Scott W. Bogue

Post–10 Ma uplift and exhumation of the northern Coast Mountains, British Columbia

Apatite (U-Th)/He ages reveal three distinct periods in the exhumation history of the northern Coast Mountains (∼54°N). A well-developed helium partial retention zone indicates little or no exhumation between ca. 30 and 10 Ma. Beginning at 10 Ma and extending to at least 4 Ma the range underwent steady but slow exhumation of ∼0.22 mm/yr, after which the exhumation rate increased by at least 70%. An 85-km-long He age traverse across the orogen at sea level shows that vertical offsets on post-10 Ma faults are minor. Furthermore, the sea-level He ages (14–2.8 Ma) inversely correlate with local mean elevation along the traverse. These data suggest that the Coast Mountains first appeared as a significant topographic feature only within the past few million years. This history is not consistent with suggestions that uplift of the range resulted from formation of the Queen Charlotte basin in early-middle Miocene time. We speculate that intense erosion by alpine and continental glaciation after 2.5 Ma triggered isostatic uplift and creation of the modern topography of the Coast Mountains.

Geochemistry of Kauai shield-stage lavas: Implications for the chemical evolution of the Hawaiian plume

We measured He, Sr, Nd, Pb, and Os isotope ratios and major and trace element concentrations in stratigraphically and paleomagnetically controlled shield-stage lavas from Kauai, Hawaii. The range of 3He/4He ratios (17–28 RA) from Kauai is similar to that reported from Loihi and thus challenges the prevailing notion that high 3He/4He ratios are restricted to the preshield stage of Hawaiian magmatism. 3He/4He ratios vary erratically with stratigraphic position, and chronostratigraphic control from paleomagnetic data indicates very rapid changes in the 3He/4He ratios (up to 8 RA in ~102 years). These variations in helium isotopic ratios are correlated with variations in radiogenic isotope ratios, suggesting rapid changes in melt composition supplying the magma reservoir. A three-component mixing model, previously proposed for Hawaiian shield lavas, does not adequately explain the isotopic data in Kauai shield lavas. The addition of a depleted-mantle (DM) component with the isotopic characteristics similar to posterosional basalts explains the isotopic variability in Kauai shield lavas. The DM component is most apparent in lavas from the Kauai shield and is present in varying proportion in other Hawaiian shield volcanoes. Shield lavas from Kauai sample a high 3He/4He end-member (Loihi component), but while lavas from western Kauai have a larger contribution from the Kea component (high 206Pb/204Pb, anomalously low 207Pb/204Pb relative to 206Pb/204Pb), lavas from eastern Kauai have a larger proportion of an enriched (Koolau) component. The systematic isotopic differences between eastern and western Kauai reflect a gradual migration of the locus of volcanism from west to east, or alternatively east and west Kauai are two distinct shield volcanoes. In the latter case, the two shield volcanoes have maintained distinct magma supply sources and plumbing systems. Our new geochemical data from Kauai are consistent with the existence of a single high 3He/4He reservoir in the Hawaiian plume and suggest that the proportion of the different mantle components in the plume have changed significantly in the past 5 Myr. The long-term evolution of the Hawaiian plume and the temporal variability recorded in Kauai lavas require more complex geochemical heterogeneities than suggested by radially zoned plume models. These complexities may arise from heterogeneities in the thermal boundary layer and through variable entrainment of ambient mantle by the upwelling plume.

Distinctive field behavior following geomagnetic reversals

The paleomagnetism of lava flows from Kauai (Hawaii), erupted about 4 my ago immediately following a geomagnetic reversal, reveals that the post-transitional field had several distinctive characteristics. While the directional dispersion was identical to that displayed by stable field over the past 5 my, the post-transitional field intensity was unusually high. In both these respects, the field at Kauai resembles the one recorded at 15 Ma by lava flows at Steens Mountain in SE Oregon. The two records also differ in an important way: the large intensity oscillations that immediately followed the Steens Mountain reversal are not apparent in the data from Kauai. These results suggest that while the stability of the newly established dipole may vary significantly from one reversal to the next, strong dipolar field and normal directional dispersion may be systematic features of the post-transitional geodynamo. Furthermore, the results lend support to the recent suggestion that the core remains in an unusual state for many tens of kyr following a reversal.

