Raymond V. Ingersoll

Tectonic history of the Altyn Tagh fault system in northern Tibet inferred from Cenozoic sedimentation

The active left-slip Altyn Tagh fault defines the northern edge of the Tibetan plateau. To determine its deformation history we conducted integrated research on Cenozoic stratigraphic sections in the southern part of the Tarim Basin. Fission-track ages of detrital apatites, existing biostratigraphic data, and magnetostratigraphic analysis were used to establish chronostratigraphy, whereas composition of sandstone and coarse clastic sedimentary rocks was used to determine the unroofing history of the source region. Much of the detrital grains in our measured sections can be correlated with uplifted sides of major thrusts or transpressional faults, implying a temporal link between sedimentation and deformation. The results of our studies, together with existing stratigraphic data from the Qaidam Basin and the Hexi Corridor, suggest that crustal thickening in northern Tibet began prior to 46 Ma for the western Kunlun Shan thrust belt, at ca. 49 Ma for the Qimen Tagh and North Qaidam thrust systems bounding the north and south margins of the Qaidam Basin, and prior to ca. 33 Ma for the Nan Shan thrust belt. These ages suggest that deformation front reached northern Tibet only ∼10 ± 5 m.y. after the initial collision of India with Asia at 65–55 Ma. Because the aforementioned thrust systems are either termination structures or branching faults of the Altyn Tagh left-slip system, the Altyn Tagh fault must have been active since ca. 49 Ma. The Altyn Tagh Range between the Tarim Basin and the Altyn Tagh fault has been a long-lived topographic high since at least the early Oligocene or possibly late Eocene. This range has shed sediments into both the Tarim and Qaidam Basins while being offset by the Altyn Tagh fault. Its continuous motion has made the range act as a sliding door, which eventually closed the outlets of westward-flowing drainages in the Qaidam Basin. This process has caused large amounts of Oligocene–Miocene sediments to be trapped in the Qaidam Basin. The estimated total slip of 470 ± 70 km and the initiation age of 49 Ma yield an average slip rate along the Altyn Tagh fault of 9 ± 2 mm/yr, remarkably similar to the rates determined by GPS (Global Positioning System) surveys. This result implies that geologic deformation rates are steady state over millions of years during continental collision.

Compositional trends in arc-related, deep-marine sand and sandstone: A reassessment of magmatic-arc provenance

Detrital modes for 524 deep-marine sand and sandstone samples recovered on circum-Pacific, Caribbean, and Mediterranean legs of the Deep Sea Drilling Project and the Ocean Drilling Program form the basis for an actualistic model for arc-related provenance. This model refines the Dickinson and Suczek (1979) and Dickinson and others (1983) models and can be used to interpret the provenance/tectonic history of ancient arc-related sedimentary sequences. Four provenance groups are defined using QFL, QmKP, LmLvLs, and LvfLvmiLvl ternary plots of site means: (1) intraoceanic arc and remnant arc, (2) continental arc, (3) triple junction, and (4) strike-slip-continental arc. Intraoceanic- and remnant-arc sands are poor in quartz (mean QFL%Q 75); they are predominantly composed of plagioclase feldspar and volcanic lithic fragments. Continental-arc sand can be more quartzofeldspathic than the intraoceanic- and remnant-arc sand (mean QFL%Q values as much as 10, mean QFL%F values as much as 65, and mean QmKP%Qm as much as 20) and has more variable lithic populations, with minor metamorphic and sedimentary components. The triple-junction and strike-slip-continental groups compositionally overlap; both are more quartzofeldspathic than the other groups and show highly variable lithic proportions, but the strike-slip-continental group is more quartzose. Modal compositions of the triple junction group roughly correlate with the QFL transitional-arc field of Dickinson and others (1983), whereas the strike-slip-continental group approximately correlates with their dissected-arc field.

