Structure of the rift basins in the central Gulf of California: Kinematic implications for oblique rifting

We use structural and seismostratigraphic interpretation of multichannel seismic reflection data to understand the structure and kinematic history of the central Gulf of California. Our analysis reveals that oblique strain in the central Gulf formed two tectono–sedimentary domains during distinct deformation stages. The eastern domain, offshore Sonora, is bounded by the East and West Pedro Nolasco faults that may constitute the southernmost segments of the Tiburón Fault System. Within this domain, the dip-slip Yaqui Fault controlled deposition of 3.9 km of sediments in the half-graben Yaqui Basin. The western domain, offshore Baja California, is bounded by the Guaymas Transform Fault, which controlled the accumulation of 1.45 km of sediments within a half-graben that formed the early Guaymas Basin. The tectono–sedimentary activity offshore Sonoran likely ranges from Late Miocene–Pliocene to Late Pliocene time, while activity in the Guaymas Basin commenced in Late Pliocene time. Extinction of the main faults offshore Sonora was nearly coeval to the initiation of the Guaymas Transform Fault. Our results suggest that oblique strain has been accommodated by strain partition since the onset of rifting in the central Gulf. The Guaymas Basin is now a nascent spreading center, but prior to this, it evolved as a half-graben controlled by the Guaymas Transform Fault; such drastic transition is not constrained, but likely occurred during the Pleistocene time and must be localized < 30 km north of the axial troughs. The faults within the central Gulf transpose the Miocene N–S oriented grabens of Basin and Range style preserved onshore in the conjugate rifted margins.     

Isla San Pedro Nolasco as a Late Miocene intrusive record at the eastern margin of the Gulf of California: Insights from geological,geochemical and geochronological studies

Isla San Pedro Nolasco (ISPN) is a structural high bounded by inactive dextral oblique-slip faults in the east-central part of the Gulf of California rift zone and is composed of intrusive rocks not exposed on other Gulf of California islands. Here we present the reconnaissance results from geological mapping, as well as first geochemical and geochronological data for the ISPN intrusive complex. The intrusive rocks compose a sheet-like body of intermediate and felsic composition intruded by an intermediate and acidic dike swarm. All intrusive rocks (host and dikes) range in age from ca. 9 Ma to 10 Ma (⁴⁰Ar/³⁹Ar) and show a hydrous ferromagnesian mineral association (amphibole and biotite) with a calc-alkalic and transitional affinity. This hydrated mineralogical association has not been recognized in the coeval rocks along the onshore western margin of the North American plate (coastal Sonora). However, such hydrous mineralogical association is found in the coeval rift transitional volcanic rocks from the Baja California Microplate at Santa Rosalía and Bahía de Los Ángeles – Bahía de Las Ánimas. The ISPN continental block, at least 40 km long, has been pulled apart by transtensional faulting of the late Miocene Gulf of California shear zone before the westward migration of the North America-Pacific plate boundary at ca. 3–2 Ma. Eventually, ISPN became isolated as an island during the late Miocene flooding of the Gulf of California seaway.     

Iron,manganese and trace metal concentrations in seaweeds from the central west coast of the Gulf of California

Concentrations of Co, Cu, Fe, Mn, Ni, Pb and Zn in four macroalgae species (Ulva lactuca, Chondracanthus squarrulosus, Sargassum sinicola and Gracilariopsis lemaneiformis) were obtained for the first time from the central part of the west coast of the Gulf of California. Generally, no differences in metal concentrations were found among the different seaweed species, although spatial differences were apparent. Iron, Mn and Cu exhibited higher concentrations at the stations located in front of Angel de la Guarda Island, probably because of high vertical mixing processes present in the zone. The results were compared with dissolved metal concentrations reported for the Gulf of California (Cd, Mn and Fe) and the North Pacific Ocean. The resulting linear regression of the results vs. North Pacific Ocean concentrations indicated that the levels of Cu, Ni and Zn measured in this study were within its 95% confidence level. Furthermore, this comparison was capable of detecting dissolved Fe and Mn enrichments in Gulf of California waters relative to the North Pacific Ocean concentrations. Calculations of total masses of metals associated with algal biomass on the west coast of the Gulf of California indicated that the lowest masses were represented by Cu (108 ± 25 kg) and Ni (184 ± 52 kg), whereas Pb (1.1 ± 0.6 ton) and Fe (10.9 ± 8.5 ton) were the elements with the highest associated masses.     

