Subsurface Constraints of an Active Detachment Fault in Laguna Salada Basin, Baja California, México, from Interpretation of GEOSPHERE; V

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GEOSPHERE Subsurface constraints of an active detachment fault in Laguna Salada Basin, Baja California, México, from interpretation of GEOSPHERE; v. 12, no. 4 seismic-reflection profiles doi:10.1130/GES01261.1 Mario González-Escobar, Clemente G. Gallardo-Mata*, Arturo Martín, Luis Munguia, and Francisco Suárez-Vidal† 9 figures; 1 supplemental file División de Ciencias de la Tierra, Centro de Investigación Científica y de Educación Superior de Ensenada (CICESE), Ensenada, Baja California C.P. 22860, México

CORRESPONDENCE: mgonzale@cicese.mx ABSTRACT coarse-grained, high-energy alluvial fan deposits prograding over the basin CITATION: González-Escobar, M., Gallardo-Mata, floor from the west in the range front of Sierra Juarez. Seismic facies 1 and C.G., Martín, A., Munguia, L., and Suárez-Vidal, F., 2016, Subsurface constraints of an active detach- The Laguna Salada Basin in northeastern Baja California, México, is an ac- 2 predominate in the east and central portions of seismic profiles where the ment fault in Laguna Salada Basin, Baja California, tive half-graben with subsidence principally controlled by two major faults depocenter accumulates thicker sedimentary sequences. México, from interpretation of seismic-reflection along the eastern basin margin—the Cañada David detachment fault and the profiles: Geosphere, v. 12, no. 4, p. 1283–1299, doi:10.1130/GES01261.1. dextral oblique Laguna Salada fault. Active-source, seismic-reflection data constrain the geometry of the active detachment fault and indicate two struc- INTRODUCTION Received 1 September 2015 tural domains. The south domain is a supradetachment basin controlled by Revision received 14 April 2016 the Cañada David detachment fault. Two seismic profiles indicate the detach- The Laguna Salada Basin (LSB) in northeastern Baja California, México, is Accepted 16 June 2016 ment fault dips 17°–20° west, has a minimum of 10.1 km of slip, and accumu- an ~20-km-wide, ~100-km-long tectonic depression at the northwestern side Published online 8 July 2016 lates a sedimentary wedge more than 2.5 km thick in the west-central part of conterminous with the Gulf of California rift system (Fig. 1). The LSB is struc- this basin domain. This estimation indicates that the subsurface portion of the turally separated from the Salton Trough in southern California by the northern Cañada David detachment accommodates 24% of extension in the western extension of the Laguna Salada fault (LSF), which splits in both left-stepping main plate boundary zone. The north domain is a dilatational stepover (or pull and right-stepping shear strands (Isaacs, 1987) and produces basement ridges apart) controlled by the northwest-trending, west-dipping, dextral-oblique up to ~660 m (Fig. 1). Southward the flat topography of the basin contrasts Laguna Salada fault and the north-trending, dip-slip Cañón Rojo fault, which with the steep relief of bounding ranges of Sierra Juárez to the west and Sierra defines the south boundary of the pull-apart basin domain. The Cañón Rojo Cucapah and El Mayor to the east (Fig. 1). Southward the modern basin be- fault accumulates more than 2 km of subsidence, but geometric consider- comes narrower and connects through a ~5–10-km-wide inlet with the modern ations indicate that the basement in the hanging wall of the Laguna Salada delta plain of the Colorado River and the tidal flats of northern Gulf of California fault projects to a depth of ~3.8 km and intersects the 70° west-dipping La- (Fig. 1). Seasonal flooding of the Colorado River inundates the LagunaSalada guna Salada fault. Several faults cut the west margin of the floodplain lagoon Basin and produces intermittent estuarine conditions now rarely observed due and the hanging wall of both the Cañada David detachment and the Laguna to dams in the upper Colorado River (Cohen and Heges-Jeck, 2001). Salada fault. The largest fault is west dipping and produces ~500 m of vertical On the basis of Quaternary fault scarp along the Laguna Salada fault offset. Its location projects south of the Cañón Rojo fault, and we speculate and gravimetric and magnetic surveys of Kelm (1972), Mueller and Rockwell these two faults may correlate. Seismic facies reflect its sedimentary environ- (1991) interpreted LSB as a “pull-apart” basin controlled by the NW-oriented, ment and processes. Seismic facies 1 is high-amplitude, laterally continuous dextral-oblique Laguna Salada fault. However, structural studies in Sierra El reflectors that represent flooding and prolonged lacustrine conditions. Seis- Mayor (Siem and Gastil, 1994) and the Sierra Juárez range front (Romero- mic facies 2 is high- to low-amplitude, laterally discontinuous reflectors also Espejel, 1997) introduced the concept of rift segmentation controlled by active representing flooding conditions. Seismic facies 3 is low-amplitude, poorly low-angle normal faults and coeval strike-slip faults (Axen, 1995; Axen and contrasted continuous to discontinuous reflectors interpreted as subaerial Fletcher, 1998; Axen et al., 1999). García-Abdeslem et al. (2001) interpreted a distal fan sandstone deposits. Seismic facies 4 is high-amplitude, discontin- strong positive gradient of the Bouguer gravimetric anomaly along the east- uous, imbricated to a chaotic pattern of reflectors. We interpret facies 4 as ern margin as related to a major structural boundary of crustal rocks with density contrasts caused by the dextral oblique Laguna Salada fault and the

For permission to copy, contact Copyright *Current address: Servicios Especializados Peñoles S.A. de C.V. Torreón, Coahuila, México Chupamirtos dextral oblique fault (Fig. 2). Two-dimensional (2D) modeling of Permissions, GSA, or [email protected]. †Published posthumously gravity data (García-Abdeslem et al., 2001; Martín-Atienza 2001; Cortés-Arroyo,

GEOSPHERE | Volume 12 | Number 4 González-Escobar et al. | Subsurface geological constraints in the Laguna Salada Basin, Baja California, México Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/12/4/1283/4178207/1283.pdf 1283 by guest on 29 September 2021 Research Paper

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Imperial Imperial faul USA Figure 1. Seismotectonic map of north- 1940(7.1) ern Baja California, Mexico, and southern Valley MEXICO California, USA. The inset map shows 1979(6.5) the major tectonic features of northwest- LSF ern Mexico and the location of the study Mexicali area (i.e., the Laguna Salada Basin— t LSB). Seismic-reflection profiles (owned MGE 49 1875(6.2) 1852(6.5) 49 Valley by Petróleos Mexicanos (PEMEX) were Sierra Cucapah processed and interpreted in this study Sierra Juárez Cerro Prieto (black lines). Red lines correspond to

32°30 ′N 1892(>7.2) 32°30 ′N PENINSULAR RANG L Volcano 5 principal faults. Yellow star is the 7.2 Mw 0 A 7 G 6 “El Mayor–Cucapah Earthquake—EMC,” -a U ELS-1 April 2010. Red stars are two historical N Cerro Prieto faul A 57 CRF earthquakes located within the eastern 49 CHF basin margin: the 1892, Mw > 7 and the ELS-2 Colorado River1980(6.4) 1934, Mw 6.5 (Ellsworth, 1990). Red dots 1934(6.5) EMC-2010 (7.2) indicate epicenters of the larger historical CDD Sierra El Mayor earthquakes reported (year and magni- 65 49 tude) by Ellsworth (1990); the yellow and S CDD Indiviso fault E A t green dots are the microseismicity re- L 1915(7.1) ported by García-Abdeslem et al. (2001) for 5 A 0 D {1891(6.0) 7 a local seismic network. The boundary of 6 A - flooding is displayed with white line. The Study Area b modern course of the Colorado River ap-

ELS-3 3 DD USA 7 C pears in blue. Abbreviations: LSF—Laguna 49 1934(7.0) MEXICO Salada fault; CRF—Cañón Rojo fault; 32°N B 32°N CHF—Chupamirtos fault; CDD—Cañada NAM A Pac S 1935(5.3) David detachment; SJFZ—Sierra Juárez I N fault zone, Indiviso fault (from Fletcher Sierra 5 et al., 2014); MGE—main gulf escarpment; 0 7 Pac—Pacific plate, NAM—North Amer- Pacific 6 Las Tinajas - Ocean c ican plate; ELS1, ELS2, and ELS3 are ex-

