Daniel R. Lux

Simultaneous Miocene Extension and Shortening in the Himalayan Orogen

The South Tibetan detachment system separates the high-grade metamorphic core of the Himalayan orogen from its weakly metamorphosed suprastructure. It is thought to have developed in response to differences in gravitational potential energy produced by crustal thickening across the mountain front. Geochronologic data from the Rongbuk Valley, north of Qomolangma (Mount Everest) in southern Tibet, demonstrate that at least one segment of the detachment system was active between 19 and 22 million years ago, an interval characterized by large-scale crustal thickening at lower structural levels. These data suggest that decoupling between an extending upper crust and a converging lower crust was an important aspect of Himalayan tectonics in Miocene time.

Thermal modelling in low-pressure/high-temperature metamorphic belts

Abstract Low-pressure/high-temperature metamorphic belts occur throughout the world. They display a characteristic metamorphic style which has remained unchanged with time. They are found in disparate tectonic settings, including magmatic arcs, regions of extension, regions of thickened crust in continent-continent collision zones, and accretionary wedges. They are commonly characterized by an abundance of granitoid plutons. Pressures of metamorphism are primarily less than or equal to that of the aluminosilicate triple point (approximately 4 kbar), while temperatures are in the range 500–750 °C. With rare exceptions, inferred PTt paths are nearly isobaric and recrystallization was rapid and prograde. Metamorphic thermal gradients are commonly in the range 60–150 °C/km. These conditions necessitate the presence of a large component of advective heat transfer and/or anomalously high basal heat flow during metamorphism. The large magmatic fluxes associated with the formation of magmatic arcs provide an adequate heat source for low pressure metamorphism (LPM). Lithospheric extension is likely to produce LPM only if the degree of thinning is large ( β > 3) and there is additional heat from magmas. The generation of LPM by crustal thickening requires either a large component of advective heating by magmas, anomalously high basal heat flow, or pre-thickening lithospheric extension. LPM in this environment must necessarily be accompanied by plutonic activity. Advection of heat by aqueous fluids may provide a significant contribution to the thermal budget but, acting along, is unlikely to produce LPM. Regional LPM induced by advective sources, rarely occurs along a geotherm which increases monotonically with depth, or synchronously across the region at a given depth; it occurs over time as a series of overlapping temporally separate and spatially localized thermal events.

A structural analysis of the Main Central Thrust zone, Langtang National Park, central Nepal Himalaya

The Main Central Thrust (MCT) is one of the most tectonically significant structures in the Himalayan orogen. Detailed geologic mapping and structural analysis of the MCT in the Langtang National Park region of central Nepal reveals that this segment of the fault zone experienced multiple episodes of south-directed movement, under both brittle and ductile conditions, during the Tertiary period. Early (mid-Miocene) movement resulted in the development of mylonitic fabrics synchronous with amphibolite-facies metamorphism. The mean orientation of the dominant mylonitic foliation is N28°W, 38°NE. An associated mineral/stretching lineation plunges 40° to N40°E. Kinematic indicators suggest hanging-wall movement to the southwest relative to the footwall along the north-dipping fault. It is not possible to constrain the magnitude of high-temperature displacement on the MCT at the longitude of Langtang. Late-stage structures in the MCT zone at Langtang include a series of imbricate, brittle thrust faults that separate different lithostratigraphic units and correspond to metamorphic discontinuities. We interpret this fault system as a duplex structure. Muscovite 40 Ar/ 39 Ar cooling ages from the MCT zone range from 8.9-6.9 Ma. Because the nominal closure temperature of Ar diffusion in muscovite (approximately 625 K) is higher than the apparent temperature conditions under which late brittle deformation occurred, we suggest that brittle deformation was a latest Miocene-Pliocene phenomenon. Another major Himalayan fault, the Main Boundary Thrust (MBT), was developing to the south of Langtang at approximately the same time. We speculate that brittle faulting within the MCT zone may have initiated as the MCT zone was transported over a ramp in the MBT.

