Rebecca J. Dorsey

Chronology of Miocene–Pliocene deposits at Split Mountain Gorge, Southern California: A record of regional tectonics and Colorado River evolution

Late Miocene to early Pliocene deposits at Split Mountain Gorge, California, preserve a record of basinal response to changes in regional tectonics, paleogeography, and evolution of the Colorado River. The base of the Elephant Trees Formation, magnetostratigraphically dated as 8.1 ± 0.4 Ma, provides the earliest well-dated record of extension in the southwestern Salton Trough. The oldest marine sediments are ca. 6.3 Ma. The nearly synchronous timing of marine incursion in the Salton Trough and northern Gulf of California region supports a model for localization of Pacifi c‐North America plate motion in the Gulf ca. 6 Ma. The fi rst appearance of Colorado River sand at the Miocene-Pliocene boundary (5.33 Ma) suggests rapid propagation of the river to the Salton Trough, and supports a lake-spillover hypothesis for initiation of the lower Colorado River.

Provenance Evolution and Unroofing History of a Modern Arc-Continent Collision: Evidence from Petrography of Plio-pleistocene Sandstones, Eastern Taiwan

ABSTRACT Plio-Pleistocene sandstones from the Coastal Range of eastern Taiwan provide a record of shifting source areas and unroofing events in adjacent arc and accretionary-wedge terranes during the early stages of are-continent collision. In latest Miocene and early Pliocene time, arc-derived sediments were deposited in a precollisional forearc basin between the Luzon volcanic arc and the Manila Trench. Sandstones of this age are rich in zoned plagioclase, volcanic lithic fragments, and pelagic foraminifera and contain only minor quartz and sedimentary-metasedimentary lithic fragments. In contrast, lower Pliocene to lower Pleistocene sandstones are poor in feldspar and volcanic fragments and contain abundant sedimentary and metasedimentary lithic fragments. This reflects a provenance shift f om the volcanic arc to the uplifted (meta) sedimentary accretionary wedge of proto-Taiwan, which resulted from the early Pliocene onset of arc-continent collision. A refined classification of sedimentary and metasedimentary lithic fragments is introduced here and used to interpret the early evolution of the collisional orogen. Lithic fragments are rich in pelitic minerals and are divided into: 1) sedimentary (Ls) = mudstone, shale, siltstone, chert and argillite; 2) low-grade metamorphic (Lm1) = slate, slatey siltstone, and quartzite; and 3) medium-grade metamorphic (Lm2) = phyllite-schist, phyllitic quartzite and quartz-mica-albite aggregate. A good correlation is observed between these lithic fragment types and bedrock lithologies exposed in the present-day Central Range of Taiwan. Based on this correlation, the change in relative abundance of sedimentary and metasedimentary lithic fragments through time has been used to interpret an unroofing sequence for the collisional fold-and-thrust belt. Lower Pliocene Taiwan-derived sandstones are rich in sedimentary lithic fragments (Ls), which were shed from the accretionary wedge during the earliest stages of arc-continent collision. From early Pliocene to early Pleistocene time, sandstones became progressively depleted in Ls lithic fragments and enriched first in Lm1 and later in Lm2 fragments, as deeper levels in the metamorphic complex were uplifted and exposed to erosion. Regional metamorphism of biotite-grade rocks (= Lm2) occurred at about 4.5 Ma at depths of 12-15 km (Ernst 1983) and were uplifted to the surface by about 1.4 Ma (this study). These ages give an uplift rate of 4 to 5 km/m.y., which is in good agreement with independent estimates based on fission-track studies, raised coral reefs, Quaternary elevation changes, and present-day denudation rates. Thus, it appears that sandstone detrital modes of clastic orogenic sequences can be used to interpret specific aspects of unroofing and tectonic evolution in nearby active mountain belts.

Stratigraphic record of basin development within the San Andreas fault system: Late Cenozoic Fish Creek–Vallecito basin, southern California