{ ploration wells by Comisión Federal de SJFZ Sierra Electricidad. Las Pintas

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2011) and stratigraphic studies in the Cerro Colorado basin and in three ex- Our study comprises the processing and interpretation of ~115 km of seis- ploratory wells of Comisión Federal de Electricidad (CFE) (Vázquez-Hernández mic-reflection profiles (Figs. 1 and 2) and the correlation of the seismic se- et al., 1996; Dorsey and Martín-Barajas, 1999; Martín-Barajas et al., 2001) are quences with stratigraphy described in three exploratory wells of CFE (Fig. 3). consistent with a northwest-trending, strongly asymmetric depocenter with a We first present the principal structural and stratigraphic characteristics to gain maximum sedimentary fill of ~3 km adjacent to the Laguna Salada fault in the insight about the architecture and the amount of subsidence and extension northwest (Fig. 2). principally in the southern LSB. We then discuss the distribution of seismic Industry seismic data collected by Petróleos Mexicanos (PEMEX) during facies as related to distinct depositional environments within the basin. the early 1980s and three exploratory wells drilled by CFE provide a unique opportunity to further investigate the structure and stratigraphy of LSB, partic- ularly the geometry of an active detachment fault in depth (Axen et al., 1999; TECTONIC AND STRUCTURAL BACKGROUND Fletcher and Spelz, 2009). Although only a few seismic lines were collected by PEMEX in the Laguna Salada Basin (Figs. 1 and 2), the available seismic data Receiver function data (Lewis et al., 2001) and gravity data modeling provide important geometric constraints for the architecture and evolution of (García-Abdeslem et al., 2001) suggest that crust-mantle interface for the La- this active supradetachment basin (Dorsey and Martín-Barajas, 1999). guna Salada region is at ~25 km depth. This contrasts with the ~35–42 km

116°0′W USA –20 MEXICOLSF -20 –40 BF IF N N 4949 LSE-1892 (>7) –10 SJFZ L A 32°30′ Cerro Prieto 32°30′ G U N LSF Volcano Figure 2. Generalized geologic map of the 5076- Laguna Salada Basin (LSB) (modified from CPF Fletcher and Spelz, 2009). The thick black BF lines running along and across the LSB are PENINSULAR RANGE a ELS-1 –20 A the seismic-reflection profiles (owned by F S Petróleos Mexicanos [PEMEX]) processed, PF interpreted, and reported in this study. The 4957 CCB yellow stars denote epicenter location of CHF ELS-2 the three major historical earthquakes re- –60 EMC (7.2) corded in the study area (two in Laguna INDF 1934 (6.5) Salada [LSE] and the El Mayor–Cucapah –20 earthquakes [EMC]), indicating year and magnitudes. Bouguer anomaly is shown A –4 with gray contours every 10 mGals (taken 0 L A 0 CDD form García-Abdeslem et al., 2001). The 4965 exploration wells drilled by the Comisión D Federal de Electricidad (CFE) in the LSB 5076-b are depicted as green dots (ELS-1 to ELS-3). A Abbreviations: Laguna Salada fault—LSF; Borrego fault—BF; Cañón Rojo fault—CRF; Pescadores fault—PF; Chupamirtos fault— CHF; Cañada David detachment—CDD; 3 CDD ELS-3 497 Sierra Juárez fault zone—SJFZ; Cerro Colo rado basin—CCB; Cerro Prieto fault—CPF; N –60 N Imperial fault—IF; Indiviso fault—INDF. B 32°0′ 32°0′ –30 A S I –40

116°0′W 115°30′W

crustal thickness beneath the peninsular ranges (e.g., Sierra Juárez) (Lewis Historical seismicity (Ellsworth, 1990; Doser, 1994; García-Abdeslem et al., et al., 2001). However, a large negative Bouguer gravity anomaly straddles 2001; Hough and Elliot, 2004) and paleoseismological studies (Mueller and the main gulf escarpment and the central part of the LSB. This suggests Rockwell, 1991, 1995; Fletcher and Spelz, 2009; Fletcher et al., 2014) demon- either a basin depocenter above homogeneous lithology of crustal rocks or strate that LSB is controlled by the active LSF and the Cañada David detach- a deep crustal root that flexurally supports topography (García-Abdeslem ment fault. These two major faults interact with other faults and accommo- et al., 2001). date transtensional shearing in the easternmost plate boundary zone (Fletcher

ELS-3 ELS-2 ELS-1 0 0 0

200 200 200 sandstone-mudstone sparse gravel 4 400 400 sandstone-mudstone 400 Sandy-gravelly

and claystone minor gravel and Unit mudstone claystone 600 600 600

800 800 H-A 800 meters coarse sandstone sandy conglomerate and gravel Crystalline basement 1000 1000 (mica schist) with minor mudstone boulder breccia and conglomerate with 1200 coarse sandstone, minor 1200 3 metric blocks conglomerate and mudstone 49 greenish claystone 49 Unit 1400 Sierra 1400 boulder conglomerate mudstone and sandstone 5076-a Cucapah and subordinate sandstone, ELS-1 1600 mudstone, shell fragments 1600 boulder breccia and meters conglomerate with

ELS-2 Sierra El Mayor H-B 1800 metric blocks 7 Crystalline basement 495 5 496 (tonalite) 2000 reddish muddy

2 50 7 6 sandstone with -b

ELS-3 Unit calcareous matrix 4973 2200

2400 meters

Figure 3. Lithostratigraphic logs (depth in meters) from wells ELS-1 to ELS-3 modified from Martín-Barajas et al. (2001). Horizons H–A (blue dashed line) and H–B (orange dashed line) are boundaries between units 3–4 and 2–3, respectively, and will appear in subsequent figures. ELS-1 to ELS-3 are the exploration wells drilled by the Comisión Federal de Electricidad (CFE).

et al., 2016). At this latitude, the active plate boundary zone is 33 km wide lated based on the intensity distribution in southern California (Strand, 1980). between the Imperial and the Laguna Salada faults and is 26 km wide between Nevertheless, this earthquake originated due to a rupture in the LSF as indi- the Cerro Prieto fault and the Cañada David detachment (Fig. 1). Several inter- cated by fault scarps that reached up to 3–4 m of vertical scrolling and sug- vening faults accommodate the transtensional strain. gested a magnitude of 7.1 (Mueller and Rockwell, 1995). Furthermore, Hough The historical seismicity in the LSB is low in the NW sector (Fig. 1), and and Elliot (2004) reported a magnitude of 7.2 using a method based on the only two major earthquakes have been previously located within the eastern distance decay of modified Mercalli intensity (MMI) values for earthquakes in basin margin—the 1892, Mw > 7, and the 1934, Mw 6.5 (Ellsworth, 1990) (Fig. 1). western North America (Fig. 1). Uncertainty remains regarding the epicentral location and the magnitude of Microearthquake and tectonic studies within the LSB region were com- the earthquake that occurred February 23, 1892, because its location was calcu- pleted in two surveys (Fig. 1) (García-Abdeslem et al., 2001). The first survey,