Helium and argon thermochronometry of the Gold Butte block, south Virgin Mountains, Nevada

One of the largest exposures of Precambrian crystalline rock in the Basin and Range province of the southwestern USA is the Gold Butte block of the south Virgin Mountains, about 15 km west of the Colorado Plateau. It has been interpreted as a largely continuous crustal cross-section about 15–20 km thick that was exhumed by a deeply penetrating normal fault during Miocene extension. To test this interpretation as well as the use of the newly developed titanite (U–Th)/He thermochronometer, we examined the low temperature thermal history of the Gold Butte block with the apatite and titanite (U–Th)/He and muscovite ^(40)Ar/^(39)Ar thermochronometers. Apatite He ages average 15.2±1.0 (2σ) Ma throughout the block, indicating that the entire section was warmer than 70°C prior to Miocene exhumation. Titanite He ages increase from 18.6±1.5 Ma near the paleobottom (west) end of the block, to 195±15 Ma near the paleotop (east) end. A rapid change from mid-Tertiary to increasingly older titanite He ages to the east is observed at about 9.3 km paleodepth, and is interpreted as a fossil He partial retention zone for titanite. Assuming a pre-exhumation geotherm of 20°C/km (consistent with earlier apatite fission track work), this depth would have corresponded to 196°C prior to exhumation, indicating that laboratory-derived He diffusion characteristics for titanite that yield a closure temperature of about 200°C are applicable and correct. Muscovite ^(40)Ar/^(39)Ar ages are 1.0–1.4 Ga near the paleotop of the block, and 90 Ma near the paleobottom. Together with ^(207)Pb/^(206)Pb ages on apatite and titanite, and an earlier apatite fission track transect across the Gold Butte block, our data indicate that the continental crust at the western edge of the Colorado Plateau resided at moderate geothermal gradients (and slowly declined in temperature) from 1.4 Ga to about 100–200 Ma. A 90 Ma cooling event clearly affected the mid-crust (deepest portions of Gold Butte), which may reflect accelerated cooling or a brief heating and cooling cycle at this time, after which gradients returned to about 20°C/km prior to rapid exhumation in the Miocene. This work thus supports previous structural and thermochronologic studies that suggest that the Gold Butte block is the thickest largely continuous cross-section of crust exposed in the southwestern USA.

Dating mylonitic deformation by the 40Ar-39Ar method: an example from the Norumbega Fault Zone, Maine

Strongly mylonitic rocks associated with the regionally extensive Norumbega fault zone in south-central Maine provide an excellent opportunity for testing the effects of mylonitization on argon isotopic systems in muscovite. 40Ar/39Ar muscovite age spectra from samples outside the zone of mylonitization are relatively undisturbed and have well defined Early Carboniferous plateau ages. In contrast to these nonmylonitized samples, all age spectra for muscovite from the mylonites are highly discordant. They are characterized by young ages at low extraction temperatures, which systematically increase to ages that equal the plateau ages for muscovite collected outside the mylonite zone. Detailed petrographic observations suggest that these systematic discordances reflect a mixing of argon components from older, relict, muscovite porphyroclasts and fine-grained white mica aggregates that recrystallized during mylonitic deformation. Total gas ages of five different grain size fractions separated from the same mylonite sample become progressively younger with decreasing grain size; indicating a larger component of the recrystallized grains in the finer grain size fractions. Although the three finest grain size fractions give different total gas ages and do not overlap in age for most of their release spectra, their initial increments do coincide, at approximately 290 Ma. This indicates a minimal older age contribution from the relict porphyroclasts in the initial increments and suggests the 290 Ma age provides a good estimate for the time of mineral growth associated with mylonitic deformation. These data, combined with kinematic analysis, reveal that the segment of the Norumbega fault zone studied, the Sandhill Corner fault, is a Late Carboniferous-Early Permian dextral strike-slip fault. A lack of significant offset in regional Early Carboniferous mineral age patterns across the fault suggests that displacement was probably less than 30 km. This study demonstrates that 40Ar/39Ar dating methods can be used to date deformational events effectively, as long as several important criteria are met. First and foremost, samples must be well characterized prior to analysis. Dynamic recrystallization must have occurred at or below the closure temperature of the mineral to be analyzed. Regional cooling patterns must also be established through detailed thermochronology so that mineral ages and age spectra from the deformed rocks can be compared to regional cooling ages of the same mineral. Finally, the effects of excess argon must be negligible.

