et al., 1973). Red circle indicates fault geometry data station for this study (more are shown in Fig. 4). Background image is from Google Earth. Abbreviations: BP—Buzzards Peak; PVF—Palo Verde ■■ REGIONAL STRATIGRAPHIC fault (introduced in this study); LRF—Lighthouse Rock fault; LTF—Lost Trigo fault. FRAMEWORK

The Bouse Formation is a thin sequence of In the southern Blythe Basin (Fig. 2), south of Dorsey et al., 2018). The UBM and unit Tfg2 gravel upper Miocene to lower Pliocene sedimentary Blythe, California, the Bouse Formation consists of together define a laterally extensive conformable deposits that are discontinuously exposed along three regionally correlative members: (1) a basal thin sequence known as “Trigo sediments” (Goo- the lower Colorado River valley in western Arizona carbonate member consisting of mixed carbonate- tee et al., 2019) that we treat as a single map unit and southeastern California (Figs. 1–3). It uncon- siliciclastic bioclastic grainstone, conglomerate, (Tbug) in the western Trigo Mountains (Fig. 4). The formably overlies variably deformed Miocene and marl; (2) a siliciclastic member comprising Bullhead Alluvium is a widespread unit of lower volcanic and sedimentary rocks that accumulated green claystone, red mudstone, siltstone, and Pliocene Colorado River gravel and sand that during and after northeast-southwest extension cross-bedded channel sandstone of the earliest locally interfingers with unit Tfg2 gravel. In most in a belt of low-angle detachment faults known Colorado River; and (3) an upper bioclastic member places, Bullhead Alluvium is erosionally inset into as the Colorado River extensional corridor (How- (UBM), which forms a coarsening-upward sequence older deposits and records incision followed by ard and John, 1987; Spencer and Reynolds, 1991; of fossiliferous sandy grainstone, pebbly grain- regional aggradation in the lower Colorado River Sherrod and Tosdal, 1991; Spencer et al., 2018). stone, and calcareous-matrix conglomerate that region that took place ca. 4.5–3.5 Ma (Pearthree and Previous studies have bracketed the age of the overlies older members of the Bouse Formation House, 2014; Howard et al., 2015). southern Bouse Formation between ca. 6.3 and along a regional unconformity that likely represents The Bouse Formation in the southern Blythe 4.6 Ma using tephrochronology, biostratigraphy, ~100–200 k.y. (Fig. 3; Homan, 2014; O’Connell, 2016; Basin has been variably interpreted to record depo- and 40Ar/39Ar methods (Sarna-Wojcicki et al., 2011; Dorsey et al., 2018, 2019). Older members of the sition in either a large saline lake (Spencer and Spencer et al., 2013; McDougall, 2008; McDougall Bouse Formation below the UBM thicken toward Patchett, 1997; Spencer et al., 2008, 2013; Pearthree and Miranda Martínez, 2014; Dorsey et al., 2018; depocenters beneath the modern Colorado River and House, 2014; Bright et al., 2016, 2018a, 2018b; Crow et al., 2019a), making these deposits an ideal axis and thin and pinch out toward basin margins Gootee et al., 2019) or a shallow-marine tidal strait target for studies of late Miocene to present defor- (Fig. 3). The UBM is gradationally overlain by locally or estuary (Buising, 1990; Turak, 2000; McDougall, mation in the paleo-ECSZ. derived alluvial-fan conglomerate, unit Tfg2 (Fig. 3; 2008; McDougall and Miranda Martínez, 2014;

Crossey et al., 2015; O’Connell et al., 2017, 2021; Tcb Alluvial-fan Bullhead Dorsey et al., 2018; Gardner and Dorsey, 2021).

conglomerate Tfg2 Interpretations for specific stratigraphic units are summarized below. Studies that support the saline Calcareous-matrix congl lake model tend to emphasize evidence from car- Tbug

bonate major- and trace-element chemistry and Coarse pebbly grainstone Tbu isotopic data (Sr, O, and C isotopes), while papers Wave-rippled grainstone, supporting the marine and estuarine model base rare C.R. mudst and sst their conclusions primarily on evidence from pro- cess sedimentology, paleontology, and trace fossils. C. R. Karst breccia and fissures sst

M Regional u ■■ METHODS d unconformity s t

a n Field work was conducted in the western Trigo d

s BOUSE FORMATION Green claystone

i Mountains, Arizona, and southeastern Palo Verde lt b s Marl a t Miocene s Mountains, California (Fig. 2). Geologic maps were a

l Tbbc Bioclastic volcanic and c Basal

a carb mbr compiled from previous work (Homan, 2014; Gootee travertine facies rb Tbbg older crystalline o et al., 2016; O’Connell et al., 2021; Dorsey et al., 2018; na Siliciclastic Member (Tbs) te Miocene fan rocks (Tv, Tvx) m Mioc. Gardner and Dorsey, 2021) and new mapping con- br Tfg1 conglomerate ducted for this study. Stratigraphic relations were Volcanics characterized by detailed geologic mapping, mea- Miocene volcanics

suring sections using a Jacob’s staff, correlation of Figure 3. Schematic stratigraphy of the Bouse Formation in the southern Blythe Basin, not to scale. Orange key contacts and marker beds, and construction of band at the top of unit Tfg1 represents a ravinement surface produced by sediment reworking and sorting geologic cross sections and stratigraphic panels to during regional transgression. Bouse upper bioclastic member (unit Tbu) overlies older deposits of the Bouse illustrate important stratal architectures and fault Formation along a regional unconformity that represents an unknown amount of time, possibly ~100–200 k.y. Fossil symbol in Tbu represents branching calcareous red algae that are common in this unit, not found in geometries. Detailed measured sections were com- the basal carbonate member. Abbreviations: carb—carbonate, C.R.—Colorado River, Bullhead—Bullhead piled from Master’s theses (Homan, 2014; O’Connell, Alluvium, mbr—member, mudst—mudstone, sst—sandstone, siltst—siltstone. See Figures 4 and 10 for 2016; Gardner, 2019). Fault geometric analysis further definition of lithologic unit symbols. was conducted using Stereonet 10 software (All- mendinger et al., 2012; Cardozo and Allmendinger, 2013). Kamb contours were calculated from poles to ■■ RESULTS onto travertine-encrusted local relief. Basal carbon- planes to decipher statistically significant geometric ate in the western Trigo Mountains is subdivided trends. Fault kinematic data were obtained by mea- Stratigraphic Summary into gravel-dominated and carbonate-dominated suring faults and striae on fault planes; shear sense facies, map units Tbbg and Tbbc, respectively was interpreted in the field using criteria outlined The basal carbonate member of the Bouse For- (Figs. 3, 4, 5A). The gravel-dominated facies in Petit (1987) and geologic observations such as mation ranges from ~1 to 25 m thick and overlies (unit Tbbg) is primarily reworked from underly- correlation of offset beds. Paleostrain was analyzed Miocene volcanic rocks and conglomerate along ing conglomerate. It is clast supported and well from fault kinematic data using FaultKin 7 software an unconformity that exhibits variable geometry sorted and displays tabular to trough cross bed- (Marrett and Allmendinger, 1990; Allmendinger et and relief around the study area (Fig. 2). In the ding formed in beach ridges, gravelly barchan al., 2012) to determine the incremental shortening western Trigo Mountains, bioclastic facies overlie dunes, delta mouth bars, and small Gilbert deltas (P) and extension (T) axes of measured faults. Strati- Miocene alluvial fan conglomerate along a sharp, (Fig. 5B; Dorsey et al., 2018; O’Connell et al., 2021). graphic and structural data were integrated with quasi-planar, laterally continuous disconformity Carbonate-dominated facies (unit Tbbc) overlie previously published age estimates (summarized (O’Connell et al., 2021). In the southeastern Palo and are interbedded with unit Tbbg and contain above) to interpret regional paleogeography, fault- Verde Mountains, a thin basal travertine unit is a wide range of sedimentary structures, facies ing controls on basin evolution, and development encrusted on steep irregular paleotopography in associations, and carbonate-siliciclastic mixtures of the paleo-ECSZ. Miocene volcanic rocks, and bioclastic facies onlap that record deposition in low- to high-energy tidal

0 km 1.0 114.60° W Qt,Qi 114.65° W Tcb Tbug Tbs Qt,Qi Tbs Qt,Qi Qt,Qi Tbug Tbbc Tbug A’ Tbug Tbs Qt,Qi Qal ? Qt,Qi 3 Tfg1 Tfg1 3 E Tbs B’ D Tbs Fig. 7A 33.30° N Tbbc Tcb 3 A10 Fig. 7B Qt,Qi Qal 24 28 Fig. 7E 15 12 Qt, Qi Qt,Qi A12 A Fig. 8 Tfg1 B67 A11 B57 Tvx B66 Monocline B33 B59 A13 at depth Fig. 9A Fig. 7D (Fig. 9A) Hart Mine Wash A24 5 A15 N Qal BTbs Qt,Qi Tcb Qt,Qi Tfg1 Tvx Qt,Qi Lost Figure 4. Geologic map of the western Tbug Tcb Tbug Qt,Qi Tbbc Trigo Mountains, compiled from Gootee EXPLANATION et al. (2016), Homan (2014), O’Connell (2016), and this study (see location in Tb- 3 Qt,Qi strike and dip of bedding Fig. 2). Dashed lines from measured Qt, 9 5 Tfg1 bc Qi 22 monocline axis section locations show projection into normal fault (faults dotted stratigraphic panels (Fig. 9). Background Tbug D’ Tbbc where concealed) image is from Google Earth.