which was carried out in 1991, was with five seismic stations located in the Basin-margin alluvial and marine conglomerate and breccia locally crop out in northern part of the basin recording from July 8 to December 17. In the sec- the north and northwestern foothills of Monte Blanco dome (Siem and Gastil, ond survey, these five stations were placed south, recording from April 27 to 1994; Vázquez-Hernández et al., 1996). September 17, 1992. The seismic networks registered ~582 microearthquakes The arrival of the Colorado River into the rift depression in the Lower Plio- with magnitudes lower than 3.6 chiefly concentrated along the LSF and the cene dramatically increased the sediment supply and deltaic progradation into mountain front of Sierra Juárez toward the southeast (Fig. 1). The seismic early marine basins (Martín-Barajas et al., 2001; Pacheco et al., 2006; Helenes activity reported by the seismological network of the northwestern México et al., 2009; Dorsey et al., 2011). The Imperial deposits in the Cerro Colorado (RESNOM-CICESE) before the El Mayor–Cucapah earthquake (EMC) of April basin grade upwards into reddish, quartzose non-marine siltstone-sandstone 10, 2010 (Mw 7.2) (Hauksson et al., 2010) indicates larger seismic activity in deposits of the Palm Spring Group (Vázquez-Hernández et al., 1996; Winker the study area concentrated along the western basin margin, after the seismic and Kidwell, 1996). During the Pliocene, uplifting of the mountain ranges of activity focused mainly along the eastern basin and continues in this sector Sierra Cucapah up to 700 m progressively isolated Laguna Salada from the (December, 2015). delta plain and from the southwestern Salton depression (Axen et al., 2000; The EMC earthquake had little influence in the subsidence of LSB but re- Martín-Barajas et al., 2001) (Figs. 1 and 2). This new structural configuration vealed the existence of a previously unidentified fault system in the southwest progressively formed the semi-closed basin with an entrance from the south part of the delta of the Colorado River, west of the Cerro Prieto fault, which was end connecting LSB with the delta plain and tidal flats of the northern Gulf considered the main plate boundary (Figs. 1 and 2). This earthquake produced of California. The modern lake basin is bordered by a 5–15-km-wide belt of a complex rupture that involved multiple major faults shearing the crustal coalescing alluvial fans derived from Sierra Juárez in the west and by a nar- block of Sierra Cucapah (Fletcher et al., 2014). The rupture propagated north rower 0.5–3-km-wide belt of alluvial fans fed from the Sierra Cucapah and along a complex dextral-oblique fault zone parallel to Laguna Salada fault Sierra El Mayor in the east. These two crystalline blocks are composed of Late through the Sierra Cucapah (Fletcher et al., 2014; Terán et al., 2015). Southeast, Cretaceous granitic rocks and pre-Cretaceous high-grade metamorphic rocks this earthquake and its aftershocks ruptured previously unidentified faults in (Barnard, 1968; Gastil et al., 1974; Siem and Gastil, 1994; Axen et al., 2000). Ter- the delta plain south of the epicenter and evidenced a wider plate boundary tiary volcanic rocks locally overlay the crystalline basement at the edges of both zone at this latitude (Hauksson et al., 2011; Chanes-Martínez et al., 2014). of these ranges, principally in the southeast in Sierra Las Tinajas (Fig. 2). This implies that pre-rift Miocene volcanic rocks may be present in depth within the LSB. In summary, the basin fill is predominantly composed of marine to deltaic GEOLOGICAL EVOLUTION OF THE LAGUNA SALADA BASIN fine-grained sediments funneled by the Colorado River into the northern Gulf of California and delta plain. Upward sandy deltaic deposits progressively al- Early extension and subsidence in the LSB likely started in Late Miocene ternate with locally derived, coarse-grained sandstone and conglomerate from as indicated by faulted ca. 10.5 Ma volcanic deposits across the southern local source alluvial fans from Sierra Juárez and Sierra Cucapah and El Mayor. range front of Sierra Juárez (Mendoza-Borunda et al., 1998). Crustal exten- Up to ~700 m of Late Pliocene to present lacustrine-estuarine deposits inter- sion in the Laguna Salada segment is principally accommodated by the low- fingered by progradational and retrogradational alluvial wedges represent the angle Cañada David detachment fault (CDD) synchronously with the Laguna modern structural and sedimentary setting in LSB (Figs. 1 and 2). Salada strike-slip fault system (Siem and Gastil, 1994; Axen, 1995; Axen and Fletcher, 1998). The lowermost stratigraphic unit in Laguna Salada crops out in the footwall block of the Cañon Rojo fault, a N-S–trending, high-angle nor- MAJOR FAULTS IN THE LAGUNA SALADA BASIN mal fault forming a dilatational stepover along the Laguna Salada fault (Fig. 2). The stratigraphic sequence exposed in the footwall block of the Cañon Rojo The Laguna Salada fault (LSF) and the low-angle Cañada David detach- fault constitutes the Cerro Colorado basin (Dorsey and Martín-Barajas, 1999) ment fault (CDD) control the basin architecture and subsidence and represent (Fig. 2). The late Neogene sedimentary sequence overlies in fault contact two distinct basin domains (Siem and Gastil, 1994; Axen, 1995; Mueller and Paleozoic and Mesozoic metamorphic and granitic intrusives that form the Rockwell, 1995; Axen et al., 1999; Spelz et al., 2010). The northern domain footwall block of the Cañada David detachment (Siem and Gastil, 1994). The is controlled by dextral-oblique LSF trends ~N45°W and dips 60°–75° to the lower unit is early Pliocene silty-clayey yellow-green marine mudstone (Siem SW. The Cañada David detachment fault controls the south basin domain. Its and Gastil, 1994; Vázquez-Hernández et al., 1996). This unit also includes metric low-angle (<20°) and Quaternary fault scarps display a curvilinear trace extend- to sub-metric evaporite deposits and locally derived conglomerate and brec- ing ~55–60 km along the western mountain front of Sierra El Mayor (Fig. 2) cia. Overall, the lower unit in the Cerro Colorado basin has similar lithological (Siem and Gastil, 1994; Fletcher and Spelz, 2009). Geometric analyses of the and chronostratigraphic characteristics to units of the Imperial Group in the fault scarps along the eastern basin margin suggest that the CDD acquires a southwestern Salton Trough (Winker and Kidwell, 1996; Dorsey et al., 2011). high angle within 10–16 km away from the Sierra Cucapah–El Mayor, and the