Neogene sedimentary evolution of Baja California in relation to regional tectonics

During the Neogene, the tectonic and sedimentary evolution of the Baja California Peninsula followed four stages: (1) during the early Miocene (22 Ma), the initiation of transform motion between Pacific and North American plates, caused a rapid subsidence in the Continental Borderland Province and in some adjacent areas.This subsidence coincided in time with with a global rise in sea level. At this time, the eastern and southern parts of the peninsula did not show any evidence of subsidence. (2) During the middle Miocene (12 Ma), normal and strike slip faulting migrated eastward, causing subsidence in the northern part of the Gulf of California, where the oldest Tertiary marine sedimentary rocks were deposited. The areas in central Baja California Sur and the central part of the Gulf itself received abundant volcanic deposits related to continental extension. (3) During the late Miocene (8 Ma), the western margin of the Peninsula changed to a slightly compressive regime, while the northern part of the Gulf contained a marine basin with upper bathyal environments. The central area of the Gulf continued receiving abundant volcanic deposits, while the Los Cabos block received marine sedimentation, correlatable with sedimentary units reported from the continental margins in Nayarit, Jalisco and Michoacán. (4) Beginning in the early Pliocene (5 Ma), the present configuration of the Gulf of California developed through right-lateral strike slip and extension in the Gulf itself. Since Pliocene times, the Gulf presents widespread marine sedimentation with deep basins reaching lower bathyal depths.     

Consag Basin: northern Gulf of California,evidence of generation of new crust,based on seismic reflection data

The Gulf of California is an excellent example of how new ocean basins form. Tectonically, the northern Gulf of California is an incipient ocean basin and studies on it have defined acoustic basement and reveal the presence of new oceanic crust and intrusive bodies. Some recent studies report fundamental differences between the basins of the northern and southern Gulf of California: that the latter have well-developed oceanic crust beneath a thin cover of sediments, whereas the northern basins show proto-ocean basins, which may reflect thermal insulation of the thick sedimentary cover, the presence of low-angle faults, and more diffuse and distributed deformation. During the 1970s, Petróleos Mexicanos (PEMEX) undertook a 2D seismic reflection survey in the northern Gulf of California, over many active rift basins, including the Consag Basin. Through the processing and interpretation of these data, we describe the structural characteristics of the Consag Basin beyond 2 km depths. Using seismic reflection data, we identified an intrusion in the central part of this basin that may represent new oceanic crust buried by more than 4 km of sediments.     

Tectonic and liquefaction structures in the Loreto basin,Baja California (Mexico) : synsedimentary deformation along a fossil fault plane

Abstract

Complex fault assemblages associated to liquefaction structures have been analyzed in a Pliocene basin located along the Gulf of California. The studied outcrop shows a fossil fault plane formed ill soft flat lying sediments. The liquefaction and fluidification structures have been recognized in voleaniclastic layers deposited ill a lagoonal environment and are potentially related to seismic wave shaking and to successive dewatering along fractures. This strati-graphic record is explained by the progressive development of a seismic fault zone, related to the transtensional regime still active in the Gulf. The present analysis can he considered as an useful case study for the reconnaissance of the different types of structures formed during synsedimentary deformation in hydroplastic conditions.     