3 Trigo oblique-slip fault Fault-kinematic data station (Fig. 13) 33.275° N Qt,Qi A4 Tbs Fault-geometric data station (Fig. 13) 3 Big Fault B31 Tbug A15 Measured section location Wash Fig. 9B A19 Fig. 7C B53 A25 Qal Quaternary alluvium C B43 3 2 A5 C’ B42 B39 2 B41 B38 B50 Quaternary terrace and Qt,Qi inset terrace gravels Tbbc Tcb Tbug unconformity B71 10 22 B40 Tcb Pliocene Bullhead Alluvium Qt,Qi B70 B13 Tfg1 Upper bioclastic member 3 Tbug

fault and younger fan gravels; 3 B24 2 thin mudstone at base. Tbbc Tvx B27 unconformity A3 Fig. 9C B26 Marl 11 Tbs Siliciclastic member Wash B3 Qt,Qi Tb Tbbc Basal member: carbonate bc 13 Tfg1 Tfg1 4

Tfg1 Bouse Formation Tbbg Basal member: gravel Tfg1 Tbbg 25 unconformity Qt, 32 Tbbg Tfg1 Miocene alluvial-fan conglomerate Qi Tbbg unconformity 33.25° N Qt,Qi Tvx Miocene volcanic and older Tfg1 Tbbc E’ Tfg1 crystalline rocks (undifferentiated)

SSE NNW Figure 5. Examples of Bouse Forma- tion and bounding units in the western Tbbc Trigo Mountains. (A) Top of Miocene con- Tbbg Tbbc glomerate (unit Tfg1) and contact with overlying Bouse Formation basal mem- Tfg1 ber conglomerate (Tbbg) and bioclastic carbonate (Tbbc) in Marl Wash. South of measured section B3 (Fig. 4). View Tfg1 looking WSW. Location of view point is 33.25628 °N, 114.64008 °W. (B) View looking southwest at interbedded bioclastic facies (Tbbc) and cross-bedded conglomerate (Tbbg) of the Bouse basal carbonate member, overlying Miocene NW SE alluvial-fan conglomerate (Tfg1). Mea- Tbbg sured section B3 (Fig. 4). Location is 33.25763 °N, 114.64189 °W. (C) Upper Tfg2 marl unit of Bouse basal carbonate Tbbc member (Tbbc) overlain by Colorado Tbs bioclastic River–derived deposits of the siliciclastic facies Tbbc member (Tbs), including thin lower upper marl green claystone, red mudstone, and pink sandstone in Big Fault Wash. Unit Tbs is sharply overlain by upper conglomerate (Tfg2). Measured section A19 in Tfg1 Big Fault Wash (Fig. 4); person (circled) for scale. View looking NE. Location of SW NE viewpoint is 33.27036 °N, 114.64295 °W. (D) Thick, multistory trough cross-bed- Qi ded Colorado River channel sandstone in the Bouse siliciclastic member (Tbs), Tcb where it is cut by a NE-dipping fault on the north side of Hart Mine Wash (Fig. 4). Fault zone is interpreted to have oblique dextral-normal offset; “R” indicates in- Mud-filled ferred Riedel shears. Northeast dip in R channel immediate hanging wall is due to local fault drag superimposed on large-scale Tbs Tbs trough cross bedding. The fault zone R is capped by undeformed Bullhead Alluvium (Tcb) along an unconformity Trough cross-bedded that correlates to the unconformity at Green channel sandstone the base of the upper bioclastic mem- claystone (earliest Colorado River) R ber (Fig. 6). Unit Qi is inset Quaternary terrace gravel (Fig. 4). Person (circled) for scale. View looking NW. Location is 33.29536 °N, 114.64143 °W.

channels, bars, and intertidal to shallow subtidal marine calcareous red algae, echinoderm spines, Blythe Basin, with concomitant thinning to the east flats (O’Connell et al., 2017, 2021; Dorsey et al., 2018; articulated barnacles, shallow-marine foraminifers, toward the basin margin (Figs. 7, 9). Gardner and Dorsey, 2021). The youngest unit of mollusks, and ostracodes that indicate deposition The Bouse Formation and older units in the the basal carbonate member consists of laterally in a high-energy shallow-marine embayment that western Trigo Mountains are cut by a prominent extensive fine-grained carbonate micrite and shale flooded the lower Colorado River valley after the north- to NW-striking fault zone that displays a dis- (marl; Fig. 5C) interpreted to have accumulated river first ran through it (Dorsey et al., 2018, 2019). tinctive anastomosing surface trace, exposed for in a pre–Colorado River subtidal marine embay- The conformable upward transition to alluvial-fan ~5 km across the map (Gootee et al., 2016; Fig. 4). ment (McDougall, 2008; McDougall and Miranda gravels of unit Tfg2 (Fig. 6D) records regional pro- This fault was first described by Metzger et al. (1973) Martínez, 2014; O’Connell et al., 2017; Dorsey et gradation of shoal-water fan deltas into the basin but was not named. A geotechnical report by San al., 2018) or low-energy saline lake fed by the early and termination of shallow-marine conditions. Diego Gas and Electric Company (1976) mapped Colorado River (Bright et al., 2016, 2018a, 2018b). and named this the Lost Trigo fault (Schell and The siliciclastic member of the Bouse Forma- Wilson, 1982), and it was included on a fault com- tion (unit Tbs), also known as the “interbedded Western Trigo Mountains pilation map for Arizona with the same fault name unit” of Metzger (1968), overlies basal carbonate (Menges and Pearthree, 1983). The Lost Trigo fault along a widespread abrupt but conformable con- Stratal geometries of the Bouse Formation also appears on maps by Sherrod and Tosdal (1991) tact (Fig. 5C) that records the sudden first arrival exposed at the western margin of the Trigo Moun- and Richard (1993) and is likely the fault that Buis- of Colorado River sediment in the study area. It is tains are revealed by the geologic map pattern ing (1990) describes in Lopez Wash, Arizona, though as much as ~200 m thick in subsurface wells and (Fig. 4) and a series of geologic cross sections and none of these publications included a fault name. thins to zero thickness at the margins of the basin stratigraphic panels (Figs. 7–9). Bedding dips in the Recent published studies have referred to this fault (Fig. 3; Metzger, 1968; Metzger et al., 1973; Dorsey Bouse Formation typically are very low (<3°–5°) but as the “Big fault” (e.g., Gootee et al., 2016; Beard et et al., 2018). The siliciclastic member displays a locally steepen to 15°–20° in Hart Mine Wash near al., 2016; Thacker et al., 2020). In this study, we use conformable up-section change from extremely a fault that cuts a thick interval of the siliciclastic the original name “Lost Trigo fault” and propose fine-grained green claystone, to red mudstone member (Figs. 7, 8). Pre-Bouse conglomerate (unit that the name “Big fault” no longer be used. Dip and siltstone with minor cross bedding and inter- Tfg1) is preferentially exposed in the south and east direction on strands of the Lost Trigo fault zone mittent weak paleosols, to thick multistory trough due to overall gentle Bouse Formation dips to the varies along strike, with steep west and SW dips in cross-bedded quartz-rich sandstone that is inter- north and west (Fig. 4). Bedding dips in unit Tfg1 the south changing to steep NE dip on the northern preted to have formed in fluvial channels of the range from 32° to 4° west, with a general up-sec- side of Hart Mine Wash (Fig. 4). earliest through-flowing Colorado River (Figs. 3, tion decrease in dip angle that suggests westward On the northern side of Hart Mine Wash, the 5D; Dorsey et al., 2018). tilting during deposition (Sherrod and Tosdal, 1991). Lost Trigo fault is a NW-striking, 4–5-m-wide fault The upper bioclastic member of the Bouse In one area ~1 km south of Hart Mine Wash, Bouse zone that dips ~70 °NE (Figs. 5D, 7A, 8). North- Formation (UBM; 0–10 m thick) rests on both the basal carbonate (unit Tbbc) overlies unit Tfg1 and east of the fault, an anomalously thick (~100 m), basal carbonate and siliciclastic members across Miocene volcanic rocks along an angular uncon- SW-dipping interval of Colorado River siltstone, a quasi-planar regional unconformity and makes formity, below which unit Tfg1 thickens markedly mudstone, and cross-bedded channel sandstone up the lower subunit of map unit Tbug (Trigo sed- to the northwest (Fig. 7D). Bouse Formation upper is exposed in the hanging wall (Fig. 4). Deposits of iments of Gootee et al., 2019) in the western Trigo bioclastic member (UBM) and unit Tfg2 gravel are this interval are observed nearby (measured sec- Mountains (Figs. 3, 4). Where the UBM overlies grouped on the geologic map (Fig. 4), geologic tion locations B33, A15; Figs. 4, 9) to overlie Bouse basal carbonate facies, marl beneath the uncon- cross sections (Fig. 7), and most stratigraphic Formation green claystone, which in turn rests on formity is altered by a distinctive 1–2-m-thick zone panels (Figs. 9A, 9B) into one geologic unit, Tbug, the basal carbonate member, and therefore these of karst-generated fissures and breccia that record due to their thin conformable nature that makes older units are inferred to be present at depth in pre-UBM subaerial exposure and weathering it impractical to separate at the scale of these dia- the hanging wall of the fault (Fig. 8). We estimate (Figs. 3, 6A, 6B; Dorsey et al., 2018). The UBM typ- grams. The unconformity at the base of unit Tbug the SW-dipping interval of Colorado River siltstone, ically coarsens up-section from sandy grainstone (Figs. 6A, 6B, 6D), also the base of the UBM, is a mudstone, and sandstone to be ~140 m thick using with centimeter-scale wave ripple cross lamina- regional sequence boundary that provides a useful measured-section techniques and accounting for tion, to pebbly grainstone with larger wave-formed stratigraphic datum for documenting stratal geom- moderate bedding dips. Southwest of the fault, the gravelly ripples (wavelengths typically 1–3 m), to etries in older units. Beneath this datum, older units older green claystone is exposed as high as 9 m calcareous-matrix pebble conglomerate (Figs. 6C, of the Bouse Formation display systematic thicken- above the floor of Hart Mine Wash (Fig. 5D). These 6D). Fossils in the UBM include upward-branching ing to the west toward the center of the southern relations indicate an apparent normal separation

East West

Regional unconformity UBM KFW

Figure 6. (A) View looking south at Lost Trigo Tfg1 Marl fault in Big Fault Wash and regional uncon- Tfg1 (subtidal) formity at base of the Bouse upper bioclastic