anti-listric geometry defines the location of the depocenter (Fletcher and Spelz, trial interest in the depth structure commonly filters the high-frequency signal 2009). The LSF and the CDD fault are kinematically linked by the Cañon Rojo during acquisition. Wells ELS-2 and ELS-3 reached the crystalline basement and Chupamirtos faults (Mueller and Rockwell, 1991) (Fig. 2) forming a releas- at 1.5 km and 0.75 km, respectively, whereas well ELS-1 located near the LSF ing stepover. The Cañon Rojo fault is responsible for the abandonment of the cut the 2.4 km of sediments and did not reach the basement (Fig. 3). The stra- northern synformal megamullion of the CDD and defines the position of the tigraphy in ELS wells provided stratigraphic and seismic velocity constraints modern range front (Mueller and Rockwell, 1995; Dorsey and Martín-Barajas, (Álvarez-Rosales and González-López, 1995; Martín-Barajas et al., 2001). Well 1999; Fletcher and Spelz, 2009). ELS-2 is closer to intersection of seismic lines 4957 and 5076-a, and wells LS-1 The main gulf escarpment in Sierra Juárez contains a northwest-trending and LS-3 are 10–15 km away from the nearest seismic line and provide indirect west- and east-dipping normal faults array, each with relatively small offset lithological and stratigraphic constraints (Fig. 3). (Romero-Espejel, 1997; Mendoza-Borunda et al., 1998). The Sierra Juarez fault zone is ~30 km long and ~5 km wide and produces a vertical relief of >1000 m in northern Sierra Juárez, although two E-dipping faults accommodate ~700 m RESULTS of the vertical relief and are likely antithetic faults of the west-directed detachment fault (Axen and Fletcher, 1998) (Fig. 2). Clusters of microseismicity lo- Acoustic Basement and Basin Configuration cated in the south escarpment of Sierra Juárez and Laguna Salada are likely related to the Sierra Juárez fault zone. The acoustic basement is a distinctive, laterally continuous, high-amplitude reflector in most profiles, except in sectors where the reflectors are rather chaotic and not distinguished from seismic noise (cf. south of profile 5076-b). SEISMIC-REFLECTION DATA ANALYSIS The two southernmost transversal profiles (4973 and 4965) clearly show the low-angle fault that controls the basement ramp and the wedge-shaped sedi In the present study, we have processed and interpreted multichannel mentary basin fill in the hanging wall of the CDD (Fig. 4). The acoustic base- two-dimensional (2D) seismic-reflection data collected by PEMEX in Laguna ment in the hanging wall deepens to a maximum of ~2100 m in profile 4973 Salada Basin from the “Delta del Colorado” prospect. The seismic source for and to ~2500 m in profile 4965 (Figs. 4A and 4B). No coherent reflectors are the acquisition of the seismic data was dynamite. A pattern of 1150–300–0- observed beneath the low-angle fault plane in the footwall block. 300–1150 m of shots was recorded on 48 channels; each receiver was spaced The acoustic basement beneath the hanging wall is disrupted by faults with every 50 m; record time was 6 seconds; and sample interval was 2 ms. tents to a few hundred meters of offset. The most obvious faults are observed Seismic processing at CICESE included: (1) edition of traces, (2) assignment in profile 4973 to form a ~2-km-wide sag in the acoustic basement (Fig. 4A). of geometry, (3) correction of static due to elevation, (4) direct wave attenuation, This sag is likely controlled by a west-dipping fault and at least two east-dip- (5) ground roll attenuation, (6) deconvolution, (7) frequency-wavenumber (FK) ping antithetic faults. Another prominent relief in the acoustic basement filter, (8) order of traces by common depth point (CDP), (9) velocity analysis, occurs at the northwest end of profile 5076-b (Fig. 5A). There, a major fault (10) normal moveout (NMO) correction, (11) stacking, (12) spherical divergence, produces a vertical offset of more than 500 m along a horizontal distance of (13) time-variable filter, (14) automatic gain control (AGC), (15) migration, and ~2 km (cdp ~6550–6750). Near the south end of profile 5076-a, a series of faults (16) depth conversion. Subsequently, the data were interpreted using the tech- with small vertical displacement disrupt the acoustic basement (Fig. 5B). The nique of Badley (1985). Processing and interpretation of the seismic data make depth of basement in well ELS-2 coincides with the depth of basement in pro- use of the ProMax anpd SeisWorks software of Landmark™ and OpendTectTM. file 5076-b (Fig. 5A) and likely maintains a similar depth as in line 5076-a (Fig. The analysis and interpretation were conducted in five seismic-reflection 5A). The southernmost longitudinal segment (line 5076-c) indicates a shallow profiles (Figs. 1 and 2). Profiles 4973, 4965, 4957, and 4949 have lengths of 14, <200-m-deep acoustic basement (see Supplemental Figures [see footnote 1]). 11, 9, and 7 km, respectively, and cross the LSB in a northeast to southwest The acoustic basement has a prominent vertical offset in the southern part direction (Figs. 1 and 2). Line 5076 is oriented northwest to southeast along of profile 5076-b (Fig. 5B) where the basement is clearly imaged at ~1700 m the west-central portion of the LSB, with a length of 70 km. Profile 5076 com- near cdp 6250. The vertical difference in depth is depicted at the south end of prises three segments (a, b, and c) with lengths of 27 km, 30 km, and 13 km, this profile, where coherent reflectors indicate basement at ~600–700 m (Fig. respectively, and crosses all four transversal profiles (Figs. 1 and 2). The seis- 5A). Between the two ends, basement loses its distinctive high- amplitude mic transversal profiles are separated ~20 km from each other and distributed characteristic and passes southward into a zone of chaotic reflectors (from cdp 1Supplemental Figures. Seismic profiles in two-way throughout the basin from north to south (Figs. 1 and 2). 5750–6150). Furthermore, the change in depth to basement in profile 5076-b travel time (TWTT) and their velocity models used Seismic lines were migrated in time and converted to depth using a stack- is confirmed at the west end of line 4973 where it crosses the longitudinal for depth conversion. Please visit http://dx.doi .org 1 /10.1130/GES01261 .S1 or the full-text article on www ing velocity model (see Supplemental Figures ). The seismic resolution in the seismic line 5076-b at its south end. Farther south, line 5076-c confirms that .gsapubs.org to view the Supplemental Figures. upper part of seismic lines (e.g., <300 m) is of lower quality because indus- crystalline basement is very shallow in the east flank of Sierra Las Tinajas.

4965 A 5076-b CDP Number B 3750 3700 3650 3600 3550 3500 3450 3400 3350 3300 0 0

500 500 4949

Sierra Cucapah 1000 1000 5076 -a ELS-1

1500 1500 4957 ELS-2 B Sierra Depth (m) 4965 A El Mayor 2000 2000 5076- ELS-3 b B Figure 4. (A) Profile 4965 (see inset map 0° 73 49 and Figs. 1 and 2 for location). The largest A SW-dipping fault to the right of the seis- 2500 Basement 2500 mic section is the Cañada David detachment (CDD). (B) Profile 4973. The yellow 30° line follows the acoustic basement along the seismic section. The fault located in 3000 60° Cañada Dabid DetachmentA 3000 75° VE ~2 the SW sector of the seismic profile coin- 1000 m cides with the western shore of the lagoon flood plain. The Cañada David detachment mapped in surface by various authors (Figs. 1 and 2) controls the basin architecture. For both (A) and (B), yellow line is the acoustic basement. Horizons in black are 4973 sequence interpreted boundaries. Other 5076-b CDP Number interpreted faults are red lines (see Fig. 3 A B for details and the stratigraphic log of 3700 3650 3600 3550 3500 3450 3400 3350 3300 3250 3200 3150 the ELS-1 well to the right of this figure). 0 0 Abbreviations, inset map: ELS-1 to ELS-3, well of Comisión Federal de Electricidad (CFE). 500 500

1000 1000

1500 1500 Depth (m ) Basement 2000 2000 0° Cañada David Detachment

2500 2500

30° 3000 60° 75° 1000 m VE ~2 B 3000

5076-a A 4949 CDP number 4957 B 8250 8150 8050 7950 7850 7750 7650 7550 7450 7350 7250 ELS-2 0 0

500 400

A UNI T 4 H-A 4949 S 1000 1000 50 ierra Cucapa 76 -a ELS-1 B h 0° 1500 of UNI T 3 Depth (m) Basement Lack 1600 4957 ELS-2 A Sierra information 2000 4965 2000 30° El Mayo 50 60° 76- ELS-3 b 75° VE ~4 A 2000 m 3 r 2500 2000 m 2500 497 B

5076-b 4973 A 4965 CDP Number ELS-2 6850 6750 6650 6550 6450 6350 6250 6150 6050 5950 5850 5750 B 0 0 4 400 500 UNI T

1000 H-A 1000 UNI T 3 1600 1500 Depth (m) 0°

2000 Basement 2000 30° VE ~4 60° B 2000 m 75° 2500 2500

Figure 5. (A) Profile 5076-a (see inset map for location) along the south basin domain. Well ELS-2 is located 1 km to the east of this seismic line. (B) Profile 5076-b. The sequence boundary H–A correlates with boundary between units 3 and 4 in well ELS-2. Crystalline basement at ~1500 m depth also matches the stratigraphic log of the exploratory well ELS-2. A NW-dipping normal fault offsets the acoustic basement and roughly coincides with the lagoon shoreline. As in previous figures, subhorizontal lines (blue and yellow): the H–A horizon reported by Martín-Barajas et al. (2001) and the acoustic basement, respectively. Note, again, the correspondence of these reflectors with the boundary between units 3 and 4 and the depth to the crystalline basement. The red lines are the interpreted fault. Abbreviations, inset map: ELS-1 to ELS-3, well of Comisión Federal de Electricidad (CFE).