Holocene laminated sediments from the southern Gulf of California: geochemical,mineral magnetic and microfossil study

Geochemical, magnetic mineral analyses and microfossils (radiolarians) are used to characterise a Holocene laminated sequence from the Alfonso Basin, northern Bay of La Paz, southern Gulf of California. In most cases, dark and light laminae show similar bulk chemical compositions and magnetic mineral properties. However, dark laminae contain relatively more terrigenous elements (Al, Si, Fe, K and Mg) supporting a pluvial origin related to the volcanic siliceous tuffs around the Bay of La Paz. In light laminae, calcium content is higher, representing biogenic input. The magnetic signal is dominated by low coercivity, fine‐grained low‐titanium titanomagnetites and magnetite. Hysteresis ratio parameters in domain state plots show that samples fall in the pseudo‐single domain field, indicating mixtures of single‐ and multi‐domain particles. Variations in hysteresis saturation magnetisation parameters for dark and light laminae reflect changes in relative mineral concentration. Results indicate a dominant volcanic source for magnetic minerals into the basin, associated with variable pluvial and/or aeolian transport. Low radiolarian abundance in dark laminae seems to be associated with larger penetration and longer residence time of oligotrophic subtropical waters into the bay that occurred during El Niño–Southern Oscillation (ENSO) events. Light laminae are the result of episodic pulses of high productivity. We suggest that dark/light laminae result from oceanographic and climatic cyclic processes, with main terrigenous input variation possibly associated with ENSO forcing of pluvial influx, and/or latitudinal migration of the Intertropical Convergence Zone. The laminated sequence of the Alfonso Basin formed a non‐annual depositional system, in contrast to the varved sediments characteristic of the central Gulf of California basins. Copyright © 2010 John Wiley & Sons, Ltd.     

Influence of friction on erosion and accretion processes in the Yavaros Bay,Gulf of California

A two-dimensional, vertically integrated, nonlinear numerical model was applied to investigate the tide-driven bed load transport of sediments and morphodynamics in the shallow coastal lagoon of Yavaros, located in the southeastern part of the Gulf of California, Mexico. Satellite imagery exposes strong sediment dynamics in this coastal region. The dynamics in the lagoon were forced by 13 tidal constituents at the open boundary. Tides are of a mixed character and they are predominantly semidiurnal. The calculations showed areas of intense tidal currents and considerable water exchange with the Gulf of California. Numerical experiments revealed an ebb-dominant tidal distortion and a net export of sediment from the lagoon to the Gulf of California. A simulation of 20 years showed that the lagoon exported about 1,600 m³ of sediment; however, the daily oscillating exchange of sediment reached values of around 8 m³. The daily averaged flux of export–import sediments oscillates principally with semiannual, monthly and fortnightly periods. By applying a threshold velocity, a variable friction coefficient and the calculated amplitude of tidal velocities, it was possible to determine that morphological changes occur in zones of sharp topographic gradients and to explain the effect of friction on the export–import process of sediments. A 10-year simulation revealed that accumulation of sediment (~20 cm) occurred in small areas, whereas erosion occurred in larger areas but with less intensity (~8 cm). Besides the importance for the morphodynamics, these kinds of erosion–accretion processes may be relevant for the marine ecology.     

Transtensional folding

Strain modeling shows that folds can form in transtension, particularly in simple shear-dominated transtension. Folds that develop in transtension do not rotate toward the shear zone boundary, as they do in transpression; instead they rotate toward the divergence vector, a useful feature for determining past relative plate motions. Transtension folds can only accumulate a fixed amount of horizontal shortening and tightness that are prescribed by the angle of oblique divergence, regardless of finite strain. Hinge-parallel stretching of transtensional folds always exceeds hinge-perpendicular shortening, causing constrictional fabrics and hinge-parallel boudinage to develop.These theoretical results are applied to structures that developed during oblique continental rifting in the upper crust (seismic/brittle) and the ductile crust. Examples include (1) oblique opening of the Gulf of California, where folds and normal faults developed simultaneously in syn-divergence basins; (2) incipient continental break-up in the Eastern California-Walker Lane shear zone, where earthquake focal mechanisms reflect bulk constrictional strain; and (3) exhumation of the ultrahigh-pressure terrain in SW Norway in which transtensional folds and large magnitude stretching developed in the footwall of detachment shear zones, consistent with constrictional strain. More generally, folds may be misinterpreted as indicating convergence when they can form readily in oblique divergence.     