Tbbc member (UBM). KFW is a zone of karst fissures and weathering in upper marl of unit Tbbc directly beneath the unconformity. Tfg1 is Miocene alluvial fan conglomerate. Measured Tfg1 section A5 (Fig. 4). Location is 33.26984 °N, 114.63174° W. (B) Close-up of sand-filled karst fissures and polygonal cracks (arrows) at base of UBM pebbly bioclastic carbonate, ~65 m northwest of the outcrop in A. Location is 3.27020 °N, Tfg2 114.63224 °W. (C) Carbonate-rich bioclastic facies of UBM showing flat-based wave-formed rip- Polygonal cracks ple cross-bedding in shelly coquina interbedded with bioturbated sandy grainstone, southeast Base of UBM (3) Calcareous-matrix conglomerate Palo Verde Mountains. Location is 33.30374 °N, 114.75718 °W. (D) UBM in the Palo Verde Moun- tains where it overlies Colorado River channel Sand- filled sandstone of the siliciclastic member (Tbs) fissures along a sharp unconformable contact. This Marl exposure reveals a typical coarsening-upward (subtidal) section of: (1) carbonate-rich sandy grainstone UBM with small-scale wave-formed ripples, (2) pebbly grainstone with larger wave ripples, (3) calcareous matrix-rich pebble conglomerate, and (2) Pebbly grainstone (4) conformable transition to alluvial fan gravels of unit Tfg2. Measured section KG12 (Fig. 10). Location is 33.29239 °N, 114.76478 °W. (1) Sandy grainstone Regional unconformity

Tbs

A A A’ SW NE Ground surface projected Line D 200 from nearby terrace Tbug Tcb Tbug

100 Tbbc VE = 2:1 Tbs ? Tfg1 0° ? 5° 0 Fig. 8 ? 10° Elevation (m) 15° ? Tvx 20° -100 0 1.0 2.0 3.0 Distance (km) B B B’ SW Measured Tbug NE section A15 Line D 200 Tbbc Tbug Tbs Tbs ? Tfg1 100 ? ? VE = 2:1 0° 5° 10° 15° Elevation (m) 0 Tvx ? ? 20°

0 1.0 2.0 3.0 Distance (km) C C’ C W E Figure 7. Geologic cross sections in the Line E western Trigo Mountains. See Figure 4 for 200 Tcb Tbug locations and lithologic unit descriptions. Tbs ? Tvx Red line is the modern-day ground surface. 100 ? Tfg1 Unit contacts are dashed where extrapo- Tfg1 VE = 2:1 lated above and below the ground surface. ? 0° VE—vertical exaggeration. 0 5°

Elevation (m) ? 10° Tvx 15° 20° 25° -100 0 1.0 2.0 3.0 Distance (km) D D D’ NW Tbbc SE Line A Tbbc 200 Line B Tbug Qt & Qi Tbbc Tbs Qt & Qi Qal Inferred Tvx Dipping paleochannel 100 Tfg1 depositional contact VE = 2:1 Inferred fanning dips in Tfg1, similar to pattern in Sherrod and Tosdal (1991) Elevation (m) 0 0 1.0 2.0 Distance (km) E E’ E bend in NW SE section N Line C S Tbug Qi 200 Tcb 100 Tbbc Tbbc

0 Tfg1 VE = 2:1

Elevation (m) -100 0 1.0 2.0 3.0 4.0 5.0 6.0 Distance (km)

16 m exposed multistorey Figure 8. Geologic cross section across trough cross-bedded ~ 60 m Lower part of Start measured Bullhead Colorado River channel measured measured section section the Lost Trigo fault in Hart Mine Wash; Alluvium Liquefaction SW sandstone (Fig. 5D) to here (all siliciclastic member) (33.29688°/ NE location in Figures 4 and 7A. See (Tcb) Zone Tcb -114.63741°) Figure 4 for lithologic unit descrip- green and tions. Thin red line shows the ground red mudstone 15°-20° bedding dip surface. Here the fault is a NE-dipping 100 e 100 dston ? ? or mu oblique dextral-normal fault with an ith min C.R. sst w anomalously thick section of Bouse For- Tbbc Colorado River bedded sst ? ? with minor mudstone mation siliciclastic member (unit Tbs) in its hanging wall. Consistent 15°–20° Colorado River-derived bedding dip toward the fault over a Bouse Fm ? ? delta-plain siltstone and mudstone distance of at least 350 m combined Siliciclastic green and red mudstone (lower Tbs? )? with known stratigraphic relations in Elevation (m) (inferred at depth, from southTbbc side of Hart Mine Wash) Member (Tbs) the map area indicate at least ~100 m 0 Tfg1 0 dip-slip component of offset on the fault ? ? Tfg1 in this area. Surface data that constrain ? ? dipping unit Tbs and an angular uncon- H:V = 1:1 formity at the base of Bullhead Alluvium -40 -40 (Tcb) are projected into the cross section 0 100 200 300 400 500 from exposures along and near the line Distance (m) of section (Fig. 4).

(throw) of roughly 140 m across the Lost Trigo fault Tbug) is supported by the map pattern and field Farther south, at Marl Wash, the Bouse Formation at this locality (Fig. 8). The fault here is unconform- observations that reveal a widespread, continuous, basal carbonate member is unconformably overlain ably overlain and capped by unfaulted Bullhead very gently dipping contact that can be traced over by Quaternary terrace and inset gravels (unit Qi) in Alluvium, showing that fault activity had ceased by large areas (5–6 km; Fig. 4) with no evidence for the west (Fig. 9C). In the eastern part of the panel, ca. 4.5 Ma (lower bounding age of Bullhead Allu- channelized or inset contact relations with under- in an area as far as 0.5 km west of the Lost Trigo vium; Howard et al., 2015). South of this location, lying deposits. These panels (Fig. 9) provide a fault, the basal carbonate member is overlain by in Big Fault Wash, the Lost Trigo fault splays into high-fidelity view of important stratigraphic rela- an expanded ~26-m-thick Tbug unit (UBM and unit an ~180-m-wide zone of mostly synthetic fault seg- tions that are not obvious at the scale of traditional Tfg2 gravel) capped by Bullhead Alluvium. Stratal ments that strike north, dip steeply west, and cut all geologic cross sections (Fig. 7) and thus provide a geometries in these three panels (Fig. 9) are inter- pre-Quaternary units including Bullhead Alluvium useful complement to the cross sections. preted further in the Discussion section below. (Figs. 4, 6A, 7C), constraining fault activity here to The Hart Mine Wash panel (Fig. 9A) reveals a ≤3.5 Ma (upper age of Bullhead Alluvium). The main monoclinal fold geometry in which southwestward- west-dipping Lost Trigo fault segment at the east- thickening subunits in unit Tbs on the SW-dipping Southeastern Palo Verde Mountains ern boundary of this zone exhibits ≥35 m of normal fold limb are progressively truncated (eroded or not displacement (Fig. 6A; Gootee et al., 2016), while deposited) along a planar unconformity beneath Geologic mapping and stratigraphic analysis in smaller synthetic and antithetic faults to the west unit Tbug. In this panel, a thin (2–3 m) interval of the southeastern Palo Verde Mountains (Gardner, display 1–3 m normal displacements. basal bioclastic grainstone defines the flat north- 2019) provide additional evidence for fault-related Stratigraphic panels constructed across the eastern upper limb of the monocline (Fig. 9A). We tilting during deposition of the Bouse Formation in eastern margin of the southern Blythe Basin (Fig. 9) speculate that a blind, west-side-down normal fault the southern Blythe Basin. In this area, the basal highlight growth-strata relations that provide a may be present at depth to explain this monocline carbonate member is subdivided into travertine record of fault-controlled tilting during deposi- geometry. Farther south, at Big Fault Wash (Fig. 4), (unit Tbbc-t) and bioclastic facies (unit Tbbc-b), and tion of the Bouse Formation. The panels were the siliciclastic member is ~20 m thick in the west the upper bioclastic member (UBM; unit Tbu) and constructed from a series of detailed measured sec- and thins gradually to the east where it is similarly locally derived alluvial-fan gravel (unit Tfg2) are tions (Homan, 2014; O’Connell, 2016) that are hung truncated beneath unit Tbug (Fig. 9B). Near the east mapped as separate units (Fig. 10). The hinge of a from the basal contact of unit Tbug, thus reveal- end of this panel, the upper bioclastic member rests segmented monocline forms an irregular boundary ing stratal architecture that existed at the time of unconformably on marl of the basal carbonate, and between a narrow belt of subhorizontal bioclastic unit Tbug deposition. An originally near-horizon- all pre-Quaternary units are cut by the Lost Trigo carbonate on the northwest and a wider belt of tal planar geometry for this datum (base of unit fault just east of measured section A5 (Figs. 6A, 9B). gently SE-dipping unit Tbs on the southeast, an

A. Hart Mine Wash Panel Stratigraphic Datum = Base of unit Tbug

SW A24 Tcb B57 NE 10 B66 A11 B59 B33 A15 A13 A12 A10 Tbug B67 Tbug 0 0 CR ms Tbs: −10 CR sst ? −10 Upper marl Bioclastic Basal grainstone −20 carbonate −20 Tbs: Tfg1 Tbbc member ? −30 −30

Height (m) Tbs: −40 green −40 claystone ? Vertical Exaggeration 10:1 −50 −50 0 500 1000 1500 2000 Distance (m)

B. Big Fault Wash Panel Stratigraphic Datum = Base of unit Tbug W E B38 10 A19 B50 A25 A5 10 Figure 9. Stratigraphic panels. See Figure 4 for B31 Tbug B39 locations and lithologic unit descriptions (Bouse 0 Tcb (inset channel fill) 0 Tbs: Colorado River sandstone B53 Formation upper bioclastic member [UBM] and Tbs: green and mudstone B41 −10 A4 claystone B42 −10 unit Tfg2 conglomerate, undifferentiated). Ver- ash B43 ash tical black lines represent measured sections; −20 ash −20 Upper marl short horizontal ticks are contacts between the Basal Bioclastic carbonate −30 main stratigraphic units. (A) Hart Mine Wash. −30 Tbbc member grainstone Lower marl (B) Big Fault Wash. (C) Marl Wash. C.R. ms, Height (m) −40 Tfg1 −40 sst—Colorado River mudstone and sandstone; −50 −50 congl—conglomerate. Vertical Exaggeration 10:1 −60 −60 0 m 500 1000 1500 2000 Distance (m)