The ~1000 m vertical difference of depth to basement in the south end of pro- The depth to basement at intersection of lines 5076-a (Fig. 5A) and 4957 file 5076-b occurs at a distance of ~10 km (Fig. 5A), but the seismic image lacks (Fig. 6A) agrees with well ELS-2, where the granitic basement was cut at the resolution to interpret any fault that may control this basement relief. For 1590 m deep (Fig. 3). Near this intersection, line 5076-a presents a series of the south basin domain, we interpret that basement is 2.5 km deep in a depo parallel, high-amplitude reflectors at depths from 1000 to 1300 m that we inter- center located 7 km west of the breakaway fault on the detachment west of pret as due to the lithological contrast in sediments (Fig. 5A). Below 1300 m, a Sierra El Mayor. ~200-m-thick interval of low-amplitude reflectors named here the “white unit,”

4957 CDP Number A 5076-a 400 350 300 250 200 150 100 50 B ELS-2 0 0 4 500

600 UNI T

1000 H-A 1000 3 UNI T

Depth (m) 1500 Basement 0° 1600

2000 2000 30° 60° A 75° 1000 m VE ~2 2500 2500 4949 A 5076-a CDP Number 4949 3650 3600 3550 3500 3450 B B A Sierr 0 0 5076- a Cucapah

a ELS-1 B A 500 500 4957 ELS-2 Sierra El Mayor

4965 0° 5076-b 100Depth (m ) 0 1000 ELS-3 Basement

4973 30° 1500 60° VE ~1 1500 75° 1000 m B

Figure 6. (A) Profile 4957 (see inset map and Figs. 1 and 2 for location) across the western end of the north basin domain. Basement ramp dips to the east as in previous figures, subhorizontal lines (blue and yellow): the H–A horizon reported by Martín-Barajas et al. (2001) and the acoustic basement, respectively. Note again the correspondence of these reflectors with the boundary between units 3 and 4 and the depth to the crystalline basement. (B) Profile 4949. Only acoustic basement and two faults are interpreted due to lower resolution of this seismic image.

overlies chaotic reflectors that we interpret as the basal conglomerate unit and center. Depocenters define lens-shaped deposits that laterally wedge out and granitic basement reported in well ELS-2 (see Fig. 3). The acoustic basement in terminate in onlap against the continuous reflectors of facies 1 below. A new the crossing of line 4957 (Fig. 6A) matches the depth to crystalline basement continuous reflector of facies 1 covers the lens-shaped deposit and defines a of well ELS-2 and supports this interpretation. Continuous high-amplitude new sequence. However, the low resolution and low number of seismic lines reflections at ~1 km deep are observed in both seismic lines. These seismic prevent a detailed interpretation and correlation of most sequence bound reflectors are cut by at least three faults dipping northwest with small vertical aries. Nevertheless, an important observation is the eastward thickening of the offset in line 5076-a; elsewhere, these reflectors are undisrupted and laterally two lowermost sequences adjacent to CDD in profile 4965, whereas the three continuous across most of the seismic image (Fig. 5B). The white unit pinches upper sequences are symmetric lenses, and the thicker intervals are located out against the acoustic basement to the northwest. 6–8 km west of the breakaway fault of the detachment (Figs. 4A and 4B). The two northern transversal profiles 4957 and 4949 (Figs. 6A and 6B) indi A 200–300-m-thick interval of low-amplitude reflectors (cf. facies 3) is lo- cate deepening of acoustic basement toward the east; from ~250 m to more cated above the crystalline basement in profile 5076-a (Fig. 5A). The south- than 1200 m in profile 4949, and to >1600 m at the eastern end of profile 4957. east end of this profile presents a series of high-amplitude and continuous Profile 4957 captures only half basin width, but depth to basement is more reflectors (facies 1) above the ~200-m-thick “white unit” interval below. The than 2.4 km farther east as indicated in well ELS-1 (Fig. 3), and thus basement acoustic basement underlies the white unit at ~1600 m as defined in well ELS-2 is likely deeper in the north basin domain near the Laguna Salada fault. (Fig. 5). Furthermore, the “white unit” is wedge shaped in seismic line 4957 and pinches out toward the northwest. Facies 4 predominates west where the basement is shallower and closer Stratigraphy and Seismic Facies to the Sierra Juárez range front. Chaotic and diffuse reflectors characterize facies 4, which laterally passes into subparallel and continuous, high-ampli- The stratigraphic units in seismic lines in Laguna Salada are interpreted on tude reflectors (facies 1 and 2). In seismic lines 4973 and 4965 (Fig. 4), facies 4 the basis of seismic facies and stratigraphic sequences limited by sequence is progradational eastward and is absent or poorly expressed in the eastern boundaries. Although seismic lines are medium to poor quality, lines 4973, side of transversal profiles, where facies 1 and 2 dominate. An independent 4965, and 4957 offer the possibility to interpret sedimentary sequences below evidence of eastward progradation of facies 4 is depicted in the northwestern 300 m depth (Figs. 4 and 6). We hereby distinguish four seismic facies. Facies 1 half of longitudinal profile 5076-b (Fig. 5A). High-amplitude continuous to dis- is characterized by a pattern of parallel high-amplitude, laterally continuous continuous reflectors (facies 1 and 2) above basement alternate at intervals reflections. Facies 2 is medium- to low-amplitude, laterally continuous imbri- tens to a few hundred meters thick. Above ~1000 m deep, seismic facies of cated to subparallel reflections. Facies 3 is low-amplitude, discontinuous wavy type 4 predominate, as well as in most of the south part of this profile (Fig. 5B). reflections (e.g., white intervals), and facies 4 is defined as discontinuous, high- to low-amplitude, imbricated to chaotic pattern of reflectors. A descrip- tion of facies is shown in the Supplemental Figures (see footnote 1). DISCUSSION We observe a systematic lateral facies change across the three transversal profiles (Figs. 4 and 6). Basin-wide continuous seismic reflections of facies 1 Geometry of the Detachment Fault at Depth distinctively represent stratigraphic sequence boundaries in the LSB. In profile 4965 (Fig. 4B), we interpret five stratigraphic sequences with basal boundaries The most important result is the direct evidence of the Cañada David defined by these continuous reflectors of facies 1. Stratigraphic sequences in- detachment fault beneath a 2–2.5-km-thick sedimentary wedge in the south clude intervals of facies 2 and 3 above sequence boundaries defined by intervals domain of LSB. Here the thickest basin fill corresponds to the site where the of continuous reflectors of facies 1. Horizon 1 is the first laterally continuous acoustic basement in the hanging wall intersects the acoustic basement in reflector across the basin. Intervals of low to medium amplitude laterally wedge the footwall block. This intersection represents the minimum amount of sub out westward and interfinger with chaotic reflectors of facies 4. Unit 1 includes sidence in LSB controlled by the detachment fault. We also estimate the mini upwards a thick interval of laterally long and continuous reflectors (sequence 1). mum displacement along the fault plane and its vertical and horizontal com- Shorter, high-amplitude continuous reflectors that lap on the west distinctively ponents (Fig. 7). From profile 4965 (Fig. 4B), the fault plane was projected to form the lower part of this interval and form local angular unconformities within the surface (dotted red line) up to the height of 285 m above sea level, which a smaller (2–3-km-wide) depocenter (Fig. 4B). Upwards, facies 1 is laterally con- is the elevation of the lower ridge in the western flank of Sierra El Mayor and tinuous both east and west and expands over a broader depocenter. corresponds to the upward projection of the CDD with an angle of dip of 160 An eastward migration of the depocenter is depicted upward in profile 4965 (Figs. 4B and 7). In this calculation, we do not consider the maximum height (Fig. 4B). Up section, the concave shape of reflectors shift toward the east, of the mountain range to the east (~700 m) or erosion in the footwall block of and sedimentary sequences thicken in the east and central parts of each depo the detachment. The minimum displacement along the fault plane is ~10.1 km,