Late Holocene lacustrine chronology and archaeology of ancient Lake Cahuilla, California

Freshwater lakes existed intermittently in the Salton Trough of southern California during the late Holocene. The lakes formed north of the subaerial Colorado River Delta whenever the Colorado River flowed west into the trough instead of south to the Gulf of California. Water filled the trough to a maximum altitude of 12 m. Stratigraphy, radiocarbon dates, and supplementary evidence document four lacustral intervals of Lake Cahuilla between A.D. 700 and 1580. Archaeological sites are associated with the 12-m shoreline and their occupation correlates with these lacustral intervals.     

Geophysical evidence for Holocene lake-level change in southern California (Dry Lake)

Bird, B. W., Kirby, M. E., Howat, I. M. & Tulaczyk, S. 2009: Geophysical evidence for Holocene lake-level change in southern California (Dry Lake). Boreas , 10.1111/j.1502-3885.2009.00114.x. ISSN 0300-9483.
Ground penetrating radar (GPR) data are used in combination with previously published sediment cores to develop a Holocene history of basin sedimentation in a small, alpine lake in southern California (Dry Lake). The GPR data identify three depositional sequences spanning the past 9000 calendar years before present (cal. yr BP). Sequence I represents the first phase of an early Holocene highstand. A regression between <8320 and >8120 cal. yr BP separates Sequence I from Sequence II, perhaps associated with the 8200 cal. yr BP cold event. Sequence II represents the second phase of the early-to-mid Holocene highstand. Sequence IIIa represents a permanent shift to predominantly low lake stands beginning ∼5550 cal. yr BP. This mid-Holocene shift was accompanied by a dramatic decrease in sedimentation rate as well as a contraction of the basin's area of sedimentation. By ∼1860 cal. yr BP (Sequence IIIb), the lake was restricted to the modern, central basin. Taken together, the GPR and core data indicate a wet early Holocene followed by a long-term Holocene drying trend. The similarity in ages of the early Holocene highstand across the greater southern California region suggests a common external forcing – perhaps modulation of early Holocene storm activity by insolation. However, regional lake level records are less congruous following the initial early Holocene highstand, which may indicate a change in the spatial domain of climate forcing(s) throughout the Holocene in western North America.     

Climate forcing,primary production and the distribution of Holocene biogenic sediments in the Gulf of California

The Gulf of California is a marginal seaway under the influence of a monsoon climate that produces cool, dry winters and warm, humid summers. Winds, tidal mixing and coastal-trapped waves forced by climate and the Pacific Ocean control nutrient advection and primary productivity (PP). Strong northwest winds from the subtropical East Pacific High Pressure system begin in November and last until April and drive coastal upwelling along the mainland margin, especially in the central and southern Gulf. In the northern Gulf, particularly around the midrift island, tidal mixing and turbulence occurs year round, advecting nutrients into the mixed layer and high productivity. During summer and early fall months, winds are variable, of less intensity and mainly blow cross-basin except in the most northern Gulf. Summer PP is generally low in the central and southern Gulf except along the mainland where coastal-trapped waves associated with tropical surges and hurricanes generate mixing over the continental shelf. Mesoscale eddies or gyres often associated with jets and filaments extend to depths of 1000 m and transport nutrient-enriched upwelled waters and plankton detritus across the Gulf. The largest and most persistent gyres rotate in an anti-cyclonic direction (east to west) and are a principal source of the plankton export to the peninsula margin.Two major biogenic sediment patterns are present in core-top sediments. Hemipelagic biosiliceous-rich muds are accumulating beneath upwelling areas of high productivity in the central Gulf and along the mainland margin. Calcium carbonate- and organic carbon-rich (OC) sediments are concentrated along the peninsula margin, generally beneath lower productivity waters with the highest OC content in areas with the lowest productivity. The high, uniform biosiliceous content in Guaymas basin, extending southward into Carmen basin reflects the redistribution by mesoscale gyres of phytoplankon debris produced in mainland coastal upwelling and tidally forced areas around the midrift islands.Holocene biogenic patterns are similar to the present day with the major difference in rates of accumulation. Phytoplankton production prior to about 8200 yr BP was significantly higher in the central and southern Gulf, decreased though the mid-Holocene and has been reasonably steady for the past 2500 yr. The strong north–south and east–west gradients in present-day phytoplankton productivity patterns are also reflected in the Holocene sediment record. A series of depositional cycles occur in the biogenic record with the strongest peaks of variability at about 150 (144±18), 200 (198±5) and 350 (350±40) yrs. Longer periodicities are present prior to 3200 yr BP but the 350 yr cycle dominates in the late Holocene where it is best expressed as productivity/dissolution cycles in the carbonate record.     