C. Marl Wash Panel Stratigraphic Datums are: (1) base of unit Tbug Tcb E (2) base of upper marl in basal carbonate member B40 Qt 30 (3) base of lower marl in basal carbonate member jog in Qt B71 panel B70 B13 20 W Tfg2 Tbug 10 10 Bioclastic B72 Tfg1 grainstone Qi C.R. ms, sst UBM 0 B3 0 Qi B27 B24 Upper marl A3 B26 Basal −10 carbonate −10 Lower marl Tbbc Lower marl member −20 Grainstone and congl −20 Height (m) −30 Tfg1 −30 Vertical Exaggeration 10:1 −40 −40 0 500 1000 1500 2000 Distance (m)

indication that the monocline axis is an important facies boundary. The stratigraphic panel (Fig. 11) 114.77° W 114.76° W Qt,Qi is hung from two datums: the base of the UBM and base of unit Tfg2. Both datums are present in 3 measured section KG12, permitting reconstruc- Tv Tbbc-t tion of stratal architecture at the time of unit Tbu Tbs deposition. The panel reveals a pronounced stratal Tbbc-b 33.30° N wedge geometry in which the Bouse Formation 1 11 siliciclastic member (unit Tbs) thickens to the Tbbc-t southeast and thins to the northwest, pinching out Tfg1 5 Tbs at the base of unit Tbu at the monocline hinge. To the northwest, unit Tbu rests unconformably on Tbbc-b Qal Bouse basal carbonate on the upper flat limb of the KG7 Tfg2 monocline, and the intervening siliciclastic mem- KG9 Tv Tbbc-t ber is absent due to erosion and/or nondeposition. Tv KG10 Tbs KG8 2 Unit Tbu thickens slightly to the southeast where it 1 6 overlies thick unit Tbs and is capped by unit Tfg2 Tbu gravel, suggesting that unit Tbu is also involved in Tbbc-t 0.5 the stratal wedge geometry. Basal carbonate and Tbbc-b Tbs 8 12 underlying Miocene conglomerate dip SE by as Tfg2 Fig. 11 Tbs KG12 much as ~8° on the limb of the monocline (Fig. 10), which restores to ~5° dip at the time of unit Tfg2 KG14 1 KG13 deposition (Fig. 11). Qt,Qi The observed relations show that monocline 1.3 Tfg2 growth took place primarily during deposition of 33.29° N Tbu Tfg2 unit Tbs, with slower rates of fold growth likely occurring during deposition of the basal carbonate Tbu Qal N and upper bioclastic members (Fig. 11). We infer that the monocline formed above the propagat- Qal Qt,Qi 0 meters 300 ing tip of a blind normal or oblique-normal fault, which we name the Palo Verde fault. Although fault unconformity EXPLANATION exposures are mostly absent in this area, one small Tfg2 Upper alluvial-fan conglomerate subvertical fault ~200 m south of measured section KG12 (Fig. 10) displays 1–3 m of apparent down- Qal Quaternary alluvium Tbu Upper bioclastic member Tbug to-the-west normal displacement in Colorado River Qt,Qi Quaternary terrace and unconformity inset terrace gravels sandstone and siltstone of the siliciclastic member Tbs Siliciclastic member (Thacker et al., 2020, their fig. 3G). This example 4 Strike and dip of bedding ( = horizontal) Tbbc-b Basal member: carbonate - bioclastic may represent a minor antithetic fault kinematically KG8 Location of measured section related to down-to-the-southeast slip inferred for Bouse Formation Tbbc-t Basal member: carbonate - travertine the blind Palo Verde fault (Fig. 11). Approximate trace, upper axis of segmented monocline Tfg1 Miocene alluvial-fan conglomerate unconformity Projection of measured sections into stratigraphic panel (Fig. 11) Tv Miocene volcanic rocks Cross-Valley Synthesis and Basin Architecture

Figure 10. Geologic map of the southeastern Palo Verde Mountains. Location is shown in Figure 2. A geologic cross section across the southern Background image is from Google Earth. Blythe Basin from the Palo Verde to the Trigo Mountains (Fig. 12) was constructed using the

NW SE 10 10 Figure 11. Stratigraphic panel in the southeast Palo Verde Mountains; location shown KG7 KG8 KG9 KG10 KG13 KG12 KG14 in Figure 10, and lithologic unit descriptions Tfg2 Tbu Quaternary gravel Datum: base of Tfg2 are shown in Figures 4 and 10. Datums (red Datum: base of Tbu 0 Upper bioclastic member (Tbu) Tbug 0 lines) are the regional unconformity at the base of Bouse Formation upper bioclastic

Reworked Conglomerate (top of Tfg1) sandstone member (UBM) and the base of unit Tfg2 Tbbc-b alluvial-fan conglomerate. Stratal wedging −10 −10 Miocene VE = 10:1 and pinch-out geometries provide evidence Bioclastic facies conglomerate Tbs 0° Heterolithic facies for tilting to the southeast during deposition

Height (m) (Tfg1) of the Bouse siliciclastic member (unit Tbs) Green claystone Red mudstone 2° and UBM (Tbu). Tilting is inferred to have −20 Palo Verde 5° −20 fault 8° been controlled by slip on the Palo Verde fault, a propagating blind normal fault at 0 100 200 300 400 500 600 700 800 900 1000 1100 depth. VE—vertical exaggeration. Vertical black lines represent measured sections. Horizontal Distance (m)

above results, supplemented with data from test thinning of unit Tbs toward basin margins where by observations that it is consistently <5 m thick well 36bbb, which penetrated 90 m of sediment it is truncated by erosion and/or nondeposition at where encountered in subsurface wells throughout including 52 m of the Bouse Formation siliciclas- the subhorizontal base of unit Tbug (Fig. 12). Bull- the Blythe Basin (Metzger et al., 1973). Although the tic member (Fig. 2; Metzger et al., 1973; Cassidy head Alluvium (unit Tcb) is not present in the valley base of the Bouse Formation was not penetrated et al., 2018; Cohen et al., 2019). The cross section center (Metzger et al., 1973), likely due to south- in well 36bbb, regional isopach analyses suggest is displayed at 5× vertical exaggeration to permit ward shallowing of the Blythe Basin and erosional that the basal carbonate member likely is <50 m visualization of key stratigraphic relations in thin truncation at the base of Quaternary to Holocene below the bottom of the well (24 m below sea level) geologic units. The basin architecture is dominated Colorado River sediment (unit Qcrs) (Crow et al., (Turak, 2000; Cassidy et al., 2018; Crow et al., 2018, by nearly symmetric, gentle (1°–4°) basinward 2019b; Cohen et al., 2019). 2019b). Thus, the available constraints indicate a bedding dips, as observed around the exposed The Bouse Formation basal carbonate member basin architecture characterized by gentle basin- margins of the southern Blythe Basin (Figs. 9–11). thickens toward the basin center from basin mar- ward bedding dips adjacent to high-angle faults at Gentle basinward dips explain the observed thick- gins in the western Trigo Mountains and Palo Verde the basin margins, with bedding dips decreasing ening of the Bouse siliciclastic member (unit Tbs) Mountains (Figs. 9, 11), but its thickness appears to to subhorizontal in the basin center beneath the into a broad axial depocenter, with concomitant decrease into deeper parts of the basin as indicated modern Colorado River (Figs. 2, 12).

Figure 12. Geologic cross section from WNW ESE the Palo Verde Mountains to the Trigo 5:1 VE Palo 0° Mountains showing basin architec- 400 Verde Trigo ture of the southern Blythe Basin. See Mountains 5° 300 10° Mountains Figure 2 for location. Unit Qcrs is Qua- ternary to Holocene Colorado River 200 Figs. 9B, 9C Tbug Qt Fig. 11 Qt Red line is sediment. Bullhead Alluvium (unit Tcb) Tbug Well 36bbb (projected topography 100 ~2.3 km into section) Tcb is not present in well 36bbb (Metzger ? Tbbc Qcrs Lost Tvx et al., 1973), probably due to removal by 0 Tbs Trigo erosion at the base of unit Qcrs. Litho- fault Elevation (m) -100 Palo Verde logic unit descriptions are the same as fault Tvx Tfg1 Tvx Tvx in Figure 4. Stratal relations exposed -200 on both sides of the valley provide 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 evidence for gentle tilting toward the Distance (km) basin center during deposition of the basal carbonate (Tbbc) and siliciclastic (Tbs) members of the Bouse Formation prior to deposition of the upper bioclastic member and alluvial fan gravels (Tbug). Syn-depositional tilting was controlled by growth of normal and oblique-slip faults at depth. The Lost Trigo fault in the Trigo Mountains propagated to the surface during deposition of unit Tbug, which is cut by the fault. We infer that the Palo Verde fault, which controlled tilting in the Palo Verde Mountains, remains at depth today and has not yet propagated to the surface. VE—vertical exaggeration.