The Fish Creek–Vallecito basin contains a 5.5-km-thick section of late Miocene to early Pleistocene sedimentary rocks exposed in the hanging wall of the West Salton detachment fault. These deposits preserve a high-fidelity record of late Cenozoic subsidence and basin filling that resulted from deformation in the San Andreas fault system of southern California. Existing and new paleomagnetic data, combined with new U-Pb zircon ages of two tuffs high in the section, show that the section ranges in age from ca. 8.0 ± 0.4 Ma at the base to ca. 0.95 Ma at the top. Geohistory analysis reveals: (1) moderate subsidence (0.46 mm/yr) from ca. 8.0 to 4.5 Ma; (2) rapid subsidence (2.1 mm/yr) from 4.5 to 3.1 Ma; (3) moderate subsidence (0.40 mm/yr) from 3.1 to 0.95 Ma; and (4) rapid uplift and erosion that has exhumed the section since ca. 1 Ma. Onset of sedimentation at ca. 8.0 ± 0.4 Ma records earliest extension or transtension in the area, possibly related to localization of the Pacific–North America plate boundary in the Salton Trough and Gulf of California. Alternatively, marine incursion at 6.3 Ma may be the earliest record of plate-boundary deformation in the Gulf of California–Salton Trough region. A thick interval higher in the section records progradation of the Colorado River delta into and across the basin starting ca. 4.9 Ma. Progradation continued during an abrupt increase in subsidence rate at 4.5 Ma, and fluvial-deltaic conditions persisted for 1.4 m.y. during the rapid-subsidence phase, indicating that delta progradation was driven by a large increase in rate of sediment input from the Colorado River. Uplift and inversion of the basin starting ca. 1.0 Ma record initiation of strike-slip faults that define the modern phase of dextral wrench tectonics in the western Salton Trough.

Rapid subsidence and stacked Gilbert-type fan deltas, Pliocene Loreto basin, Baja California Sur, Mexico

Abstract Pliocene nonmarine to marine sedimentary rocks exposed in the Loreto basin, Baja California Sur, provide a record of syntectonic subsidence and sedimentation in a transform-rift basin that developed along the western margin of the Gulf of California. A thick sequence of twelve Gilbert-type fan deltas, having a total measured thickness of about 615 m, accumulated near the fault-bounded southwestern margin of this basin. Based on stratal geometries and lithofacies associations, sedimentary rocks are divided into Gilbert-delta topset, foreset and bottomset strata, shell beds and background shallow-marine shelf deposits. Topset strata of each Gilbert-type delta cycle are capped by laterally persistent molluscan shell beds containing diverse assemblages of bivalves, pectens, oysters, gastropods and echinoids. These shell beds are interpreted to be condensed intervals that record sediment starvation during abandonment of the fan-delta plain. Delta abandonment may have been caused by large episodic faulting events, which submerged each pre-existing fan-delta plain, substantially slowed detrital input by drowning of alluvial feeder channels, and created new accommodation space for each new Gilbert-type fan delta. Alternatively, it is possible that delta-plain abandonment was caused by upstream avulsions and autocyclic lateral switching of fan-delta lobes during relatively uniform rates of slip along the basin-bounding fault. Two contrasting, plausible basin models are proposed for the Loreto basin: (1) asymmetric subsidence along a high-angle oblique-slip normal fault, producing a classic half-graben basin geometry with vertically stacked Gilbert-type fan deltas; or (2) lateral stacking and horizontal displacement of strata away from a relatively fixed depocenter due to fault movement in the releasing bend of a listric strike-slip fault. We favor the first model because field relations and simple geometric constraints suggest that most of the total measured section represents a true vertical stratigraphic profile. Assuming vertical sediment accumulation and using ages of interbedded tuffs obtained from high-precision 40 Ar/ 39 Ar dating of plagioclase and biotite, quantitative decompaction and geohistory analysis was carried out for the Loreto basin sequence. Tuff ages range from 2.61 ± 0.01 Ma in the lower part of the basinal sequence to 1.97 ± 0.02 Ma near the top, with two intermediate tuffs dated at 2.46 ± 0.06 and 2.36 ± 0.02 Ma that are separated by 782 m of measured section. Basin subsidence initially took place at moderate rates of 0.43 ± 0.17 mm/yr and accelerated dramatically at 2.46 Ma to 8.1 ± 5.1 mm/yr. This phase of extremely rapid subsidence lasted for only about 100 ka, and it produced much of the total accomodation space and sedimentary thickness in the basin. Accumulation of Gilbert-type fan deltas took place only during the short pulse of very rapid subsidence, between 2.46 and 2.36 Ma. Prior to this time interval, alluvial-fan and shelf-type fan-delta depositional systems prevailed; afterwards no fan deltas of any kind were deposited, and the basin evolved to a slowly subsiding low-energy carbonate shelf setting. This suggests that very rapid subsidence, combined with rapid sediment input, may be required to maintain steep basin-margin slopes and continually create new accommodation space, conditions that seem necessary for the development of thick sequences of stacked Gilbert-type fan deltas.

Global distribution of late Paleogene hiatuses

Six global late Paleogene hiatuses (PHa to PHe) have been identified from deep-sea sequences. These hiatuses occurred at the middle/late Eocene boundary, late Eocene, Eocene/Oligocene boundary, late early Oligocene, late Oligocene, and Oligocene/Miocene boundary horizons. Paleodepth distribution of hiatuses shows hiatus maxima characterized by major mechanical erosion below 4800 m, at mid-depth between 2000 and 3000 m, and in shallower water above 1600 m paleodepth. The geographic distribution and paleodepth of these hiatus maxima suggest that flow paths of major water masses and currents are the principal cause. Widespread short hiatuses due to carbonate dissolution or nondeposition occurred primarily during global cooling trends or climatic instability and appear to correlate to sea-level transgressions or onlap sequences. These hiatuses may have been caused by basin-shelf fractionation of carbonates.