Tectonic framework of Tiburon Basin,Gulf of California,from seismic reflection evidence

Tiburon Basin is characterized by a thick sedimentary fill that records the evolution of one of the rift segments of the East Pacific Rise. Its structure corresponds to an echelon pull-apart basin bounded by two dextral-oblique faults. Unlike basins in the southern Gulf of California that are underlain by oceanic crust, rift basins in the northern Gulf of California contain sedimentary thickness (up to 6 km) that masks the structure of the crust. To study the architecture of the Tiburon Basin, two-dimensional, multichannel seismic reflection data collected by Petróleos Mexicanos (PEMEX) in the early 1980s were used. The data base is a grid of lines, 5–20 km apart, with 6 s of record in 48 channels. Additional seismic data of the Ulloa 99 project were also interpreted. Our results indicate that the general structural pattern of the Tiburon Basin is controlled by two dextral-oblique faults: De Mar and Tiburon. De Mar lies to the east and ends in elevated basement transferring the stress to the Desemboque fault. The latter borders the incoming basement from the Sonora and Tiburon faults to the west, ending to the north in an antiform. Four structural domains are recognized: (1) the northern Tiburon domain is a high basement that divides the Delfin Basin to the northeast and exhibits extensional folds with their axes parallel to the basement and its flanks; (2) the Libertad domain is a sheared basement high along the margin of Sonora and forms the right step of the Tepoca Basin to the north; (3) the Tiburon central domain defines a broad sag cut by a dense NE-striking pattern of normal faults with opposed dips in the depocentre and abruptly ends to the west against the Tiburon fault; and (4) the southern Tiburon domain forms a basement ramp offshore Isla Tiburon and is controlled by a pattern of NNE-striking normal faults on the south that likely connect at an oblique angle (?60°) to the De Mar fault. We propose a rhombochasm basin model with more than 6 s of sedimentary record in the depocentre, in which the basement is not recorded. The NW-trending faults in the Libertad domain possibly continue towards the Sonora coastal plain. The principal NW-trending dextral faults and the secondary NNE-striking pattern of normal faults cut the shallow strata of this domain.     

Sand supply to the Algodones dunefield, south-eastern California, USA

In an effort to gain a better understanding of the relationship between climate, sand supply and aeolian sand deposition, an investigation was undertaken in the Algodones dunefield of south-eastern California to determine: (1) the source of the sand and (2) the mode of sand supply to the Algodones dunefield. Textural, geochemical and mineralogical analyses show that the dune sands were mainly derived from Colorado River sands during episodes when the Colorado River drained west to the Salton Basin instead of south to the Gulf of California. A model is presented which relates episodic aeolian deposition in the Algodones dunefield to the growth of the resulting lake under a persistent, intense north-westerly wind regime and to subsequent desiccation of the lake following diversion of the Colorado River to the Gulf of California. Late Holocene radiocarbon ages for relict lake shoreline deposits correlate with documented episodes of extreme flooding and increased winter storminess across the south-western United States. These conditions are likely to have been the cause of intensified north-westerly winds which coincided with high stands of Lake Cahuilla. Although diversion of the Colorado River to the Salton Basin occurred at least three times during the late Holocene, it is evident that the river did not change its course in response to the majority of the floods. It appears that the critical control on diversion of the river was the difference in gradient between courses leading to the Salton Basin and Gulf of California, which in turn was influenced by fluvial deposition and tectonic activity. Flooding was able to cause diversion of the Colorado River to the Salton Basin only when the difference in gradient between the two courses was relatively small.     