Minimum length ~9.7 km 17° for the CDD, which is similar to the 15° dip angle of detachment in profile 4965 (Fig. 4A). NE SW The microseismic activity, according to García-Abdeslem et al. (2001), is 0.285 km 16° 0 0 located along the Laguna Salada fault, Cerro Prieto geothermal field, and the Sedimentary eastern front of Sierra Juárez, and only 17 events lie within the Laguna Salada Basin. These last events are concentrated in the northwestern basin domain, 1 deposits 1 and no correlation with faults antithetic to the CDD is observed. 2.5 km CDD Sierra El Mayor In profile 4973 (Fig. 4B), the lowermost stratigraphic unit is lenticular, and Depth (km) 2 2 seismic reflectors are parallel to the acoustic basement. The top of this lower Basement Basement 0° otal displacement ~10 km unit is an erosional unconformity underlying a narrow basin depocenter. in hangingwall block T in footwall block 30° Above the lower sequence boundary, the sedimentary sequences have a 3 60° 75° 3 quasi-symmetric synform shape (cf. from ~1300–2000 m in profile 4973). This 1 km depocenter broadens upwards and probably represents a broader zone of subsidence and/or an increase in sediment supply. We interpret that sequence 1 in Figure 7. Geologic cross section derived from the seismic profile 4965 (Fig. 4A) and geometric constraints of the basin depth and thickness of the sedimentary fill. The geologic cross section profile 4973 (Fig. 4B) was partially eroded by inflow along the estuarine chan- also shows the total displacement and minimal extension produced by the Cañada David de- nel. Sequences 3, 4, and 5 maintain their thickness across the seismic profile tachment (CDD), which dips 16° west. 4973, and seismic reflections gently dip toward the east, whereas sequence 2 is wedge shaped and nearly 1000-m-thick sediments juxtapose the CDD. The CDD likely includes synthetic and antithetic faults that merge at depth and the minimum horizontal displacement is ~9.7 km. The vertical component into the master fault. The two deeper synthetic faults are likely inactive and (e.g., subsidence) of this geometric analysis is of ~2.8 km. The 9.7 km of exten- do not propagate upwards (Fig. 4), and they do not offset a thick interval sion (e.g., horizontal displacement) represents 53% of the basin width from the of high-amplitude continuous reflectors observed at ~600 m deep. Above range front of Sierra Juárez in the west to the range front of Sierra El Mayor ~400 m, the poor resolution prevents further seismic interpretation, and the in the east and represents 24% of extension across the 40-km-wide zone of activity of faults located farther east in the sedimentary wedge is not imaged extension from Sierra El Mayor (285 m above mean sea level [amsl]) to the (cf. Fletcher and Spelz, 2009). summit of Sierra Juárez (1596 m amsl). The ~10.1 km minimum displacement The north domain is controlled by the high-angle, dextral oblique Laguna along the buried fault plane of CDD can be added to 14–18 km of extension re- Salada fault and the high-angle, dip-slip Cañón Rojo fault. Both control the ported in the lower plate of the CDD across Sierra El Mayor and Monte Blanco modern depocenter and subsidence in the northern half of the basin. The dome, respectively (Axen and Fletcher, 1998). This yields nearly 25–29 km of Laguna Salada fault is high angle (~60° to 70°) and forms a releasing stepover displacement in the CDD. in the Cañón Rojo fault (Fig. 8). Mueller and Rockwell (1991) proposed that The basal nonconformity of sediments over the acoustic basement ob- the Cañón Rojo fault is a dilatation stepover in a pull apart bounded by the served in the seismic lines constitutes, up to now, the most reliable piercing dextral oblique Chupamirtos fault. The Chupamirtos fault apparently consti- point to estimate the minimum amount of extension and the ~2.8 km of sub- tutes the hard link between the Laguna Salada and the active portion of CDD. sidence controlled by the CDD in the south domain of Laguna Salada Basin. The Chupamirtos fault bounds the Cerro Colorado basin along the west-south- A distinctive feature in the two seismic images of the CDD at depth is the west and likely intersects the CDD north of seismic profile 4965 (Fig. 8). The eastward shift of the depocenter through time. Interestingly, the two lower se- Chupamirtos fault likely represents the structural boundary between two basin quences are wedge shaped with a maximum thickness adjacent to the fault domains. South of the Chupamirtos fault, Laguna Salada Basin is an active plane (Figs. 4A and 4B). The upper three units are quasi-symmetric in shape supradetachment basin, whereas north of the Chupamirtos fault, the Cañón and thicken in the central synform. We interpret that depocenters in the two Rojo stepover produces a ~10-km-wide pull-apart basin laterally controlled by lower sequences developed closer to the fault plane likely with a higher fault the Laguna Salada and Chupamirtos faults. dip. As the detachment fault becomes low angle, the horizontal component in- Profile 5076-b (Fig. 5B) contains a west-dipping fault that produces ~500 m creases and displaces the depocenter basinward as proposed by Fletcher and vertical offset of the acoustic basement in the hanging wall of the CDD. Al- Spelz (2009). Furthermore, in the southernmost seismic image (profile 4973, though fault orientation is not defined in the seismic line, the Cañón Rojo fault Fig. 4B), the CDD has an anti-listric shape as proposed by Fletcher and Spelz projects south into the position of the largest fault in profile 5076-b (Fig. 5B). (2008) as inherent to the development of a rolling hinge during footwall uplift. The correlation of the Cañón Rojo fault and the largest fault in profile 5076-b Profile 4973 (Fig. 4B) shows an apparent steeper angle of the CDD, but the (Fig. 8) although speculative implies that the Cañón Rojo fault would have calculation of the fault dip using the same procedure as in profile 4965 yields a smaller vertical offset to the south. Near its intersection with the Laguna

Salton Trough Imperial fault USA Mexicali

MEXICO Figure 8. Structural map of the Laguna N Salada Basin (LSB) and surrounding areas. LSF The thick blue line indicates where the Cañada David detachment observed in the seismic profiles (CDDS) cut the 500 m 4949 depth in the seismic profile in map view and is interpreted to be a structure con- 5076-a Sierra Cucapa tour on the detachment at depth. Faults interpreted in the geo-seismic sections LSF Cerro Prieto faul are indicated by a yellow line with a dot in Cerro Prieto the downthrown block. These yellow lines Sierra Juárez BF Volcano (faults) roughly follow the west shoreline P F of the lake. Hypothetical correlation of ELS-1 F h Cañón Rojo fault and the fault interpreted flood plain in line 5076-b is indicated in the orange of lagoon CR LS t discontinuous line. The red lines are faults F mapped by various authors (e.g., Mueller PENINSULAR RANGE and Rockwell, 1991; Siem and Gastil, 1994; CHF Indiviso faul ELS-2 MBD Romero-Espejel, 1997; Axen et al., 1999; Dorsey and Martín-Barajas, 1999; Fletcher CDD 4957 et al., 2016). The CCD produces fault scarps EMC MBD along the west side of Sierra El Mayor Sierra El Mayo (from Axen et al., 1999; Fletcher and Spelz, t 2009; Spelz et al., 2010). The red star Sea level 4965 1 CMF denotes the epicenter of the El Mayor– 3 CD Hardy Cucapah earthquake (EMC, M 7.2). Abbre D viations: Laguna Salada fault—LSF; MBD 5076-b R Borrego fault—BF; Cañón Rojo fault—CRF; CDDS iver Pescadores fault—PF; Chupamirtos fault—

DD CHF; Monte Blanco detachment—MBD; C Central Mayor fault—CMF; red bars where ELS-3 4 the displacement was greater than 0.5 m 1 D (Fletcher et al., 2014); Comisión Federal

r de Electricidad exploratory well—ELS-1 2 CD to ELS-3. The navy-blue lines are the modern flooding channel within LSB and the 4973 Hardy River east of Sierra El Mayor. 1 Faul interpreted at 1400 m depth modern channel CDD 4 Fault interpreted at 450 m depth Sierra 2 2140 meters, deeper part of basin Las Tinajas 3 2500 meters, deeper part of basin

Salada fault, the Cañón Rojo fault has a vertical offset of ~1.3 km measured section with the Laguna Salada fault. Southward, the Cañón Rojo fault likely from the stratigraphic thickness of Plio-Pleistocene deposits, including the lo- loses vertical displacement because it transfers part of the slip into the Chupa cally derived Red Beds, the Grey Gravel units, and the Palm Spring and Im- mirtos fault. perial deposits (Dorsey and Martín-Barajas, 1999). The stratigraphic thickness The minimum depth to depocenter in the northern domain of the Laguna of the Cerro Colorado basin is a minimum of vertical offset in the Cañón Rojo Salada Basin is well ELS-1, which drilled ~2.4 km of deltaic, lacustrine-estua- fault. Additional ~700-m-thick lacustrine deposits cut in well ELS-1 suggest rine and alluvial fan sedimentary deposits (Martín-Barajas et al., 2001). This that the vertical slip of the Cañón Rojo fault may attain ~2000 m near its inter- well did not reach the Imperial marine mudstone unit inferred to lie at a depth