Fault Kinematic Analysis is dominantly a normal fault that formed due to (1) syn-depositional tilting toward a fault produces approximately east-west extension (Figs. 13A–13D), anomalously thick deposits in the immediate hang- Fault orientations and kinematic data were similar to extension directions in the lower Colorado ing wall of the fault; and (2) growth of a monocline collected from several locations in the study area River region over the past ~10 m.y. (Thacker et al., above the upward-propagating tip of a normal or (Figs. 2, 4) and analyzed to estimate the orientations 2020). The Lost Trigo fault zone changes northward oblique-slip fault causes strata to be tilted and of the principal strain axes responsible for faulting to a NW-striking segment that displays complex thicken away from the fault, followed by subse- (Fig. 13). In the western Trigo Mountains, geometric anastomosing fault strands, a change in overall dip quent tilting and thickening toward the fault after data for major and minor faults along the Lost Trigo direction, and variable sense of dip-slip displace- it propagates up to Earth’s surface. fault zone are grouped according to their location in ment along the trend of the fault zone (Figs. 4, 7), The first type of response is documented at Hart the northern, NW-striking segment of the fault zone features that are commonly observed in strike-slip Mine Wash where the Bouse siliciclastic member (north of latitude 33.275°N) versus the southern, fault zones (e.g., Wilcox et al., 1973; Sylvester, 1988; (unit Tbs) dips 15°–20°SW toward the Lost Trigo north-striking segment (south of latitude 33.275°N) McClay and Bonora, 2001; Bergh et al., 2019) and fault, and a nearly continuous exposure of the (Fig. 4). The results reveal a conjugate fault pattern related sedimentary basins (Christie-Blick and Bid- section reveals ~100 m of dipping siltstone and in both groups (Figs. 13A, 13B). Northern faults have dle, 1985; Nilsen and Sylvester, 1995; McNabb et al., mudstone overlain by cross-bedded Colorado River an average strike of approximately N20°W (340°), 2017). In the absence of fault kinematic data from channel sandstone near the fault (Fig. 5D, 7A, 8). and southern faults strike on average approximately the Lost Trigo fault in Hart Mine Wash, we speculate The close association of steeper bedding dips and due north (Figs. 13A, 13B). Although slickenlines that it would have acted as a dextral oblique-nor- excessive local thickness adjacent to the fault and were not observed in the NW-striking segment of mal fault under the same approximately east-west the observation that the fault is unconformably the Lost Trigo fault zone exposed in Hart Mine Wash, extensional strain regime recorded along the south- capped by undeformed Bullhead Alluvium (Figs. 5D, subvertical Riedel shears, drag folds, and offset ern segment of the fault zone. 7A) indicate that the NW-striking northern seg- Bouse Formation strata indicate a large component ment of the Lost Trigo fault experienced ~100 m of of normal slip (Figs. 5D, 8). At three fault-slip data dip-slip and an unknown magnitude of strike-slip localities farther south (red circles on Fig. 4), fault ■■ DISCUSSION offset during deposition of unit Tbs at this locality. striae within the Lost Trigo fault zone have steep to While we interpret this NW-trending segment of slightly dextral oblique rakes, and kinematic crite- Data and results presented above provide a the Lost Trigo fault to be a dextral oblique-normal ria indicate dominantly normal slip (Fig. 13C). Fault record of fault-controlled tilting, subsidence, and fault, the dip-slip component of fault displacement kinematic analysis yields a subhorizontal incremen- sedimentation during deposition of the Bouse For- is well constrained by stratigraphic data in Hart tal extension axis oriented 262°/14° (trend/plunge) mation in the lower Colorado River valley (Fig. 1). In Mine Wash. and a subvertical incremental shortening axis this section, we integrate stratal geometries and tilt The second type of response is syn-depositional oriented 040°/72°, indicating east-west–directed patterns with fault-kinematic data and prior studies growth of fault-related monoclines that results in extension with a very minor component of dextral in the region to decipher the basinal and surface tilting away from a blind propagating fault tip. shear (Fig. 13D). In the western part of the study response to late Miocene–early Pliocene transten- This pattern is observed around the margins of area, several small normal faults that cut Bouse sional faulting in the paleo-ECSZ. the southern Blythe Basin and is associated with Formation carbonate near Buzzards Peak (Fig. 2) gentle tilts toward a regional depocenter beneath have a similar northerly strike with an average steep the modern Colorado River (Figs. 9, 11, 12). Mono- dip to the east (Figs. 13E, 13F), kinematically com- Faulting and Folding during Bouse Formation cline growth related to the Lost Trigo fault in the patible with east-west extension. No fault kinematic Deposition western Trigo Mountains is documented at Hart indicators (slickenlines) were observed on faults at Mine Wash where thin basal carbonate rests on a the Buzzards Peak locality. Syn-depositional growth of normal faults is flat upper limb in the northeast and passes laterally We interpret the fault geometric and kinematic commonly documented with detailed geologic into a wedge of southwestward-thickening marl results and along-strike variations in fault strike and mapping, structural cross sections, and strati- and unit Tbs on the SW-dipping limb of the mono- dip direction to suggest that the Lost Trigo fault graphic analysis (Gawthorpe et al., 1997; Young cline (Fig. 9A). Farther south, a similar pattern of zone accommodated roughly east-west extension et al., 2003; García-García et al., 2006; Lewis et al., westward thickening and truncation of unit Tbs is with a small component of transtensional defor- 2017; Bennett et al., 2017; Muravchik et al., 2018; observed at the base of unit Tbug (Fig. 9B), provid- mation on linked normal and dextral-normal fault Serck and Braathen, 2019). Using this approach, ing further evidence for tilting and monoclinal fold segments. The southern segment of the Lost Trigo we have identified two types of basinal response growth during deposition of the Bouse Formation. fault zone strikes approximately north-south and to fault growth in the southern Blythe Basin: The Lost Trigo fault at this location cuts the eastern

M.P. 1 = 156°, 61°SW N Lost Trigo fault zone N Eigenvalue = 0.8262 M.P. 2 = 345°, 62°NE Eigenvalue = 0.8741

North of 33.275°N South of 33.275°N

Figure 13. Geometric and kinematic fault data and paleostrain analysis for selected localities in the study area; all plots are lower-hemisphere, equal-area stereo- graphic projections. Eigenvalue—statistical fit to data; C.I.—Kamb contour interval (in sigma); S.L.—signifi- C.I. = 2 cance level (in sigma); M.P.—mean plane determined M.P. 1 = 174°, 67°W S.L. = 2 Eigenvalue = 0.8803 from maximum eigenvector (diamonds). (A–B) Geomet- M.P. 2 = 008°, 65°E ric data for major and minor faults from 16 locations n = 72 Eigenvalue = 0.9137 Average map trend of along the Lost Trigo fault zone (blue and red circles in N N northern fault segment Fig. 4). Data are grouped according to location in either the northern, NW-striking segment of the fault zone (north of latitude 33.275°N), or the southern, N-striking segment of the fault zone (south of latitude 33.275°N) (Fig. 4). (A) Stereoplot of fault planes for northern (red) and southern (black) fault segments. (B) Stereoplot of poles to faults in A with Kamb contours colored by fault P segment. The mean planes for each group define a conjugate geometry that strikes NNW-SSE (northern group) T and north-south (southern group) and dips 61°–67° to the west and east. Dashed lines are mean planes for each population of poles to planes determined from a cylindrical best fit and shown with red (northern) and black (southern) diamonds. (C–D) Fault kinematic data n = 12 and analysis for the Lost Trigo fault zone from three n = 12 n = 12 locations (red circles in Fig. 4) based on 12 faults that displayed kinematic criteria. (C) Stereoplot of fault planes T-axis = 262°/14° Fault 1: 162°, 60°W; 77° rake from N and their respective slip lineations; arrows show hang- P-axis = 040°/72° Fault 2: 008°, 33°E; 68° rake from N ing-wall slip direction. (D) Paleostrain analysis for data in C. Incremental shortening (P; blue) and extension (T; red) Buzzards Peak locality axes (linked Bingham axes; squares) and pseudo–fault N N plane solution indicate approximately east-west extension (T-axis trend = 262°; bold arrows) with a relatively E minor component of dextral shear. (E–F) Geometric data for faults near Buzzards Peak (Fig. 2). (E) Stereoplot of fault planes. (F) Kamb-contoured stereoplot for poles to faults in E; mean fault plane = 357°, 63°E, similar to east-dipping fault surfaces in the southern segment of the Lost Trigo fault (panel B).