Cretaceous to Cenozoic sequential kinematics in the forearc–arc transition: effects of changing oblique plate convergence and the San Andreas system with implications for the La Paz fault (southern Baja California, Mexico)

We studied metasediments and mylonitic arc granitoids from the forearc–arc transition of southern Baja California, Mexico. Thin section analyses and field evidence show that metamorphism of the forearc–arc transition is of the high T/P active margin type. The heat was provided by Cretaceous arc intrusions. Field observations and thin section analyses, including the time/temperature deformation path, demonstrate that the study area was first affected by dextral, ductile shearing followed by ductile, sinistral, possibly transpressive strike-slip parallel to the magmatic arc during the Cretaceous. Both intervals are related to changing oblique plate convergence and, thus, identified as trench-linked strike-slip effects. The geometric relationship between arc-dipping foliation, stretching lineation and shear sense indicates that the arc may have been pressed onto the rocks of the study area during sinistral shearing. The sinistral interval lasted up until regional cooling (Early Cenozoic?). Because the La Paz fault is closely associated with the forearc–arc transition, it must have the same Cretaceous to Early Cenozoic kinematic history. The northern segment of the La Paz fault is a modern, brittle, strike-slip fault interpreted as a dextral synthetic fault of the San Andreas system which opened the Gulf of California (Mar de Cortés/Golfo de California). We found no evidence for Miocene Basin and Range extension.     

Fine-scale relief related to Late Holocene channel shifting within the floor of the upper Redondo Fan, offshore Southern California

Erosional and depositional bedforms have been imaged at outcrop scale in the upper Redondo Fan, in the San Pedro Basin of offshore Southern California in ≥600 m water depths, using an Autonomous Underwater Vehicle developed by the Monterey Bay Aquarium Research Institute. The Autonomous Underwater Vehicle is equipped with multibeam and chirp sub-bottom sonars. Sampling and photographic images using the Monterey Bay Aquarium Research Institute Remotely Operated Vehicle Tiburon provide groundtruth for the Autonomous Underwater Vehicle survey. The 0·3 m vertical and 1·5 m lateral bathymetric resolution and 0·1 m sub-bottom profile resolution provide unprecedented detail of bedform morphology and structure. Multiple channels within the Redondo Fan have been active at different times during the Late Holocene (0 to 3000 yr bp ). The currently active channel extending from Redondo Canyon makes an abrupt 90° turn at the canyon mouth before resuming a south-easterly course along the east side of the Redondo Fan. This channel is floored by sand and characterized by small steps generally <1 m in relief, spaced 10 to 80 m in the down-channel direction. A broader channel complex lies along the western side of the fan valley that was last active more than 850 years ago. Two distinct trains of large scours, with widths ranging from tens to a few hundred metres and depths of 20 m, occur on the floor of the western channel complex, which has a thin mud drape. If observed in cross-section only, these large scours would probably be misidentified as the thalweg of an active channel.     

High-pressure–low-temperature metamorphism and deformation in the Bündnerschiefer of the Engadine window: implications for the regional evolution of the eastern Central Alps

A study of the metamorphic and tectonic evolution of the Bündnerschiefer of the Engadine window shows that the individual nappes have been thinned by a large amount and that extension was active during and soon after nappe stacking.
Based on contrasting P–T  histories the Penninic Bündnerschiefer can be divided in two major units bounded by a horizontal contact. The lower (Mundin) unit shows typical high- P /low- T  parageneses in metapelites (Mg-carpholite) and in metabasites (glaucophane); metamorphic conditions are estimated around 12  kbar, 375  °C. The upper (Arina) unit contains no specific high- P minerals; metamorphic conditions are estimated around 7  kbar, 325  °C. A minimum pressure gap of 5  kbar is thus observed. The contact between the two units is marked by a mappable normal shear zone with top-to-the-north-west sense of shear. Near the shear zone, fresh carpholite fibres trend parallel to the regional stretching lineation, implying that the detachment is an early structure active from the depth of stability of the carpholite and persisting during subsequent exhumation. The good preservation of carpholite and the absence of retrograde chloritoid below the shear zone show that exhumation occurred along a cooling path, whereas the deeper units are exhumed along an isothermal path. Exhumation probably occurred during convergence and further nappe stacking during the earlier Eocene. These results suggest that pre-collisional tectonic thinning of the Penninic oceanic units may be more widespread and significant than generally recognized.
     