Paleomagnetism, magnetic anisotropy, and mid‐Cretaceous paleolatitude of the Duke Island (Alaska) ultramafic complex

We report paleomagnetic results from layered igneous rocks that imply substantial post mid-Cretaceous poleward motion of the Insular superterrane (western Canadian Cordillera and southeast Alaska) relative to North America. The samples studied are from the stratiform zoned ultramafic body at Duke Island, which intruded rocks of the Alexander terrane at the south end of the southeastern Alaska archipelago at about 110 Ma. Thermal and alternating field demagnetization experiments show that the characteristic remanence of the ultramafic rocks has high coercivity and a narrow unblocking temperature range just below the Curie temperature of magnetite. This remanence is likely carried by low-Ti titanomagnetite exsolved within clinopyroxene and perhaps other silicate hosts. The Duke Island intrusion exhibits a well-developed gravitational layering that was deformed during initial cooling (but below 540°C) into folds that plunge moderately to the west-southwest. The characteristic remanence clearly predates this early folding and is therefore primary; the Fisher parameter describing the concentration of the overall mean remanence direction improves from 3 to 32 when the site-mean directions are corrected by restoring the layering to estimated paleohorizontal. All samples exhibit a magnetic anisotropy that is strong but nonuniform in orientation across the intrusion, and we show that it has no significant or systematic effect on the site-mean directions of remanence. At least some of the anisotropy derives from secondary magnetite formed during partial serpentinization. The mean paleomagnetic inclination (56°±10°) corroborates paleomagnetic results from five coeval silicic plutons of the Canadian Coast Plutonic Complex to the south and southeast and implies 3000 km (±1300 km) of poleward transport relative to the North American craton. Between mid-Cretaceous and middle Eocene time, the Insular superterrane and Coast Plutonic Complex shared a common paleolatitude history, with more poleward transport than coeval inboard terranes.

Climatic and tectonic controls on sedimentation and erosion during the pliocene-quaternary in the qaidam basin (China)

The Qaidam Basin is an internally drained basin located in the northeastern Tibetan Plateau. Presently, over 50% of the basin floor exposes thick (>1 km) sections of Pliocene–Quaternary strata that are deformed by folding and faulting. We investigated this nearly continuous Pliocene–Quaternary sedimentary record for the effects of global climate change and deformation on basin sedimentation. New detailed stratigraphic, magnetostratigraphic, and stable isotope (δ 18 O, δ 13 C) data from the Pliocene Shizigou and Pleistocene Qigequan Formations along the southwestern flank of an intrabasin fold within the north-central Qaidam Basin are presented here. Strata reveal climatically controlled, meter-scale parasequences within shallow-lacustrine, marginal-lacustrine, and deltaic lithofacies. Paleocurrents shift from eastward at the base to southwestward for the majority of the section, but they abruptly shift toward the south-southeast in the upper 400 m. Twenty-two magnetozones constrain deposition between 5.2 Ma and ca. 0.8 Ma and reveal that sedimentation rates were fairly constant (474 ± 34 m/m.y.) from 5.2 to 3.0 Ma, after which time rates abruptly decreased to 154 ± 40 mm/yr before increasing again to ∼750 m/m.y. since 1.2 Ma. The δ 18 O values shift from relatively constant values (avg. −6.8‰, range −9.6‰ to −4.5‰ relative to Vienna Peedee belemnite [VPDB]) to less negative values (avg. -1.2‰, range -1.2‰ to -2.7‰ VPDB) between 3.1 and 2.6 Ma and to extremely variable values (avg. –2.9‰, range −8.3‰ to 4.0‰) after ca. 2.6 Ma. The post–2.6 Ma extreme variability in stable isotopes reflects the same timeframe of enhanced climatic cyclicity associated with Northern Hemisphere glaciation. The δ 13 C values remain relatively constant (average −4.0‰, range −5.7‰ to -1.0‰) until ca. 0.9 Ma, when the values increase to -0.3‰ (range -1.0‰ to 1.5‰) VPDB. The appearance of growth strata at 3.0 Ma, shallow-water, evaporite deposition after 2.6 Ma, and the observation of paleoyardangs (buried, wind-sculpted landforms) within lake-marginal strata at 2.4 Ma imply that emergence of the adjacent anticline was followed by the shallowing and partitioning of the lake basin and subaerial exposure and erosion of marginal lake sediments. These data reflect a significant change to a more arid climate in the Qaidam Basin between ca. 3.1 Ma and 2.6 Ma, overlapping with the onset of significant Northern Hemisphere glaciations and basin-floor deformation. Lake-level cycles were on ∼230,000 yr frequency from 2.6 Ma to 1.2 Ma, before increasing to ∼12,000 yr frequency, suggesting increased aridity and broad, subaerial exposure of lake sediments after the Pliocene-Quaternary transition. Deformation of intrabasin sediments by at least 3.0 Ma caused uplift of the basin floor, which provided an environment rich in friable material for wind deflation of the Qaidam Basin, a likely source for sediments on the Chinese Loess Plateau and nutrients for the Pacific Ocean carried by westerly winds. By 2.6 Ma, deformation of the Qaidam Basin created closed depressions that facilitated evaporite sedimentation that continues today. Coeval intrabasin deformation, combined with increasing aridity after 3.1 Ma, thus controlled both deposition and erosion within the region.