n = 7 M.P. = 357°, 63°E C.I. = 2 Eigenvalue = 0.7315 S.L. = 3

end of the west-dipping strata, indicating that fault observed thickening of the Bouse Formation from River to the northern Gulf of California (Metzger activity continued after deposition of the Bouse For- basin margins to basin center (Fig. 12) as well as et al., 1973; Buising, 1990; Howard et al., 2015; mation and the tip of the fault propagated up to the the distribution of Bouse Formation exposures at Bennett et al., 2016a). Fault-controlled tilting and Earth’s surface near the end of Bouse deposition. In higher elevations around the margins of the basin subsidence were ongoing prior to deposition of Marl Wash, the Bouse Formation upper bioclastic and at lower elevations near the basin center. the Bouse Formation, as indicated by widespread member (UBM) and overlying unit Tfg2 (collec- This conclusion contrasts with those of previous fault-bounded Miocene alluvial fan conglomerates tively, unit Tbug) thicken and dip ~3° toward the studies that treated elevation as a proxy for strati- that locally display distinctive fanning-dip geom- fault (Figs. 4, 9C). Although the record is incomplete graphic position and age in the Bouse Formation etries (e.g., Sherrod and Tosdal, 1991; Fig. 7D). due to erosion in this area, the map pattern and and asserted that deposits at lower elevations are Late Miocene faulting and subsidence produced stratigraphic data reveal some thickening and gen- older than higher-elevation deposits (Spencer et al., topographic basins that later became the site of tle dip of unit Tbug toward the fault, opposite the 2008, 2013; Roskowski et al., 2010; McDougall and deposition for the Bouse Formation (e.g., House predominant direction of westward thickening and Miranda Martínez, 2014). An assumed correlation et al., 2008; Pearthree and House, 2014; Howard et dip direction in older basal carbonate and siliciclas- of age with elevation implies no syn- or post-Bouse al., 2015). In addition, results presented above show tic members (Figs. 4, 9B, 9C). Taken together, these tectonic deformation and a stratigraphic model in that fault-controlled tectonic subsidence continued relations imply a double-wedge geometry that which horizontal beds of the Bouse siliciclastic into Pliocene time, as recorded in syn-depositional results when strata first tilt and thicken away from member onlap onto preexisting steeper basin mar- growth structures of the Bouse Formation in the a normal fault in a growth monocline as the fault gins draped by the Bouse basal carbonate member lower Colorado River region. tip propagates upward at depth, followed by tilting (e.g., Pearthree and House, 2014; Crow et al., 2019b; The 3-D pattern of syn- to post-Bouse fault- and thickening back toward the fault after the fault Cohen et al., 2019). In contrast, we document sys- related tilting and subsidence documented in this ruptures Earth’s surface (Gawthorpe et al., 1997; tematic gentle bedding dips that were produced study offers a unique perspective on interactions Lewis et al., 2017). The stratigraphic relations thus by fault-related monoclinal folding and basinward among crustal deformation, sedimentation, and indicate that the tip of the Lost Trigo fault breached tilting, with the result that younger members of landscape evolution in the paleo-ECSZ. Late Mio- the surface after deposition of unit Tbs, just prior to the Bouse Formation commonly are exposed at cene transtensional fault activity continued into or during deposition of the lower part of the UBM. lower elevations over distances of 1–3 km (Figs. 4, Pliocene time and influenced the evolution of the Blind strands of the Lost Trigo fault may also be 7, 10, 11). The observed basinward dips thus define a Blythe Basin as well as the geometry and extent present beneath the upper reaches of Hart Mine complex three-dimensional (3-D) basin architecture of the Bouse Formation (Figs. 1, 14). Our results Wash (Fig. 9A). The Palo Verde fault represents produced by fault-controlled, syn-depositional tilt- suggest that the present-day distribution and ele- another growth structure in the southeastern Palo ing on opposite sides of the southern Blythe Basin vation of Bouse deposits (up to 330 m elevation in Verde Mountains, similarly controlled by growth (Fig. 12). the Blythe region) reflect long-wavelength tectonic of a propagating normal fault that remains buried tilting and post-depositional uplift (e.g., Beard et al., in the subsurface today (Fig. 11). Our analysis thus 2016; Bennett et al., 2016b; Karlstrom et al., 2017; provides evidence for two stages of syn-deposi- Structural Controls on Basin Evolution Thacker et al., 2020). This provides an alternative to tional fault development on opposing sides of the suggestions that the Bouse Formation accumulated southern Blythe Basin (Fig. 12). Syn-depositional faulting, tilting, and basin sub- on a static landscape that has not experienced any Fault-related growth monoclines in the southern sidence documented in this study took place during significant post-depositional uplift and that depos- Blythe Basin provide a clear record of syn-deposi- a protracted history of late Miocene to Pliocene its of the Bouse Formation in Blythe Basin formed in tional tilting away from mountain range fronts and transtension in the Gulf of California shear zone lakes at their present-day elevations (Spencer and toward a regional depocenter beneath the mod- in northwestern Sonora (Mexico) and the ECSZ Patchett, 1997; Spencer et al., 2013; Pearthree and ern Colorado River valley (Fig. 12). Importantly, of western Arizona and southeastern California House, 2014). We find that diffuse transtensional fine-grained lime mud and heterolithic facies of (Fig. 1; Buising, 1990; Sherrod and Tosdal, 1991; strain in this region was active before, during, and the Bouse basal carbonate record deposition in Bennett et al., 2016a, 2017; Thacker et al., 2020; after deposition of the Bouse Formation and was low-energy tidal flats on widespread horizontal Mavor et al., 2020). Oblique extension in a diffuse sufficient to modify the elevation of Bouse For- surfaces (O’Connell et al., 2017, 2021; Gardner and network of strike-slip, oblique-slip, and normal mation deposits by subsidence in modern basins Dorsey, 2021). We therefore conclude that 1°–5° faults led to tectonic subsidence in the lower Col- and broad uplift in flanking mountain ranges over basinward bedding dips in this area are tectonic orado River region (Jachens and Howard, 1992; the past ~5 m.y. This interpretation is consistent in origin, not original primary bedding dips. Struc- Howard and Miller, 1992; Richard, 1993) and set with evidence for deposition of the Bouse basal turally controlled gentle bedding dips explain the the path for integration of the earliest Colorado carbonate member in a tidal to subtidal marine

basin (McDougall, 2008; McDougall and Miranda The Palo Verde fault is a previously unmapped geometries (Fig. 15). Because the ECSZ formed Martínez, 2014; O’Connell et al., 2017, 2021; Dorsey blind, basin-bounding fault at the range front of adjacent to an evolving transform plate boundary et al., 2018; Gardner and Dorsey, 2021). the southeastern Palo Verde Mountains, western (Fig. 1), it did not undergo the protracted phase of Indirect evidence for syn- and post-Bouse defor- margin of the southern Blythe Basin, and provides post-rift thermal subsidence that is commonly doc- mation in the Blythe Basin comes from recognition additional direct evidence for syn- and post-Bouse umented at rifted continental margins. As a result, of age-correlative deposits that occur at different deformation (Figs. 10, 11, 12). The NE-striking the deposits and bounding structures of these elevations across the basin. Deposits of the Bouse Palo Verde fault strikes subparallel to the NNE- to basins remain at shallow crustal levels providing Formation and Bullhead Alluvium have subsided north-striking Lighthouse Rock fault that links north access for direct observation around the basin mar- or been faulted down in the central Blythe Basin to the Lost Trigo fault (Fig. 2). All of these normal gins (this study). (Fig. 12; Metzger et al., 1973; Howard et al., 2015; and oblique-slip faults dip toward the southern Bly- Cassidy et al., 2018; Cohen et al., 2019; Crow et the Basin depocenter (Fig. 12). We infer that this al., 2019b), suggesting that structures in the Blythe network of basin-facing faults, taken together, is Implications for the Paleo-ECSZ Basin were active during late Miocene to Pliocene related to a diffuse right stepover between the dex- time. Direct evidence for syn- and post-Bouse fault- tral Laguna and Cibola fault zones (Fig. 1). We thus Based on results of this and previous studies ing in the Blythe Basin includes basin-bounding interpret the southern Blythe Basin to be an oblique (Bennett et al., 2016a; Beard et al., 2016; Thacker faults such as the Lost Trigo fault mapped along the dilational basin that subsided within this transten- et al., 2020), we present a palinspastic reconstruc- eastern margin of the basin (Metzger et al., 1973; sional fault system during deposition of the Bouse tion for the ECSZ and lower Colorado River region San Diego Gas and Electric Company, 1976; Menges Formation (Fig. 14). This is similar to the geome- at ca. 5 Ma (Fig. 15). The southern Blythe Basin and Pearthree, 1983; Sherrod and Tosdal, 1991; try and kinematics of other fault systems in the formed as a transtensional sag basin in a releasing Richard, 1993; this study). Detailed stratigraphic ECSZ and Gulf of California shear zone that have stepover between the dextral Laguna fault system analysis (this study) indicates that the Lost Trigo developed in response to northwest-directed dex- in the south and the Cibola and Big Maria dex- fault played a fundamental role in syn-depositional tral transtension and related east-west extensional tral faults in the north. This interpretation builds basinward tilting and thickening of the lower mem- strain along the Pacific–North America plate bound- on previous studies that document similar timing bers of the Bouse Formation (units Tbbc, Tbs), local ary (e.g., Burchfiel and Stewart, 1966; Howard and and kinematics of deformation in two NW-trending tilting and thickening of upper Bouse Formation Miller, 1992; Lonsdale, 1989; Pacheco et al., 2006; belts of dextral faults (Table 1): (1) Iron Mountain members toward the fault, and local control on the Aragón-Arreola and Martín-Barajas, 2007; Guest et and Bristol-Granite Mountains fault zones that con- eastern extent of Blythe Basin deposits (Figs. 4, 7, al., 2007; Mahan et al., 2009; Thacker et al., 2020). nect from the Blythe Basin northwestward to the 9). The Lost Trigo fault likely links south-southwest- We therefore conclude that the Bouse For- eastern end of the Garlock fault (Dokka and Travis, ward to the Lighthouse Rock fault (Fig. 2; Sherrod mation and Bullhead Alluvium accumulated 1990; Howard and Miller, 1992; Brady, 1992, 1993; and Tosdal, 1991; Beard et al., 2016), along which in a subsiding fault-bounded basin (southern Lease et al., 2009; Langenheim and Miller, 2017); the southern Blythe Basin pinches out toward the Blythe Basin) that formed as a result of transten- and (2) Bill Williams River fault zone (Sherrod, 1988) south. This basin-bounding system of normal faults sional deformation in a diffuse network of linked and a series of NW-striking dextral faults distrib- appears to link southward to the NW-striking dex- right-stepping dextral, normal, and oblique-slip uted across western and south-central Arizona tral Laguna fault system in the southeastern Trigo faults in the lower Colorado River region (Fig. 14). (Singleton, 2015; Singleton et al., 2019) that may Mountains and northward to the NW-striking dex- The flat base, shallow depth, and moderate width link northward to the Stateline fault system across tral Cibola fault zone, a zone of NW-trending dextral (~5–20 km) of the southern Blythe Basin (Fig. 12), its a complex releasing stepover represented by exten- and oblique-slip faults concealed beneath central association with low-strain bounding faults, and its sional faults systems in Mohave Valley and Piute Blythe valley (Fig. 1; Richard, 1993; Beard et al., stratigraphic position overlying older extensional Valley (Figs. 1, 15; Sherrod, 1988; Howard et al., 2016) that may link northwestward to other ECSZ fault systems all suggest a rift-sag style of basin 1999; Bennett et al., 2016a; Singleton et al., 2019; dextral fault zones (Langenheim and Miller, 2017; formation. Rift-related sag basins are common in Thacker, 2019; Thacker et al., 2020). These two fault Mavor et al., 2020; Umhoefer et al., 2020). These zones of orthogonal extension at now-submerged belts define a ~100-km-wide zone of diffuse exten- linked normal and dextral faults form a distributed passive margins (Lambiase and Morley, 1999; Davi- sion and transtension (light red area in Fig. 15) that right-stepping fault geometry across the southern son and Underhill, 2012; Pang et al., 2018; Zhang et accommodated a portion of Pacific–North America Blythe Basin. The change of strike in the northern al., 2019). In this case, dilation and subsidence of plate motion during late Miocene to early Pliocene part of the Lost Trigo fault (Fig. 4) likely represents a the southern Blythe Basin and coeval basins of the time (see also Bennett et al., 2016a; Thacker et al., smaller-scale example of this style of linked normal paleo-ECSZ occurred as a result of diffuse transten- 2020). Below we summarize published evidence and dextral faults. sional fault kinematics and releasing-stepover for the slip history in these two fault belts (Table 1).