as recorded in stratigraphy of the Cerro Colorado basin (Vázquez-Hernández fill observed in seismic profile 4965 represents a minimum subsidence rate of et al., 1996). The 2D gravity model of García-Abdeslem et al. (2001) indicates ~0.4 mm/yr. This is 25% of the 1.5 mm/yr calculated for the Cañón Rojo fault that the basin fill adjacent to the LSF is ~3 km thick, which is a reasonable (Dorsey and Martín-Barajas, 1999) and sedimentation rates estimated at ~1.5 estimate of basement depth. An independent estimate of depth to basement mm/yr from spectral analysis of gamma ray log in well ELS-1 (Contreras et al., is the eastward projection of acoustic basement in profile 4957. This geometric 2005). These estimates are consistent with a faster subsidence in the Laguna projection suggests that basement in the hanging wall intersects the Laguna Salada and Cañón Rojo dilatational stepover compared to subsidence in the Salada fault at ~3.8 km below the surface, assuming that the Laguna Salada supradetachment basin domain controlled by the Cañada David detachment. fault maintains a ~70° dip to the west (Fig. 9). This amount of subsidence Mio-Pliocene detachment faults and coeval strike-slip faults occur south would be ~3.5 km, if LSF dips 60° to the west. For these estimates, we infer that of Laguna Salada in Sierra San Felipe (Bryant, 1986; Seiler et al., 2010) and the the acoustic basement is a flat ramp that deepens at an angle of ~20° similar Altar basin in northwestern Sonora (Pacheco et al., 2006, González-Escobar to average basement dip in profile 4957 (Fig. 6A). This calculation suggests a et al., 2013). Offshore, the Angel de la Guarda detachment coexisted during somewhat deeper depocenter as proposed by the 2D gravity modeling. dextral shearing in the Tiburon and Amado dextral oblique faults that con- The contrast in structural style and the amount of subsidence among the trolled the early separation of the Baja California peninsula from mainland north basin domain controlled by the dextral oblique Laguna Salada fault México (Martín-Barajas et al., 2013). Elsewhere, examples of concurrent strike- and the south basin domain controlled by the CDD fault requires a structural slip faults and low-angle normal faults are reported in Mormon Mountains– boundary likely in the Chupamirtos fault (Mueller and Rockwell, 1991). We pro- Tule Springs Hills, Nevada (Wernicke, 1995), Panamint Valley in California pose that the Chupamirtos fault separates the active supradetachment basin (Wernicke, 1995; Numelin et al., 2007; Haines et al., 2014; among others). It domain in the south from the pull-apart basin domain controlled by the Cañón seems that detachment faults and coeval strike-slip faults constitute a com- Rojo and Laguna Salada faults. mon and efficient way to partition oblique strain in the northern Gulf of Califor- Several faults cut the basement along the west side of LSB (Figs. 4–6), and nia (Axen and Fletcher, 1998). Laguna Salada is unique among these examples only the principal fault in this sector in each of these profiles is presented in because it is the only documented site of coeval active deformation. It is possi- Figure 8 (yellow mark). We interpret that these faults are a clear expression of ble that detachment faults initiated during the early phase of transtension and several synthetic and antithetic faults cutting the hanging wall of the detach- produced a broader supradetachment depocenter that was subsequently over- ment and probably accommodating significant amounts of basin subsidence. printed by the Laguna Salada fault. The Cañón Rojo and Chupamirtos faults Curiously, these faults roughly follow the west shoreline of the lake, and Figure produced the abandonment of the Cerro Colorado synformal domain of the 8 shows the direction of apparent dip in each of these faults in the seismic sec- Cañada David detachment and reduced in ~25% its original length (Siem and tions as indicated by yellow marks of apparent strike and dip. Gastil, 1994; Fletcher and Spelz, 2008). The CDD and Laguna Salada faults are, thus, a common example of coexistence of two fundamental modes of deformation and strain partition in the northern Gulf of California rift. Magnitude of Extension and Subsidence

The two seismic images of the supradetachment basin domain capture the Seismic Facies and Sedimentary Sequences hanging-wall basement ramp that subsided ~2.7 km below sea level, whereas the detachment fault has accumulated a minimum of ~10 km of finite extension. Transversal profiles in southern Laguna Salada Basin domain show that The amount of subsidence is also a minimum because mechanical compaction seismic facies 4 dominate the western portion of the seismic images, and facies reduces the original porosity and sedimentary thickness and underestimates 1 and 2 dominate the eastern part, where depocenters define the thicker sedi- the original volume of sedimentary deposits and the amount of subsidence mentary fill. We interpret that facies 4 is produced by anastomosing channels (Giles, 1997). Nevertheless, a crude estimate of the rates of extension and verti and bars of alluvial fan deposits from Sierra Juárez (Figs. 4 and 6). Profiles 4965 cal subsidence suggests a ratio of 3:1, respectively. The lower sedimentary (Fig. 4A) and 4957 (Fig. 6B) clearly show that laterally continuous reflectors unit reported in Laguna Salada is the Imperial mudstone unit, which may cor- (seismic facies 1 and 2) penetrate westward and interfinger with facies 3 and relate to either the Latrania Formation dated 5.2–6.1 Ma or to the lower part of 4 produced by alluvial fan deposits. In profile 4957, the modern lakeshore is Deguyinos Formation (5.1–4.2 Ma) (Dorsey et al., 2011) based on similar distinc- ~7 km from the mountain front, and lacustrine facies 1 and 2 are found beneath tive lithology and paleodepths (Vázquez-Hernández et al., 1996). Although this the modern distal fan where eolian, alluvial, and lacustrine deposits interfinger. unit overlies crystalline basement in fault contact, and a slip in the detachment Due to their lateral continuity, facies 1 and 2 are interpreted to represent flood- may have started synchronously with marine deposition. If we conservatively ing and prolonged lacustrine conditions produced by the Colorado River enter- assign 7 Ma for the onset of extension, the 9.7 km of horizontal slip estimated ing Laguna Salada Basin. We speculate that the prolonged lake condition must in the CDD represents an extensional rate of ~1.4 mm/yr. The 2.8 km of basin have occurred during major sea level highstands, similar to the present time.

SEISMIC REFLECTION PATTERN USA A SEMI PARALLELAND IF A′ MEXICO Mexicali GOOD LATERAL CONTINUITY LS 380 m F 5076-b Modern Channel 4949 Sierra Cucapah 250 m CDD LSF 0.3 5076-a 0.3 ALLUV IAN F ALLUVIAN FACIES C′ C 0 Sierra AN Sierra ELS-1 P RF F C

Las Tinajas 4957 C El Mayor HF D ELS-2 D S hanging wall PENINSULAR RANG C IN –1.0 C B′ier D 5 ra F 496 CDD

El 9 km 5076-b basement –2.0 M A′ ayor

B C 4973 DD Cañada David fault ELS-3 –3.0 A E

5 076- 5 10 15 20 25 Sierra c SEISMIC PROFILE 4973 Las Tinajas

Figure 9. Geologic cross sections derived B B′ from the seismic profiles: 4973 (A–A′), 1519 m 4965 (B–B′), and 4957 (C–C′). We inter- 1.5 SEISMIC REFLECTION PATTERN pret the geometry of the basin at depth ) 1.0 ELS-3 (TD 830 m) SEMI PARALLELAND and the thickness of the sedimentary fill, m GOOD LATERAL CONTINUITY Modern Channel with seismic-reflection pattern semi-par-

k 9 Km south 400 m 227 m allel and good lateral continuity, defining Sierra Juárez CDD ( Sierra Juárez 0 ALLUVIAN FAN stratigraphic sequences representing e basement at 0.75 km ALLUVIAN FACIES major flooding periods and lithological d Sierra constraints from the correlation of wells –1.0 in the well H-A u hanging wall (ELS-1 to ELS-3). In profile C, the projec-

t El Mayor i

t H-B tion of acoustic basement to intersect l basement –2.0 the Laguna Salada fault yields a depth

A ada David fault ñ to basement of ~3800 m assuming the Ca –3.0 fault dips 70° west. CDD—Cañada David detachment; TD—total depth. The hori 5 10 15 20 25 30 35 zontal and vertical units are km, for all profiles. Abbreviations, inset map: Laguna SEISMIC PROFILE 4965 Salada fault—LSF; Cañón Rojo fault—CRF; Pescadores fault—PF; Chupamirtos fault— C C′ CHF; Imperial fault—IF; Cerro Prieto fault— 1536 m CPF; Indiviso fault—INDF (from Fletcher 1.5 et al., 2014). 1.0 ELS-2 (TD 1700 m) ELS-1 (2400 m) 730 m well ends in basement well ends in sediments 4km Sierra 0 Sierra Juárez t Cucapah –1.0 basement at 1.59 km hanging wall in the well 0 –2.0