Actualistic sandstone petrofacies: Discriminating modern and ancient source rocks

Actualistic models relating plate-tectonic setting to sedimentary basins and provenance successfully predict modal compositions of sand and sandstone at the scale of continents and ocean basins (third-order models). Second-order models may be used to identify source regions within a given tectonic setting, such as source terranes within the Rio Grande rift of New Mexico. First-order models relate the composition of modern alluvial sand directly to specific source rocks (e.g., granite). Petrographic data from locally derived sand of known provenance may be used to statistically discriminate compositions according to their source rocks, thus defining actualistic petrofacies. Ancient petrofacies may be compared to the actualistic petrofacies, in order to better constrain provenance and paleotectonic reconstructions. This approach may be applied at first-, second-, or third-order scales. At first- and second-order scales, stepwise discriminant analysis reveals the Tertiary provenance history of the Rio Grande rift of north-central New Mexico by comparing Tertiary petrofacies with modern petrofacies of known provenance. Application of this technique provides needed objectivity in source-rock and paleotectonic reconstructions.

Paleogeographic and paleotectonic evolution of the Himalayan Range as reflected by detrital modes of Tertiary sandstones and modern sands (Indus transect, India and Pakistan)

Detrital modes of sandstones derived from the Himalayan suture belt record the history of the mountain range since initial collision between India and Asia, which began in latest Paleocene time. Tertiary clastic wedges deposited in fore-arc, foreland, and remnant-ocean basins, and exposed along the Indus transect from northernmost India to the Arabian Sea, represent the best opportunity to study sedimentary responses to successive tectonic events during continental collision. Quartzose “continental-block” and feldspatholithic “magmatic-arc” sandstones were deposited, respectively, on the passive Indian (Tethys Himalayan succession) and active Asian (Indus Group) continental margins during Late Cretaceous–Paleocene time. Closure of the Neotethys was marked by sudden arrival of volcanic and ophiolitic detritus on the passive continental margin of the Indian plate during deposition of sediments dated at foraminiferal zones P6 (Pakistan) to P8 (India). Starting in early Eocene time (deposition of Chulung La Formation and Murree Supergroup), volcanic and ophiolitic to metasedimentary detritus was accumulated in rapidly subsiding “piggy-back” and foreland basins. Homogeneous petrographic composition within the Eocene–lower Miocene Murree Supergroup, with only slight progressive increase of detrital feldspars, suggests erosion of largely supracrustal rocks involved in thrusting in the north. In middle Miocene time, marked enrichment in medium- to high-grade metamorphic detritus in foreland sandstones (Siwalik Group) reflects rapid uplift of a warm wedge of Indian crust, which was carried southward along the Main Central thrust. This major paleogeographic change was recorded also by quartzolithic remnant-ocean turbidites, which were fed great distances along transverse fracture zones and later accreted in the coastal Makran subduction complex (Panjgur association and Makran Group). Recycled-orogen detritus derived from the elevated Himalayan chain is still accumulating today in the Indus fan. Enrichment in feldspars with respect to ancient sandstones reflects deep erosion levels into mid-crustal rocks along the core of the growing orogen.

Evolution of the Late Cretaceous forearc basin, northern and central California

The Upper Cretaceous part of the Great Valley Sequence of California provides a unique opportunity to study deep-marine sedimentation, petrologic evolution, and tectonic evolution of a forearc basin. Actualistic models of submarine fan sedimentation and arc-trench evolution provide the basis for unraveling the complex depositional history of the bathyal to abyssal sediment deposited between the Sierra Nevada volcano-plutonic arc to the east and the Franciscan subduction complex to the west. Submarine fan components are lenticular stratigraphic units which can be correlated along strike on the basis of both paleontologic and petrologic data. The following depositional components are present: basin plain, outer fan, midfan, inner fan, slope, and shelf. Vertical successions of fan facies associations constitute retrograding and pro-grading suites that correspond, respectively, to onlapping and offlapping relations in the basin. Sedimentation rates are similar to those of other tectonically active flysch basins. Paleocurrents are predominantly southerly and westerly in the Sacramento Valley, and predominantly westerly in the San Joaquin Valley. Microfossil evidence and the lack of carbonate material suggest deposition below the Late Cretaceous calcite compensation depth. Dimensions and geometries of tectono-stratigraphic components of the Late Cretaceous arc-trench system are similar to those of modern arc-trench systems. The Late Cretaceous arc-trench gap widened by the prograde accretion of the Franciscan Assemblage (subduction complex) and the retrograde migration of the Sierra Nevada volcanic front (arc). Sediment dispersal systems expanded as the basin widened. The Java arc-trench system provides a modern analogue for the Late Cretaceous forearc basin, with sediment fed laterally from the arc and dispersed longitudinally along the basin axis.