Figure 14. Interpretation of structural controls on subsid- A. Bouse Basal Carbonate Member (ca. 6.3–5.4 Ma) ence in the southern Blythe Basin during deposition of P North the Bouse Formation (modified from Dorsey et al., 2018). BMM Background image is from Google Earth. See Figures 4 and DRM B 10 for definition of lithologic unit descriptions. C.R. in part Tidal B is Colorado River. Abbreviations: B—Blythe, California; flats? Shallow inland sea BMM—Big Maria Mountains; C—Cibola, Arizona; DRM— Dome Rock Mountains; P—Parker, Arizona; PVM—Palo PVM Verde Mountains; TM—Trigo Mountains; MW—Milpitas C Wash. Approximate ages are from Dorsey et al. (2018). Lime-mud Tidal strait tidal flats? TM MW

Carbonate Miocene conglomerate & Tbbc shelf volcanics, and pre-Miocene The Iron Mountain and Bristol-Granite Moun- Upward propagating crystalline rocks normal fault tains fault zones form a NW-trending dextral fault system along strike of the Cibola and Big Maria Marine tidal strait, pre-Colorado River fault zones along the northeastern margin of the modern ECSZ (Fig. 15; Sherrod and Tosdal, 1991; Richard, 1993; Beard et al., 2016, and references B. Bouse Siliciclastic Member Colorado River (ca. 5.4–5.0 Ma) channel therein). Richard (1993) estimated dextral offset on Playa lakes? P Playa the overlapping Laguna (2 km), Cibola (7 km), and BMM lakes? ? DRM Big Maria (4.5 km) faults. This NW-trending zone Playa lakes? B ? of linked transtensional faulting likely continues lakes? C.R. flood Playa plain to the northwest across the northern Blythe Basin, where Umhoefer et al. (2020) suggested 30–35 km PVM C of dextral offset. Estimates of cumulative dextral Playa lakes? TM slip on the Bristol-Granite Mountains fault zone MW range from 9–15 km to 24 ± 3 km (Dokka and Travis, Tbs 1990; Howard and Miller, 1992; Brady, 1992, 1993; Miocene conglomerate & volcanics, and pre-Miocene Tbbc Lease et al., 2009; Langenheim and Miller, 2017). crystalline rocks Upward propagating normal fault Some or all of this dextral shear is transmitted northwestward along strike to the Soda-Avawatz Arrival of Colorado River delta and through-flowing river fault zone (Fig. 1; Schermer et al., 1996; Langen- heim and Miller, 2017), where cumulative dextral C. Bouse Upper Bioclastic Member (ca. 5.0–4.8 Ma) offset estimates range from 8–9 km (Langenheim and Miller, 2017) to as much as 20–28 km (Brady, Playa lakes? P BMM retreating 1984; Schermer et al., 1996). This transtensional ? delta DRM strain likely continues north past the eastern tip of Playa lakes? B ? the Garlock fault onto the dextral-oblique Death lakes? Playa Valley fault zone (Davis, 1977; Glazner et al., 2002). Although all of these fault zones are thought to PVM Prograding C fan deltas have been active during late Miocene to Pliocene Shallow marine time, only the faults northwest of the Bristol-Gran- MW estuary TM ite Mountains fault zone remained unambiguously Tbu Normal fault Tbs breaches surface active into Quaternary time. Miocene conglomerate & The Stateline fault system and Bill Williams River volcanics, and pre-Miocene Tbbc crystalline rocks fault zone, and related faults, form another NW- trending belt of dextral faults along the northeastern Re-flooding of lower Colorado River valley by shallow marine water margin of the paleo-ECSZ (Fig. 15). Significant late

Miocene–Pliocene dextral shear is documented on the Stateline fault system, which experienced as to Stateline fault much as 30 ± 4 km of dextral displacement after Death IV ? ca. 13 Ma (Fig. 15; Guest et al., 2007). At least Valley Bristol-Granite lt Mountains fault u PV ~30% of this slip appears to be transmitted south- a ? f n eastward across Piute Valley and Mohave Valley o t p BlM i MV early to the Bill Williams River fault zone and Buckskin- N FV SM Pliocene Rawhide Mountains in western Arizona, where a minimum of 9–11 km of distributed dextral shear ChM Colorado BWR F.Z. River is documented in the Buckskin-Rawhide Mountains OWM Extension - Edge of model (~7–9 km; Singleton, 2015; Singleton et al., 2019) dominated Iron Mtn fault WM Edge of model faults and and in other ranges to the southeast (~2–4 km; Sin- BRM Future San basins gleton et al., 2019). We speculate that two small Bernardino ~200-km-wide paleo-ECSZ Mountains transtensional stepovers north and northeast of RM Northern BMM Blythe the Whipple Mountains connect dextral strain from LSBM Eastern Basin Cibola the Buckskin-Rawhide Mountains and Bill Williams

Transverse DRM Big Maria fault River fault zone to the Needles deformation zone Future San Gabriel Mountains Ranges fault SPM S an in Mohave Valley (Fig. 15; Thacker, 2019; Thacker et A nd al., 2020). The remaining ~20 km of dextral shear on re as PVM Southern – the Stateline fault system may step to the southwest Blythe Al Fig. 2 go Basin via sinistral-oblique extension on the Nipton fault do CM TM ne CDM in Ivanpah Valley (Fig. 15; Miller and Wooden, 1993; s f Future Path of au Mahan et al., 2009; Miller et al., 2019) and in valleys lt Colorado River

sy flanking the Old Woman Mountains. Slip on these U.S.A. Jacinto Mountains s Laguna fault system Future San te Mexico m faults appears to be transferred south to NW-strik- Gulf ofs California seaway ing dextral fault zones such as the Big Maria fault Baja California microplate zone (Miller and McKee, 1971), southern Plomosa Mountains (Strickland et al., 2018), and lower Col- U.S.A. orado River region (Richard, 1993; Thacker et al., Mexico 2020) as a diffuse transtensional system of linked GCSZ dextral, normal, and oblique faults and related FCVB basins, including faults around the margins of the southern Blythe Basin (Fig. 15). These and other published constraints on the timing, magnitude, and style of strain across the Figure 15. Palinspastic reconstruction of the ca. 5 Ma, ~200-km-wide paleo–Eastern California shear zone (paleo-ECSZ), southern San Andreas fault system, the greater which was kinematically linked to the Gulf of California shear zone (GCSZ) across the lower Colorado River and northern Gulf of California regions during deposition of the Bouse Formation. Modified from Bennett et al. (2016a) and Beard et al. ECSZ, and the Gulf of California shear zone inform (2016). Faults (red lines) are shown in their ca. 5 Ma architecture. The southern Blythe Basin (within Fig. 2 box) formed our palinspastic tectonic reconstruction for this part in a releasing stepover from the Laguna fault system in the south to the Cibola and Big Maria faults in the north. A wide of the Pacific–North America plate boundary at ca. belt of northwest-southeast–directed extension and transtension (transparent red region with large white arrows) likely 5 Ma (Fig. 15). We estimate 74 ± 7 km of post–12 Ma was bounded by discrete fault systems near the western and eastern margins of the paleo-ECSZ. Orange areas are tectonic basins that formed due to late Miocene to Pliocene extension and transtension: FV—Fenner Valley; IV—Ivanpah dextral shear across the modern ECSZ (Bennett et Valley; MV—Mohave Valley; PV—Piute Valley. Political boundaries are fixed in present-day coordinates for reference. al., 2016a), a value that includes more recently Mountain ranges (yellow): BMM—Big Maria Mountains; BlM—Black Mountains; BRM—Buckskin-Rawhide Mountains; documented and larger offset than the ~65 km CM—Chocolate Mountains; CDM—Castle Dome Mountains; ChM—Chemehuevi Mountains; DRM—Dome Rock Mountains; estimate of Dokka and Travis (1990) and is indis- LSBM—Little San Bernardino Mountains; OWM—Old Woman Mountains; PVM—Palo Verde Mountains; RM—Riverside tinguishable from an estimate of 74 ± 17 km offset Mountains; SM—Sacramento Mountains; SPM—southern Plomosa Mountains; TM—Trigo Mountains; WM—Whipple Mountains. Other abbreviations: BWR F.Z.—Bill Williams River fault zone; FCVB—Fish Creek–Vallecito Basin. Animation along strike to the northwest in the southern Walker of this reconstruction from 11 to 0 Ma (Bennett et al., 2016a) is available at: https://youtu.be/htzdDLW3-aQ. Lane–Death Valley region (Renik and Christie-Blick,

TABLE 1. PUBLISHED ESTIMATES OF DEXTRAL SLIP IN THE PALEO–EASTERN CALIFORNIA SHEAR ZONE Fault zone or region* Cumulative dextral slip Timing of slip References (km) (Ma) Big Maria 4.5 Uncertain Miller and McKee (1971); Richard (1993) Bill Williams River ~1 <10 Sherrod (1988); Thacker (2019); Thacker et al. (2020) Bristol-Granite Mountains 24 ± 3 ≤11 Lease et al. (2009) 12 ± 3 ≤16 Langenheim and Miller (2017) Cibola 7 Uncertain Richard (1993) Iron Mountain 5.5 Uncertain Howard and Miller (1992); Richard (1993) Laguna 2 Uncertain Richard (1993) Needles deformation zone# <1 (?) ≤2.58 Pearthree et al. (2009); Thacker (2019); Thacker et al. (2020) Soda-Avawatz 20–28 ≤10 Brady (1984); Schermer et al. (1996) 8–9 ≤16 Langenheim and Miller (2017) Southern Plomosa Mountains 6 to >24 <19 to <4.6 Miller and McKee (1971); Strickland et al. (2018) Stateline fault system 30 ± 4 ≤13.1 Guest et al. (2007) Western to south-central Arizona ≥9–11 ≤12 Singleton (2015); Singleton et al. (2019) *Italicized text refers to regions. #Pre-Quaternary dextral slip uncertain.