Dip 70 –3.0 basement SEISMIC PROFILE 4957 –4.0 Laguna Salada Faul –5.0 0 5 10 15 20 25 30 40 Distance (km) 35

This condition reduces the length and topographic relief for the fluvial runoff sponds to a change in seismic facies from low-amplitude, poorly contrasted in the delta plain, and fluvial discharges reach supratidal flats and the delta seismic reflections of facies F3 to high-amplitude and continuous reflectors front closer to the southern inlet that connects Laguna Salada and the northern of facies F1. Horizon B–A likely corresponds to the boundary between units 3 Gulf of California. In contrast, during lowstands (>100 m), the delta front shifts and 4 in well ELS-2 (Martín-Barajas et al., 2001). south, and a large portion of the submarine delta (pro-delta) is exposed. Fluvial The south part of longitudinal profile 5076-b (Fig. 5B) shows nearly 11 km of channels acquire a steeper profile, preventing Laguna Salada to retain water chaotic reflectors (facies F4) that we also interpret as alluvial fan deposits close during prolonged periods of time. During lowstand sea level, Laguna Salada to the north end of Sierra Las Tinajas (Fig. 4B). Modern alluvial fans progradate is intermittently dry, and playa-lake deposits must enhance progradation of and narrow the flood plain and channel in the southernmost part of the basin. alluvial fan and eolian deposits into the basin floor. These conditions might However, interpretation of seismic sequences and facies distribution is lim- be similar to the modern situation in LSB produced by damming the Colorado ited due to low resolution and low number of seismic lines and is beyond the River since the early part of the twentieth century. In 1984, flooding in the delta scope of this paper. Nevertheless, it is clear that the sedimentary record in LSB occurred due to the release of excess water in the river dam system. The event responds to both tectonic and climatic controls and constitutes an important produced estuarine conditions in Laguna Salada for nearly five years and then archive yet to be explored in detail. dried out by ca. 1990 (Cohen and Henges-Jeck, 2001). Two main processes likely cause the broad belt of alluvial fan deposits along the western margin. One is the strong asymmetric subsidence that CONCLUSIONS maintains depocenters along the eastern margin near the Laguna and Cañón Rojo faults. The other is a higher coarse-grained sediment input due to higher The Laguna Salada Basin is an active asymmetric depression structurally topography and higher runoff in the range front of Sierra Juárez. Lacustrine controlled by the Laguna Salada fault and the Cañada David detachment fault. deposits likely prevail for a longer time along the eastern basin margin due to The processing and interpretation of five seismic profiles indicate that these higher subsidence rates and intermittent flooding of the Colorado River and two master faults define two distinctive basin domains. The south domain is locally from the Sierra Juarez mountain range. During flooding events, mostly a supradetachment basin controlled by the Cañada David detachment fault. silt and clay are transported in suspension into the lake basin and continu- Two seismic profiles indicate the detachment fault dips 16°–20° west and has ously accumulate over larger areas in the Laguna Salada. Climatic forcing and a minimum of 10.1 km of total slip. The supradetachment basin domain accu- changes in sea level likely control the shift from estuarine conditions (flooding) mulates a sedimentary wedge more than 2.5 km thick in the west-central part to hyper-arid playa lake conditions (e.g., sea level lowstand) in Laguna Salada of the basin, and the subsurface portion of the Cañada David detachment rep- (Contreras et al., 2005). resents 24% of extension in the western main plate boundary zone. The north The older, deeper, and narrower depocenters depicted in seismic lines 4965 domain is a pull apart controlled by the northwest-trending, west-dipping, and 4973 (Figs. 4A and 4B) show erosional features probably related to lateral dextral-oblique Laguna Salada fault. The pull-apart forms a dilatational step shifts of estuarine channels during flooding. The most important erosional over with the north-south–trending, dip-slip Cañón Rojo fault, which defines feature is observed in profile 4973. The lower lens shape sequence is ~4 km the southern boundary of the pull-apart basin domain. The Cañón Rojo fault wide and ~500 m thick, and the seismic reflections are parallel to the acoustic accumulates more than 2 km of subsidence, but seismic profiles in the north basement (Fig. 4). Sequences 2 and 3 unconformably overlie the lower len- domain indicate the acoustic basement is an east-dipping ramp in the hanging ticular sequence and have a strong asymmetric thickness controlled by the wall of the Laguna Salada fault. Geometric considerations indicate the base- detachment fault (Fig. 4). In both 4965 and 4973 profiles, the upper sedimen- ment in the hanging wall projects to a depth of ~3.8 km to 3.5 km and intersects tary sequences have a broader distribution and uniform thickness across the the west-dipping Laguna Salada fault. This estimate assumes that the Laguna basin, although slightly thicker to the east. We interpret the eastward shift of Salada fault maintains its surface dipping angle of 60° to 70° west, respectively. the depocenter as related to the widening of the basin and to a lower angle in We recognize four seismic facies representing the dominant sedimentary the detachment fault. environments. Facies 1 and 2 are high-amplitude, laterally continuous reflec- The low-amplitude wavy reflectors of seismic facies 3 are commonly ob- tors that represent flooding and prolonged lacustrine conditions. Facies 3 is served above facies F1 to lateral interfingering on intervals of facies F2 and low-amplitude, poorly contrasted continuous to discontinuous reflectors inter- F1. This seismic facies implies a small lithological contrast among strata and preted as distal alluvial fan sandstone deposits, whereas facies 4 is high-ampli- probably represents sandstone-siltstone facies, as indicated in profile 4957 lo- tude, discontinuous, imbricated to a chaotic pattern of reflectors. We interpret cated 1.7 km to the south of well ELS-2 (Fig. 6). The white interval matches facies 4 as high-energy, alluvial-fan coarse-grained deposits prograding over unit 3 in well ELS-2 and consists of an ~200-m-thick sandstone that underlies the basin floor from the west in the range front of Sierra Juarez. Seismic facies a thick interval of mudstone with subordinate siltstone and sandstone (unit 4) 1 and 2 predominate in the east and central portions of seismic profiles where (Martín-Barajas et al., 2001). This lithological change in well ELS-2 also corre- the depocenter accumulates thick, fine-grained sedimentary sequences.

ACKNOWLEDGMENTS Fletcher, J.M., and Spelz, R.M., 2009, Patterns of Quaternary deformation and rupture propaga- tion associated with an active low-angle normal fault, Laguna Salada, México: Evidence of a We want to thank Consejo Nacional de Ciencia y Tecnología (the National Council of Science and rolling hinge?: Geosphere, v. 5, p. 385–407, doi:10.1130/GES00206.1. Technology [CONACYT]), México, for financial support, to PEMEX Exploration and Production for Fletcher, J.M., Teran, O.J., Rockwell, T.K., Oskin, M.E., Hudnut, K.W., Mueller, K.J., Spetz, R.M., allowing the use of seismic data and Halliburton/Landmark, OpendTect, and Google Earth Pro Akciz, S.O., Masana, E., Faneros, G., Fielding, E.J., Leprince, S., Morelan, A.E., Stock, J., for the use of their software through the University Grant Program to Centro de Investigación Lynch, D.K., Elliott, A.J., Gold, P., Liu-Zeng, J., Gonzáles-Ortega, A., Hiojosa-Corona, A., and Científica y de Educación Superior de Ensenada (CICESE). We thank Sergio Arregui for technical Gonzáles-García, J.G., 2014, Assembly of a large earthquake from a complex fault system: support, Martín Pacheco and Ramón Mendoza-Borunda for fruitful discussion on interpretation. Surface rupture kinematics of the 4 April 2010 El Mayor–Cucapah (Mexico) Mw 7.2 earth- Constructive comments and suggestions by reviewer Dr. Gary Axen improved this manuscript. quake: Geosphere, v. 10, no. 4, p. 797–827, doi:10.1130 /GES00933 .1. Fletcher, J.M., Oskin, M.E., and Teran, O.J., 2016, The role of a keystone fault in triggering the complex El Mayor–Cucapah earthquake rupture: Nature Geoscience, v. 3, p. 303–307, doi:10 REFERENCES CITED .1038/ngeo2660. 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