Provenance of Franciscan graywackes in coastal California

A systematic comparison of available detrital modes for graywacke sandstones of the Franciscan subduction complex and for coeval sandstones of the Great Valley sequence in the California Coast Ranges indicates that both were apparently derived from the same general sources. The inferred provenance terrane was the ancestral Sierran-Klamath magmatic arc, from which mixed volcanic and plutonic detritus readily entered the adjacent Great Valley forearc basin. At intervals along the trend of the arc-trench system, arc-derived detritus also bypassed the forearc region through submarine canyons that fed the Franciscan trench. Longitudinal flow along both the Great Valley trough and the Franciscan trench achieved wide dispersal of the turbidite sediment. Suites of both Franciscan and Great Valley samples include an array of subquartzose compositions ranging from feldspatholithic to lithofeldspathic. Mean framework modes of 17 Franciscan suites comprising 203 individual samples, and of 23 Great Valley suites comprising 410 individual samples, range from 14% to 44% quartz grains, 15% to 54% feldspar grains, and 7% to 71% total lithic fragments. The ratio of quartz to feldspar remains relatively constant as the proportion of lithic fragments changes. The compositional variations reflect differences mainly in the admixture of lithic fragments derived principally from volcanic cover with quartz, and feldspar derived principally from erosion of underlying plutons. Despite major overlap in the compositions of the two sets of samples, some Franciscan sandstones are somewhat more feldspathic and less lithic than any known Great Valley counterparts and were probably derived from segments of the arc terrane where exposures of plutons were more extensive than within typical Great Valley sources. Higher proportions of non-volcanic to volcanic lithic fragments in some Franciscan sandstones probably reflect complex recycling processes on the trench slope. Diagenetic effects in many Franciscan suites include apparent wholesale replacement of K-feldspar by albite. Present age control is inadequate to test fully for time-dependent trends in the compositions of Franciscan sandstones analogous to the known stratigraphic variations in the composition of Great Valley sandstones. This question ought to be investigated in future studies.

Three-stage evolution of the Los Angeles basin, southern California

We propose that an episode of transtension dominated development of the Los Angeles basin area from 12 to 6 Ma, following middle Miocene transrotation and prior to the modern transpressional regime. Transtension resulted from the releasing bend of the San Gabriel–Chino Hills–Cristianitos fault, which acted as the primary transform boundary in southern California during this episode. Such an interpretation implies that significant transform motion did not occur on the southern San Andreas fault prior to 6 Ma and that the Gulf of California has opened primarily since 6 Ma. We propose a three-stage model for evolution of the Los Angeles basin and vicinity within the evolving transform-fault system: transrotation (18–12 Ma), transtension (12–6 Ma), and transpression (6–0 Ma). Timing of these stages correlates with microplate-capture events, which occurred during conversion from a convergent margin to a transform margin.

Triple-junction instability as cause for late Cenozoic extension and fragmentation of the western United States

Regional extension in the western United States began soon after 30 m.y. B.P., the time of first interaction between the Pacific and North American plates. Previous explanations for extension include lithospheric-active processes and asthenospheric-active processes; the former are favored because of coincidence of timing with lithospheric interactions, relative timing of volcanism and deformation, and coincidence with pre-existing structures. Although extension has occurred, in part, in a back-arc setting, it cannot be attributed to age of subducted crust or azimuth of subduction, because both attributes favor back-arc contraction (characteristic behavior before 40 m.y. B.P. The Mendocino triple junction has been unstable since its inception. The continental margin was relatively straight before 30 m.y. B.P., and it has become more convex westward as the triple junction has migrated northward. The continental margin and arc have been anchored to the subducting slab (Farallon-Juan de Fuca), whereas the triple junction must move parallel to the San Andreas transform. The combined result has been the northwestward and clockwise movement of coastal blocks relative to the continental interior and the eastward stepping of the San Andreas transform relative to the coast. These effects result from the unstable geometry of the Mendocino triple junction. Lack of a subducted slab (slab window) beneath the extended lithosphere enables asthenospheric rise but does not cause extension.