Pliocene Paleoclimate and East Antarctic Ice-Sheet History from Surficial Ash Deposits

The preservation, age, and stratigraphic relation of an in situ ashfall layer with an underlying desert pavement in Arena Valley, southern Victoria Land, indicate that a cold-desert climate has persisted in Arena Valley during the past 4.3 million years. These data indicate that the present East Antarctic Ice Sheet has endured for this time and that average temperatures during the Pliocene in Arena Valley were no greater than 3�C above present values. One implication is that the collapse of the East Antarctic Ice Sheet due to greenhouse warming is unlikely, even if global atmospheric temperatures rise to levels last experienced during mid-Pliocene times.

Minimal Pliocene-Pleistocene uplift of the dry valleys sector of the Transantarctic Mountains: A key parameter in ice-sheet reconstructions

We propose that there has been little or no Pliocene-Pleistocene uplift of the dry valleys sector of the Transantarctic Mountains, on the basis of isotopic dating and mapping of in situ cinder-cone deposits on the walls of Taylor Valley. Twenty-seven 40Ar/39Ar incremental heating analyses on whole-rock samples from subaerially erupted olivine basanite volcanic outcrops of known elevations define 14 eruptions ranging in age from 3.89 to 1.50 Ma. Because Taylor Valley opens directly onto the Ross Sea, these results show that any surface uplift during the past 2.57 m.y. was <300 m. Our conclusion of minimal uplift contradicts previous models of dramatic uplift (up to 3000 m since 3 Ma) and associated hypotheses that uplift caused climatic cooling and growth of a polar East Antarctic ice sheet.

Sm-Nd dating of multiple garnet growth events in an arc-continent collision zone, northwestern U.S. Cordillera

Integrated petrologic and Sm−Nd isotopic studies in garnet amphibolites along the Salmon River suture zone, western Idaho, delineate two periods of amphibolite grade metamorphism separated by at least 16 million years. In one amphibolite,P−T studies indicate a single stage of metamorphism with final equilibration at ∼600°C and 8–9 kbar. The Sm−Nd isotopic compositions of plagioclase, apatite, hornblende, and garnet define a precise, 8-point isochron of 128±3 Ma (MSWD=1.2) interpreted as mineral growth at the metamorphic peak. A40Ar/39Ar age for this hornblende indicates cooling through ∼525°C at 119±2 Ma. In a nearby amphibolite, garnets with a two-stage growth history consist of inclusion-rich cores surrounded by discontinuous, inclusion-free overgrowths. Temporal constraints for core and overgrowth development were derived from Sm−Nd garnet — whole rock pairs in which the garnet fractions consist of varying proportions of inclusion-free to inclusion-bearing fragments. Three garnet fractions with apparent “ages” of 144, 141, and 136 Ma are thought to represent mixtures between late Jurassic (pre-144 Ma) inherited radiogenic components preserved within garnet cores and early Cretaceous (∼128 Ma) garnet overgrowths. These observations confirm the resilience of garnet to diffusive exchange of trace elements during polymetamorphism at amphibolite facies conditions. Our geochronologic results show that metamorphism of arc-derived rocks in western Idaho was episodic and significantly older than in arc rocks along the eastern margin of the Wrangellian Superterrane in British Columbia and Alaska. The pre-144 Ma event may be an expression of the late Jurassic amalgamation of marginal oceanic arc-related terranes (e.g., Olds Ferry, Baker, Wallowa) during the initial phases of their collision with North American rocks. Peak metamorphism at ∼128 Ma reflects tectonic burial along the leading edge of the Wallowa arc terrane during its final penetration and suturing to cratonic North America.