Geomagnetic field behavior before and after the Kauai reverse-normal polarity transition

New paleomagnetic results from 4 m.y. old lava flows from Kauai, Hawaii, suggest that strong poloidal field is associated with an unusual state of the geodynamo that follows attempts at polarity reversal (successful or not). The new data comprise 50 paleomagnetic sites from superposed lava flows occurring just below and above the Kauai reverse-normal polarity transition. A composite record of 45 distinct field determinations was constructed by combining sites that record similar ancient field and correlating them to previously published results from Kauai. Of the 45 data, 25 include paleointensity estimates derived from double-heating experiments. A comparison of the composite record from Kauai with two similar data sets from volcanic sequences shows that field variability (in direction, intensity, or both) can change substantially across a polarity transition. These changes, however, do not appear to be systematic in sign or magnitude. The only feature common to all three records is high field strength in the posttransitional interval, interpreted here as a transient phenomenon associated with the reversal process.

Paleogeography of the Insular and Intermontane terranes reconsidered: Evidence from the southern Coast Mountains Batholith, British Columbia

New geologic and paleomagnetic data from Knight Inlet in the southwestern Coast Mountains Batholith, British Columbia, support signifi cant revision to the paleogeography of the Insular and Intermontane terranes. Recompilation of radiometric ages confi rms that after 100 Ma, a magmatic arc migrated northeastward across the Coast Mountains Batholith at ~2 km/m.y. Magmatic age patterns suggest that plutons older than 100 Ma intruded the Intermontane terrane, not the expected Insular terrane. The distribution of brittle faults along Knight Inlet defi nes a structurally intact central domain, ~45 km wide, fl anked to the SW and NE by faulted domains, with no evidence of the widespread Tertiary extension affecting the batholith farther north. Al-in-hornblende geobarometry yields emplacement depths of ~2.5‐4 kbar and does not reveal systematic postemplacement tilting. Plutons in the central structural domain yield a consistently oriented paleomagnetic remanence presumably acquired as the Late Cretaceous arc cooled from ca. 110 to 85 Ma. In the absence of recognizable tilting, this result indicates ~1700 km of northward translation since ca. 85 Ma, which is signifi cantly less than predicted for the Insular terrane in the “Baja British Columbia” model but similar to results from the Intermontane terrane. The pluton ages and the paleomagnetic results suggest that the Intermontane terrane, not the Insular terrane, underlies the southwestern fl ank of the Coast Mountains Batholith. This conclusion is compatible with a paleogeographic model in which the Vancouver Island fragment of Wrangellia was juxtaposed against the Intermontane terrane prior to ca. 120‐100 Ma and emplaced in southern British Columbia after ca. 75 Ma.