Fault tectonics of the Baja California Peninsula and the opening of the Sea of Cortez,Mexico

A detailed field analysis of Neogene and Quaternary faults in Baja California has enabled us to reconstruct the stress pattern and the tectonic evolution of the central and southern parts of the peninsula. The deformation, which is related to the opening of the gulf, affects the whole peninsula, but decreases from east to west. Most observed faults, normal and/or dextral, strike NNW-SSE to WNW-ESE; their mechanisms include both strike-slip and dip-slip, as well as intermediate motions. Compressional events have occurred since Late Neogene times, but were probably of minor quantitative importance because reverse faults are rare and small.The principal fault pattern includes dextral NNW-SSE Riedel shears and N-S tension faults induced by dextral strike-slip along two main NW-SE fault zones bordering the peninsula: the Gulf of California to the east, which is the most important, and the Tosco-Abreojos fault to the west. The resulting pattern of deformation shows that the eastern part has been a complex transform-extensional zone since Late Miocene-Early Pliocene times.     

Climate and ocean variability during the middle and late Holocene recorded in laminated sediments from Alfonso Basin, Gulf of California, Mexico

A laminated sequence (core BAP96-CP 24°38.12′N, 110°33.24′W; 390 m depth) from the Alfonso Basin in Bay of La Paz, southern Gulf of California, contains a record of paleoceanographic and paleoclimatic changes of the past 7900 yr. Radiolarian assemblages and magnetic susceptibility are used as proxies of oceanographic and climatic variability. The records provide a regional scenario of the middle and late Holocene, suggesting two major climatic regimes and several millennial-scale events. Conditions relatively warmer and drier than today occurred from 7700 to 2500 cal yr BP, promoting the intensification of evaporation processes and the prevalence of the Gulf of California water in the Basin. These conditions correlate with strong droughts in the middle Holocene of North America and with minimal incursion of tropical waters into the Gulf of California. Proxies indicate a warm scenario and the dominance of the Equatorial Surface Water in the Alfonso Basin from 2400 to 700 cal yr BP, suggesting the intensification of ENSO cycles. A climatic signal between 1038 and 963 cal yr BP may be correlated with global signal of the “Medieval Warm Period.” Several cooling events are recognized at 5730, 3360, 2700, 1280 and 820 cal yr BP and are associated with intensification of northwest winds leading to upwellings and enhanced productivity in the Basin.     

Structural characteristics of the Altar Basin,Northwest Sonora,Mexico

Eight two-dimensional, multichannel seismic reflection lines were acquired, processed, and interpreted to study the structure of the Altar Basin, which is part of the Salton Trough tectonic province. We identified two basin-bounding zones characterized by different degrees of strain: the Cerro Prieto–Altar deformation zone (CPADZ) and the Altar–Caborca deformation zone (ACDZ). The CPADZ is bounded on the west by the Cerro Prieto fault and on the east by the Altar fault. To the north, the strike of both faults changes slightly from a NW to more NNW direction. In the CPADZ, the thickness of the crust decreases southward towards the Gulf of California, and is associated with a deformation-developing fault. The CPADZ has a rotation component orientating these faults in an oblique direction to the Cerro Prieto fault, whereas within the ACDZ, a geometric coherence of synthetic and antithetic faults exists, creating horsts and graben striking N37° W. The Altar fault is recognized by basement interruption, with a vertical component of ～1 km, striking at N37° W and dipping 83° SW. On the northeastern side of the Altar Basin, the basement configuration shows that the minimum time of basement record (～0.4 s of two-way travel time) and the time curve gradient decrease in the NE–SW direction. The depocentre is ～6 km deep in the central-west portion of the basin. We identified a graben between the Rosario and Tinajas Altas mountains (Rosario Basin). The extension–connection of the Altar and Rosario basins to the south is not well defined; nevertheless, these basins could represent the link between the Colorado River and the Gulf of California during the late Miocene, whereas this link was abandoned in the Pliocene as subsidence migrated towards the northwest into the Cerro Prieto and Laguna Salada basins.