Thermal and barometric constraints on the intrusive and unroofing history of the Black Mountains: Implications for timing, initial dip, and kinematics of detachment faulting in the Death Valley Region, California

Unroofing of the Black Mountains, Death Valley, California, has resulted in the exposure of 1.7 Ga crystalline basement, late Precambrian amphibolite facies metasedimentary rocks, and a Tertiary magmatic complex. The 40Ar/39Ar cooling ages, obtained from samples collected across the entire length of the range (>55 km), combined with geobarometric results from synextensional intrusions, provide time-depth constraints on the Miocene intrusive history and extensional unroofing of the Black Mountains. Data from the southeastern Black Mountains and adjacent Greenwater Range suggest unroofing from shallow depths between 9 and 10 Ma. To the northwest in the crystalline core of the range, biotite plateau ages from ∼13 to 6.8 Ma from rocks making up the Death Valley turtlebacks indicate a midcrustal residence (with temperatures >300°C) prior to extensional unroofing. Biotite 40Ar/39Ar ages from both Precambrian basement and Tertiary plutons reveal a diachronous cooling pattern of decreasing ages toward the northwest, subparallel to the regional extension direction. Diachronous cooling was accompanied by dike intrusion which also decreases in age toward the northwest. The cooling age pattern and geobarometric constraints in crystalline rocks of the Black Mountains suggest denudation of 10–15 km along a northwest directed detachment system, consistent with regional reconstructions of Tertiary extension and with unroofing of a northwest deepening crustal section. Mica cooling ages that deviate from the northwest younging trend are consistent with northwestward transport of rocks initially at shallower crustal levels onto deeper levels along splays of the detachment. The well-known Amargosa chaos and perhaps the Badwater turtleback are examples of this “splaying” process. Considering the current distance of the structurally deepest samples away from moderately to steeply east tilted Tertiary strata in the southeastern Black Mountains, these data indicate an average initial dip of the detachment system of the order of 20°, similar to that determined for detachment faults in west central Arizona and southeastern California. Beginning with an initially listric geometry, a pattern of footwall unroofing accompanied by dike intrusion progresses northwestward. This pattern may be explained by a model where migration of footwall flexures occur below a scoop-shaped hanging wall block. One consequence of this model is that gently dipping ductile fabrics developed in the middle crust steepen in the upper crust during unloading. This process resolves the low initial dips obtained here with mapping which suggests transport of the upper plate on moderately to steeply dipping surfaces in the middle and upper crust.

The tectonic significance of pre-Scandian 40Ar/39Ar phengite cooling ages in the Caledonides of western Norway

Pre-Silurian continental-margin deposits in western Norway, non-conformably overlying allochthonous continental orthogneisses retain Ordovician 40Ar/39Ar cooling ages for phengites, implying either rapid cooling immediately after a Late Ordovician orogenic event, or less likely, a slow cooling following an Early Ordovician or older orogeny. The Dalsfjord Suite–Høyvik Group basement–cover pair are probably a lateral equivalent to Late Proterozoic sandstones (‘sparagmites’) covering the Jotun Nappe gneisses of the Middle Allochthon in central-south Norway. The Høyvik Group underwent polyphase deformation, greenschist-facies metamorphism (Tmax<450°C) and exhumation prior to deposition of the unconformably overlying Wenlockian continental-margin deposits of the Herland Group. The Høyvik Group was only weakly metamorphosed during obduction of the Solund–Stavfjord Ophiolite and the Scandian continental collision between Baltica and Laurentia. Phengitic white micas from the Høyvik Group yield cooling ages of 446.1± 3.0, 449.1±2.2 and 447.5±4.0 Ma, respectively, identical within experimental error. One sample gives a plateau over 72% of the gas analysed, whereas the other samples were slightly disturbed after initial cooling, as indicated by systematically lower apparent ages at low experimental extraction temperatures. Minor 40Ar loss probably occurred during subsequent Scandian deformation and late to post-orogenic extension.The Høyvik Group rocks were unroofed before the Wenlock time (423–428 Ma) and cooled through the temperature for argon retention in phengite at c. 447±4 Ma, indicating a maximum cooling rate between 14 and 22C/Ma-1 through Ashgill and Llandovery times before being subjected to low-grade metamorphism during the Scandian orogeny. Rapid pre-Scandian cooling, combined with peak metamorphic conditions of 450C or less, may indicate that the Dalsfjord–Høyvik basement–cover pair were aVected by an orogenic event during the Late Ordovician (Caradoc) time. The data also suggest that the Caledonian margin of Baltica may have experienced a more protracted tectonism during the Caledonian cycle than previously models focusing on Early Caledonian and Tremadoc (or older) ophiolite obduction and the Scandian continental collision between Baltica and Laurentia.