Deformation of continental crust along a transform boundary, Coast Mountains, British Columbia

New structural, paleomagnetic, and apatite (U-Th)/He results from the continental margin inboard of the Queen Charlotte fault (~54°N) delineate patterns of brittle faulting linked to transform development since ~50 Ma. In the core of the orogen, ~250 km from the transform, north striking, dip-slip brittle faults and vertical axis rotation of large crustal domains occurred after ~50 Ma and before intrusion of mafic dikes at 20 Ma. By 20 Ma, dextral faulting was active in the core of the orogen, but extension had migrated toward the transform, continuing there until <9 Ma. Local tilting in the core of the orogen is associated with glacially driven, post-4 Ma exhumation. Integration with previous results shows that post-50 Ma dextral and normal faulting affected a region ~250 km inboard of the transform and ~300 km along strike. Initially widespread, the zone of active extension narrowed and migrated toward the transform ~25 Ma after initiation of the transform, while dextral faulting continued throughout the region. Differential amounts of post-50 Ma extension created oroclines at the southern and northern boundaries of the deformed region. This region approximately corresponds to continental crust that was highly extended just prior to transform initiation. Variation in Neogene crustal tilts weakens interpretations relying on uniform tilting to explain anomalous paleomagnetic inclinations of mid-Cretaceous plutons. Similarities to the Gulf of California suggest that development of a transform in continental crust is aided by previous crustal extension and that initially widespread extension narrows and moves toward the transform as the margin develops.

Erg deposition and development of the ancestral Taklimakan Desert (western China) between 12.2 and 7.0 Ma

The Taklimakan Desert in western China contains the second largest shifting sand desert on earth. The onset of this desert formation has been debated between the Eocene, early Miocene, late Miocene, or Pliocene, with each hypothesis having profound implications for the climatic and tectonic evolution of this region. We provide stratigraphic evidence for desert formation based on a new 3800-m-thick stratigraphic section in the northwestern Tarim Basin. Magnetostratigraphy defines 50 magnetozones and constrains the age of these strata to between ca. 15.1 and 1.5 Ma. Fluvial and lacustrine strata at the base of the section change abruptly to eolian sandstone (~1100 m thick) at 12.2 Ma and persist until 7.0 Ma, implying development of an erg system that represents the ancestral Taklimakan Desert. The appearance of sand dunes at 12.2 Ma has no global climate parallel, and resulted from aridification in the rain-shadow behind a growing Tian Shan and Pamir that isolated the Tarim Basin.

Directional change during a Miocene R‐N geomagnetic polarity reversal recorded by mafic lava flows, Sheep Creek Range, north central Nevada, USA

Recurring transitional field directions during three Miocene geomagnetic reversals provide evidence that lateral inhomogeneity of the lower mantle affects flow in the outer core. We compare new paleomagnetic results from a composite sequence of 15.2 Ma lava flows in north central Nevada (Sheep Creek Range; 40.7N, 243.2E), erupted during a polarity reversal, to published data from Steens Mountain (250 km to the northwest in Oregon) and the Newberry Mountains (650 km to the south in California) that document reversals occurring millions of years and many polarity switches earlier. Alternating field demagnetization, followed by thermal demagnetization in half the samples, clearly isolated the primary thermoremanent magnetization of Sheep Creek Range flows. We correlated results from our three sampled sections to produce a composite record that begins with a single virtual geomagnetic pole (VGP) at low latitude in the Atlantic, followed by two VGPs situated near latitude 30N in NE Africa. After jumping to 83N (one VGP), the pole moves to equatorial South America (one VGP), back to NE Africa (three VGPs), to high southern latitudes (two VGPs), back to equatorial South America (three VGPs), and finally to high northern latitudes (nine VGPs). The repeated visits of the transitional VGP to positions in South America and near NE Africa, as well as the similar behavior recorded at Steens Mountain and the Newberry Mountains, suggest that lower mantle or core-mantle boundary features localize core flow structures, thereby imparting a discernible regional structure on the transitional geomagnetic field that persists for millions of years.