Sedimentation and crustal recycling along an active oblique-rift margin: Salton Trough and northern Gulf of California

Transtensional basins embedded in the San Andreas fault system of Southern California (United States) and northwestern Mexico are fi lled with sediment derived from the Colorado River, which drains a large area of the western U.S. interior. The sediment is rapidly buried, heated, and mingled with intrusions in the deep basins to form a new generation of recycled crust along the active plate boundary. Using a range of values for total basin depth, relative volume of mantle-derived intrusions, and composition of early rift deposits, the volume of Colorado River‐derived sediment in the basins is bracketed between 2.2 and 3.4 ◊ 10 5 km 3 , similar to the volume of rock that likely was eroded from the Colorado River catchment over the past 5‐6 m.y. The volumetric rate of crustal growth by sedimentation is ~80‐130 km 3 /m.y./km, comparable to growth rates in subductionrelated island arcs and slow seafl oor spreading centers. Sedimentary and basinal processes thus play a major role in crustal evolution and recycling in this setting, and may be important at other rifted margins where a large river system is captured following tectonic collapse of a prerift orogenic highland.

River-evolution and tectonic implications of a major Pliocene aggradation on the lower Colorado River: The Bullhead Alluvium

The ∼200-m-thick riverlaid Bullhead Alluvium along the lower Colorado River downstream of Grand Canyon records massive early Pliocene sediment aggradation following the integration of the upper and lower Colorado River basins. The distribution and extent of the aggraded sediments record (1) evolving longitudinal profiles of the river valley with implications for changing positions of the river’s mouth and delta; (2) a pulse of rapid early drainage-basin erosion and sediment supply; and (3) constraints on regional and local deformation. The Bullhead Alluvium is inset into the Hualapai and Bouse Formations along a basal erosional unconformity. Its base defines a longitudinal profile interpreted as the incised end result after the Colorado River integrated through lake basins. Subsequent Bullhead aggradation, at ca. 4.5–3.5 Ma, built up braid plains as wide as 50 km as it raised the Colorado River’s grade. We interpret the aggradation to record a spike in sediment supply when river integration and base-level fall destabilized and eroded relict landscapes and Tertiary bedrock in the Colorado River’s huge catchment. Longitudinal profiles of the Bullhead Alluvium suggest ≥200 m post-Bullhead relative fault uplifts in the upper Lake Mead area, >100 m local subsidence in the Blythe Basin, and deeper subsidence of correlative deltaic sequences in the Salton Trough along the Pacific–North American plate boundary. However, regionally, for >500 km along the river corridor from Yuma, Arizona, to Lake Mead, Arizona and Nevada, the top of the Bullhead Alluvium appears to be neither uplifted nor tilted, sloping 0.5–0.6 m/km downstream like the gradient of a smaller late Pleistocene aggradation sequence. Perched outcrops tentatively assigned to the Bullhead Alluvium near the San Andreas fault system project toward a Pliocene seashore or bayline twice as distant (300–450 km) as either the modern river’s mouth or a tectonically restored 4.25 Ma paleoshore. We conclude that Bullhead aggradation peaked after 4.25 Ma, having lengthened the delta plain seaward by outpacing both 2 mm/yr delta subsidence and 43–45 mm/yr transform-fault offset of the delta. Post-Bullhead degradation started before 3.3 Ma and implies that the river profile lowered and shortened because sediment supply declined, and progradation was unable to keep up with subsidence and plate motion in the delta.

Stratigraphic record of Triassic-Jurassic collisional tectonics in the Blue Mountains province, northeastern Oregon

Sedimentary and volcanic rocks in the Blue Mountains province (BMP) of northeastern Oregon preserve a well studied record of Triassic–Jurassic magmatism, basin evolution, and terrane accretion. Terranes of the BMP represent two magmatic arcs (Wallowa and Olds Ferry terranes), an intervening oceanic subduction and accretionary complex (Baker terrane), and a complex thick succession of sedimentary rocks commonly known as the Izee terrane. We divide volcanic and sedimentary rocks into two regionally correlative, unconformity-bounded megasequences: (1) MS-1, Late Triassic to Early Jurassic deposits that change up section from (1a) older volcanic and volcaniclastic deposits of the Wallowa and Olds Ferry arcs to (1b) marine turbidites, shale, and argillite with chert-clast conglomerate and olistostromes derived from the emergent Baker terrane; and (2) MS-2, Early to Late Jurassic marine deposits that overlap older rocks and structures and record ∼20 to 40 m.y. of deep crustal subsidence in a large marine basin. Many of the known stratigraphic relationships in the Blue Mountains cannot be explained using the existing model for a Middle Triassic to Late Jurassic west-facing, non-collisional volcanic arc and forearc basin. We propose a new tectonic model for the BMP based on prior studies and comparison to modern analogues, which includes: (1) Middle Triassic magmatism in the Wallowa and Olds Ferry arcs during subduction and progressive closure of an ocean basin; (2) Late Triassic collision between facing accretionary wedges of the Wallowa and Olds Ferry arcs, and growth of marine basins on both sides of the emergent Baker terrane thrust belt; (3) Early to Middle Jurassic terrane-continent collision which resulted in closure of a wide back-arc basin, crustal thickening and loading in Nevada, and growth of a large marine collisional basin in the BMP; and (4) Late Jurassic thrusting, regional shortening, and final accretion of the basin and underlying terranes to western North America. This analysis suggests that collisional tectonics may have played a significant role in plate interactions that drove Triassic–Jurassic crustal thickening, mountain building, and basin development in the western North American Cordillera.