Nd-Sr isotopic, geochemical, and petrographic stratigraphy and paleotectonic analysis: Mesozoic Great Valley forearc sedimentary rocks of California

Measurements of Nd-Sr isotopes, major and trace elements, and model mineralogy were made on Upper Jurassic and Cretaceous Great Valley forearc sedimentary rocks to test models for the temporal and spatial evolution of Sierra Nevada arc sources. Isotopes and major and trace elements are sensitive provenance indicators because of the large west-east isotopic, geochemical, and age gradients in the plutonic rocks of the Sierra Nevada batholith, and because petrographic models indicate that source areas moved east during the Cretaceous. Isotopic and chemical variations are correlated in the forearc sandstone; as ϵNd decreases, Th, U, La, Nb, Zr, Hf, Pb, Rb, SiO2, and K2O concentrations increase, and FeO, MgO, TiO2, Ni, and Cr concentrations decrease. This relation is the same as that observed in the plutonic rocks and indicates that the arc was the primary source of sediment and that the sandstone chemistry was not disturbed by sedimentary processes. The ϵNd-ϵSr relation of San Joaquin Valley sandstone is the same as the plutonic rocks, but Sacramento Valley sandstone is elevated in ϵSr because of seawater exchange, weathering, and diagenesis. Whole-rock sandstone decreases in ϵNd from +7 to -5 and increases in 87Sr/86Sr from 0.7045 to 0.7073 with decreasing stratigraphic age. The Nd-Sr isotopic composition is correlative with the plagioclase to feldspar ratio and indicates that source areas moved inland during the Cretaceous. Upper Cretaceous San Joaquin Valley shale is similar in ϵNd to the sandstone, indicating that sandstone and shale were derived from the same source and that the Nd isotopic composition is independent of grain size. The shale is higher in 87Sr/86Sr than the sandstone, possibly due to concentration of biotite in the fine fraction during transport and subsequent Rb loss during diagenesis. Nd-Sr isotopes were used to construct models to locate source areas. Parameters include lithology, drainage basin geometry, and erosion rate. The age and isotopic compositions of the calculated igneous component of the sandstone correspond to the age and isotopic compositions of the plutonic rocks of the batholith; this correspondence indicates that (1) the isotopic composition of the plutonic rocks and the coeval volcanic cover were similar, (2) the volcanic front was denuded within a few million years, and (3) the sediment was derived from the head of the drainage basin, located at the migrating volcanic front.

Initiation and evolution of the Great Valley forearc basin of northern and central California, U.S.A.

Summary The late Mesozoic Great Valley forearc basin of northern and central California evolved from a residual forearc basin formed on top of young oceanic crust to a composite forearc basin resting on both oceanic and continental crust. Depositional environments preserved in outcrop along the west side of the Sacramento Valley evolved from deep oceanic floor and slope in the Late Jurassic to basin plain in the Early Cretaceous to submarine fan complexes in the Late Cretaceous to slope and shelf in the Palaeogene. The forearc basin widened and enlarged, and was supplied with voluminous sediment primarily derived from the coeval magmatic arc to the east. However, significant quantities of ophiolitic detritus (chert, and sedimentary, mafic volcanic and mafic metavolcanic lithic fragments) occur in the lower Great Valley strata, thus indicating erosion of ‘tectonic highlands’ formed during arc-arc collision immediately preceeding initiation of the late Mesozoic subduction regime. The Great Valley forearc owes its geometry and history to the peculiarities of subduction initiation and to the shapes of the continental margin and magmatic arcs extant before initiation of the Great Valley subduction phase. The residual forearc developed on oceanic crust that had formed previously by spreading behind an east-facing intraoceanic arc. After this east-facing arc collided with the west-facing continental-margin arc, a new trench formed west of the suture belt. The shape of the new continental margin was irregular, so that a wide residual forearc basin formed in the Great Valley area, whereas no residual forearc basin formed to the north (Klamath area) or possibly, to the south (southern California borderland area that has been dislocated by Neogene strike-slip motions). The northern coastal promontory (Klamath area) provided much of the ophiolitic detritus within the lower part of the Great Valley strata. Sediment derived from the Klamaths was transported southward directly into the forearc basin, as well as westwards and then southwards into the trench west of the forearc basin. The Great Valley forearc basin is preserved and displayed so well today due to its mode of initiation (residual forearc formed in a previously backarc area) and its mode of termination (northward migration of a triple junction that converted a convergent margin into a transform margin).