2013). Northwest-striking faults with ~74 km of dex- diminishes southeastward along strike into central that significant dextral faults initiated ca. 7–9 Ma tral shear project northwestward toward the eastern Arizona (Singleton et al., 2019); (2) a central exten- (Seiler et al., 2010, 2011; Bennett et al., 2013, 2015, part of the Garlock fault, consistent with a sugges- sion-dominated, low-shear domain including the 2016b, 2017; Bennett and Oskin, 2014), synchro- tion that this broad zone of distributed shear has Fenner Valley, Old Woman Mountains, Riverside nous with the onset of major plate-boundary strain produced at least 60 km of oroclinal bending and Mountains, and northern Blythe Basin; and (3) the in the Salton Trough region (Dorsey et al., 2011; right-lateral deflection of the eastern Garlock fault >400-km-long transtensional belt of linked dextral Mason et al., 2017). Although this timing is slightly (Dokka and Travis, 1990). Oroclinal bending of the and normal faults including the Bristol-Granite younger than the ca. 10 Ma initiation of the sinistral Garlock fault provides a mechanism to transmit Mountains, Big Maria, Cibola, and Laguna fault Cave Mountain fault in the modern ECSZ based dextral shear strain from the modern ECSZ in the zones (Fig. 15). Late Miocene faults in the paleo- on U-Pb dating of fault opal (Nuriel et al., 2019), it Mojave Desert northward across the Garlock fault ECSZ linked southeastward to dextral faults in is consistent with estimates for the onset of dex- into the southern Walker Lane (Andrew and Walker, northwestern Sonora that accommodated at least tral shear along the Camp Rock fault in the central 2017), where active strike-slip structures occur in ~50 km of cumulative slip (Nourse et al., 2005) and Mojave Desert (ca. 7 Ma; Nuriel et al., 2019) and the Death Valley, Panamint Valley, and Indian Wells Val- other right-stepping transform faults in the north- southern San Andreas fault system based on strati- ley, including faults that ruptured in the 2019 CE ern Gulf of California (e.g., Pacheco et al., 2006; graphic analysis and thermochronology (ca. 8 Ma; Ridgecrest earthquakes (Fig. 1) (DuRoss et al., 2020). González-Escobar et al., 2013) that comprise the Dorsey et al., 2011; Mason et al., 2017). The onset Our tectonic reconstruction suggests that Gulf of California shear zone (Bennett and Oskin, of rapid exhumation in the Little San Bernardino dextral transtension related to the late Miocene– 2014; Bennett et al., 2017). Mountains (Fig. 15; Spotila et al., 2020) at ca. 5 Ma Pliocene Pacific–North America plate boundary The timing of earliest dextral shear and trans records tectonic activity in the eastern Transverse was distributed across a ~200-km-wide zone east rotation in the ECSZ is uncertain, although most Ranges that likely was related to transtensional of the southern San Andreas fault that included studies suggest that major northwest-dextral shear faulting, further suggesting early development both the paleo-ECSZ and the modern ECSZ (Figs. 1, began between ca. 10–12 Ma (Carter et al., 1987; and antiquity of the western part of the modern 15). In the northeastern part of this belt (shaded Dokka and Travis, 1990; Schermer et al., 1996; ECSZ. The kinematic compatibility and along-strike green and labeled “paleo-ECSZ” in Fig. 1), at Reheis and Sawyer, 1997; Nuriel et al., 2019) and linkage between faults of the paleo-ECSZ and the least ~30 km of late Miocene to early Pliocene ca. 5–7 Ma (Gan et al., 2003; Langenheim and Pow- Gulf of California shear zone (Figs. 1, 15) and the deformation apparently was distributed across ell, 2009). In contrast, the onset of dextral shear is absence of compelling evidence for major dextral three northwest-trending domains: (1) an eastern well constrained along strike in the Gulf of Cal- shear in the Mojave Desert region prior to late Mio- belt of dextral transtension and shear, including ifornia shear zone of coastal Sonora and Baja cene time suggest that the paleo-ECSZ may have the Bill Williams River fault zone, in which slip California (Mexico) where structural studies show also initiated at ca. 7–9 Ma. While this timing is

compatible with available constraints from field- Our results from the southern Blythe Basin thus integration of the Colorado River into the northern based studies (Table 1), future work is needed to support an emerging picture of late Miocene defor- Gulf of California at ca. 5 Ma. test this hypothesis with improved age constraints mation across the lower Colorado River region in on faulting in the ECSZ. which non-uniformly distributed dextral shear and Past studies have pointed to the lack of recent transtension affected the entire ~200-km-wide ECSZ ACKNOWLEDGMENTS fault activity in the paleo-ECSZ and evidence for between the San Andreas fault and the Califor- Research for this study was supported by grants from the National Science Foundation (EAR-1546006), Society for Sedi- Quaternary-active faults in the modern ECSZ to nia-Arizona border. Kinematically linked normal mentary Geology, Geological Society of America, and the U.S. suggest that dextral shear across the plate bound- and strike-slip faults led to local tectonic subsidence Geological Survey National Cooperative Geologic Mapping ary has migrated to the west or southwest through in fault-controlled transtensional basins between Program. Support for Thacker was provided by the National time (e.g., Dokka and Travis, 1990; Andrew and zones of dextral shear, as we have documented Science Foundation (EAR-1545986 to Drs. Crossey and Karl- strom at the University of New Mexico). Glenn Sharman and Walker, 2017). This inference is consistent with in the southern Blythe Basin. Additional detailed Sue Beard are thanked for thoughtful reviews of an earlier draft models for westward migration of plate bound- studies are needed to better understand the distri- of this paper. This study benefited from discussions with Sue ary–related dextral strain in the San Bernardino bution and pattern of strain in the paleo-ECSZ and Beard, Andy Cohen, Laura Crossey, Ryan Crow, Brian Gootee, Kyle House, Keith Howard, Karl Karlstrom, Kris McDougall, and Mountains (Cochran et al., 2020), Salton Trough evaluate their influence on the evolution and inte- Phil Pearthree. Any use of trade, product, or firm names is for (McNabb et al., 2017), and northern Gulf of Cali- gration of the Colorado River and Gulf of California. descriptive purposes only and does not imply endorsement by fornia (Aragón-Arreola and Martín-Barajas, 2007; the U.S. Government. Bennett et al., 2013). In addition, the highest strain rates deduced from Quaternary slip rates (Oskin et ■■ CONCLUSIONS REFERENCES CITED al., 2008), historic surface-rupturing earthquakes Allmendinger, R.W., Cardozo, N., and Fisher, D.M., 2012, Struc- (Zeng and Shen, 2016), and highest GPS velocity Stratal wedge geometries in the Bouse Forma- tural Geology Algorithms: Vectors and Tensors: Cambridge, gradients (e.g., Thatcher et al., 2016) appear to tion south of Blythe, California, provide a record of UK, Cambridge University Press, 302 p., https://doi.org/10 presently be focused in a narrow belt of strike-slip latest Miocene to early Pliocene basinward tilting .1017/CBO9780511920202. Andrew, J.E., and Walker, J.D., 2017, Path and amount of dex- faults near the middle of the modern ECSZ, ~50 km in response to growth of syn-depositional normal tral fault slip in the Eastern California shear zone across west of the eastern boundary of the modern ECSZ and oblique-slip faults of the paleo-ECSZ. 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Some Mexico: Geology, v. 35, p. 571–574, https://doi.org/10.1130 Mojave block and its many brittle strike-slip faults faults formed at depth and propagated up to the /G23360A.1. over a fixed, vertical shear zone in the ductile lower Earth’s surface during Bouse deposition, while Beard, L.S., Haxel, G.B., Dorsey, R.J., McDougall, K.A., and Jacobson, C.E., 2016, Late Neogene deformation within crust (Dixon and Xie, 2018), rather than a true west- others evolved as blind faults that remain buried the Chocolate Mountains Anticlinorium: Implications for ward shift in location of dextral shearing relative in the subsurface today. Transtensional deforma- deposition of the Bouse Formation and the course of the to stable North America. However, the concept of tion across a diffuse system of normal, dextral, Lower Colorado River, in Reynolds, R.E., ed., Going LOCO: Investigations along the Lower Colorado River: 2016 Desert westward migration of the ECSZ—be it relative or and oblique-slip faults played a protracted role in Symposium Field Guide and Proceedings: Zzyzx, California absolute—implies that faults in the modern ECSZ the structural development and evolution of the State University Desert Studies Center, p. 176–184. initiated after faults in the paleo-ECSZ, a prediction southern Blythe Basin. We interpret these faults Bennett, S.E.K., and Oskin, M.E., 2014, Oblique rifting ruptures continents: Example from the Gulf of California shear zone: that contradicts documented late Miocene activity to be the structural boundaries of a late Miocene Geology, v. 42, p. 215–218, https://doi .org /10 .1130 /G34904 .1 . on faults of the modern ECSZ (Carter et al., 1987; to early Pliocene transtensional sag basin that sub- Bennett, S.E.K., Oskin, M.E., and Iriondo, A., 2013, Transten- Dokka and Travis, 1990; Nuriel et al., 2019). Instead sided within the paleo-ECSZ. Diffuse transtensional sional rifting in the proto–Gulf of California near Bahía Kino, of a westward shift in dextral strain, we propose strain in the paleo-ECSZ was connected southward Sonora, México: Geological Society of America Bulletin, v. 125, p. 1752–1782, https://doi.org/10.1130/B30676.1. that the late Miocene paleo-ECSZ accommodated a to the Gulf of California shear zone, now preserved Bennett, S.E.K., Oskin, M.E., Dorsey, R.J., Iriondo, A., and Kunk, fraction of distributed Pacific–North America plate in coastal Sonora and coastal Baja California, which M.J., 2015, Stratigraphy and structural development of the boundary shear across the entire ~200-km-wide initiated at ca. 7–9 Ma when relative plate motion southwest Isla Tiburón marine basin: Implications for latest Miocene tectonic opening and flooding of the northern Gulf region east of the San Andreas fault (Fig. 15) and became established in the northern Gulf of Califor- of California: Geosphere, v. 11, p. 977–1007, https://doi.org that structures in the eastern half of this domain nia and Salton Trough region. These results support /10.1130/GES01153.1. became mostly inactive as the plate boundary prior suggestions that regional subsidence in a Bennett, S.E.K., Darin, M.H., Dorsey, R.J., Skinner, L.A., Umhoefer, P.J., and Oskin, M.E., 2016a, Animated tectonic reconstruc- became more localized and focused into the mod- fault-bounded tectonic lowland controlled latest tion of the Lower Colorado River region: Implications for ern ECSZ through time (Fig. 1). Miocene shallow-marine inundation followed by Late Miocene to Present deformation, in Reynolds, R.E., ed.,

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