Earthquake clustering inferred from Pliocene Gilbert-type fan deltas in the Loreto basin, Baja California Sur, Mexico

A stacked sequence of Pliocene Gilbert-type fan deltas in the Loreto basin was shed from the footwall of the dextral-normal Loreto fault and deposited at the margin of a marine basin during rapid fault-controlled subsidence. Fan-delta parasequences coarsen upward from marine siltstone and sandstone at the base, through sandy bottomsets and gravelly foresets, to gravelly nonmarine topsets. Each topset unit is capped by a thin shell bed that records marine flooding of the delta plain. Several mechanisms may have produced repetitive vertical stacking of Gilbert deltas: (1) autocyclic delta-lobe switching; (2) eustatic sea-level fluctuations; (3) climatically controlled fluctuations in sediment input; and (4) episodic subsidence produced by temporal clustering of earthquakes. We favor hypothesis 4 for several reasons, but hypotheses 2 and 3 cannot be rejected at this time. Earthquake clustering can readily produce episodic subsidence at spatial and temporal scales consistent with stratigraphic trends observed in the Loreto basin. This model is supported by comparison with paleoseismological studies that document clustering on active faults over a wide range of time scales. Earthquake clustering is a new concept in basin analysis that may be helpful for understanding repetitive stratigraphy in tectonically active basins.

Evolution of the margin of the Gulf of California near Loreto, Baja California Peninsula, Mexico

The Gulf of California is a prime example of a young oblique-divergent plate boundary. This type of plate boundary is much less well understood than classic rifts and passive margins in divergent plate settings. A complexity in the Gulf of California is that the modern oblique rifting was preceded by a stage of orthogonal rifting. We have used extensive mapping as well as stratigraphic, geomorphic, and structural analysis to determine the history of deformation near Loreto on the southern Baja California peninsula during formation of the Gulf of California. Our data support the suggestion that middle to late Miocene (protogulf stage, 12 to ca. 6 Ma) orthogonal rifting was overprinted by transtensional structures during Pliocene to Quaternary time, though we cannot closely define the time of change. We further demonstrate that the plate margin was structurally segmented in the protogulf stage and then complexly overprinted by transtensional structures that suggest one type of process for initiating transform faults. The Loreto segment is 85 km long along the rift and is bounded on the west by a discontinuous series of aligned, down-tothe-east extensional monoclines and normal faults. These structures lie along, or as far as 4 km in front (east) of, a steep, 1000‐ 1600-m-high Main Gulf Escarpment that defines the western topographic margin of the rift zone. The Loreto segment is further defined by a rise in elevation of the Main Gulf Escarpment of up to 1 km and an ;500‐800 m increase in structural relief of prerift strata in the escarpment from the segment boundaries to the center of the segment. Structural analysis of secondary faults of the Loreto segment shows that fault populations that are known to be Pliocene to Quaternary in age are mixed normal and dextral-normal faults with a bulk extension direction of west-northwest‐eastsoutheast (2808‐1008). In contrast, fault populations that cut only prerift, Oligocene‐middle Miocene volcanic and sedimentary rocks are mainly normal faults with northeast-southwest to east-northeast‐ west-southwest bulk extension directions that average 2458‐658. These faults are interpreted to be late Miocene in age and formed during protogulf orthogonal rifting. Thus, the Pliocene faults record a major change of extension direction of ;358 from latest Miocene to Pliocene time, compatible with the onset of oblique rifting in the Gulf of California. Oblique-divergent overprinting is mainly expressed as the Loreto fault and Loreto basin in the northern part of the segment. The Loreto fault and basin evolved with major fault reorganizations and partial basin inversion. Many observations suggest that the Pliocene Loreto fault was linked to two nascent transform faults in the gulf and that Carmen Island rotated ;358‐408 clockwise within the complex fault system.