High-elevation Plateau Research Articles

Estimating Xanthoparmelia (Parmeliaceae) population density in subalpine communities in southern Utah, U.S.A. using two distance methods, with implications for assessing community composition

Lichens are often used to monitor air quality and ecosystem health, and to define ecological gradients. Quantitative methods for estimating lichen population density and community composition are essential to effectively assess species and community response to ecological and environmental change. In this study we used two distance methods (point-centered quarter and quartered-neighbor methods), along with a compound estimator (Diggle's) to remove bias from non-random spatial patterns, to estimate Xanthoparmelia population density at a generic level on the summit of a high elevation plateau in southern Utah, USA. The sampling was extended to assess species composition and diversity. Population densities and relative species abundance were evaluated between meadow areas, sites, cardinal directions, and general substrate classes. We found that Xanthoparmelia populations varied widely in density and community composition across a relatively homogeneous landscape. Microhabitat, meadow-wide, and plateau-wide factors significantly influenced density and community structure in Xanthoparmelia. Our data reemphasize the inherent variability found in lichen communities and the importance of effective sampling for accurate ecological and biomonitoring studies. The two distance methods implemented here, moderated by Diggle's population density estimator, provide an efficient alternative to cover methods for estimating density. This study also indicates that these methods provide an effective approach for estimating population density and community structure in conjunction with ecological and bio-monitoring studies of some types of terricolous and saxicolous lichen communities.

Denudational response to surface uplift in east Tibet: Evidence from apatite fission-track thermochronology

East Tibet preserves broad areas of low relief that occur at elevations >3000 m and are dissected by major strike-slip faults that have been repeatedly reactivated since the late Mesozoic. Apatite fission-track samples from these low-relief, high-elevation surfaces indicate Cretaceous cooling (ca. 90 Ma). Fast rock uplift of the 110 °C isotherm and a fossilized partial annealing zone began in the plateau interior of east Tibet in the Miocene. Low denudation rates in the plateau interior during the Cenozoic, which are constrained by thermal history modeling and a consistency in ages from apatite fission tracks, suggest that dip-slip displacement on faults has been minor. Some localized areas have experienced the effects of structurally enhanced rock uplift and denudation or fault-related hydrothermal reheating. Cooling ages in Eocene sediments deposited on the older plateau surface suggest that rapid denudation occurred from the Miocene to Holocene. Other evidence of significant Cenozoic denudation comes from river valleys, in particular, the Yalong River valley, where incision was initiated in the Oligocene to Miocene (28–12 Ma). Estimated mean denudation in east Tibet during the Cenozoic was ~1–2 km in the low-relief, high-elevation plateau interior and at least 5 km at the high-relief plateau margin in the Longmen Shan. The region south of the Longmen Shan and the Sichuan Basin was not uplifted with the main Tibetan Plateau in the Miocene, but it has undergone enhanced denudation since the Eocene. Due to an orographic effect, most of the denudation in the east Tibetan Plateau interior has occurred in major river valleys. Incision in the major rivers of the Longman Shan occurred simultaneously and has kept pace with surface uplift that began in the middle–late Miocene (12 Ma), with acceleration of denudation rates in the late Miocene (>5 Ma).

Plateau collapse model for the Transantarctic Mountains–West Antarctic Rift System: Insights from numerical experiments

The high elevation and considerable length of the Transantarctic Mountains have led to speculation about their origin. To date, no model has been able to adequately reconcile the juxtaposition of the high, curvilinear Transantarctic Mountains with the adjacent West Antarctic Rift System, a broad region of thin extended continental crust exhibiting wide rift characteristics. We present a fi rst-order investigation into the idea that the West Antarctic Rift System‐ Transantarctic Mountains region was a high-elevation plateau with thicker than normal crust before the onset of continental extension. With major Cretaceous extension, the rift underwent a topographic reversal, and a plateau edge with thickened crust, representing the ancestral Transantarctic Mountains, remained. In the Cenozoic, minor extension and major denudation reduce the crustal root while simultaneously uplifting peak heights in the mountains. The Cretaceous stage of this concept is investigated using two-dimensional numerical models to determine under what conditions plateau collapse is plausible. Model results indicate that elevation of a remnant plateau edge decreases with increasing initial Moho temperature. Very cold initial Moho temperatures, <675 °C, under the plateau leave a thick plateau edge but do not exhibit wide rifting. A cold to moderate initial thermal structure, Moho temperatures of 675‐850 °C, is needed to retain the plateau edge and still exhibit wide rifting in the middle of the plateau. We conclude that this plateau collapse concept is possible using these numerical experiments, and that application of this idea to the West Antarctic Rift System‐Transantarctic Mountains system is also supported by geological and geophysical evidence.

Constraining landscape development of the Sri Lankan escarpment with cosmogenic nuclides in river sediment

Escarpments are prominent morphological features along high-elevation passive margins. Recent studies integrating geomorphology, thermochronology, and cosmogenic nuclide-based denudation rate estimates suggest a rapid phase of denudation immediately after the earliest stages of seafloor spreading, and subsequent slow denudation rates since. To constrain the geomorphic evolution of passive margins, we have examined the development of the Sri Lankan escarpment. Cosmogenic nuclide data on river sediment along a north–south transect across the southern escarpment reveal that the landscape is eroding ten times more rapidly in the escarpment zone (26 to 71 mm kyr − 1 ) than in the high-elevation plateau above it and in the lowland plain beneath it (2.6 to 6.2 mm kyr − 1 ). Unlike these low denudation rate areas, the escarpment denudation is strongly and linearly hill slope-dependent. This shows that denudation and retreat are tightly interlinked within the escarpment, which suggests that the escarpment is evolving by rift-parallel retreat, rather than by escarpment downwearing. Supporting evidence is provided by the morphology of rivers draining the escarpment zone. These have steep bedrock channels which show sharp and prominent knickpoints along their longitudinal profiles. It appears that fluvial processes are driving escarpment retreat, as rivers migrate headwards were they incise into the high-elevation plateau. However, the average catchment-wide denudation rates of the escarpment zone are low compared to the denudation rates that are estimated for constant escarpment retreat since rifting. In common with other escarpments worldwide, causes for this slow down can be tectonic change related to flexural bending of the lithosphere, climate change that would vary the degree of precipitation focused into the escarpment, or the decrease in the contributing catchment area, which would reduce the stream power available for fluvial erosion.

Crustal balance and crustal flux from shortening estimates in the Central Andes

The Central Andes of South America form the second largest high elevation plateau on earth. Extreme elevations have formed on a noncollisional margin with abundant associated arc magmatism. It has long been thought that the crustal thickness necessary to support Andean topography was not accounted for by known crustal shortening alone. We show that this may in part be due to a two-dimensional treatment of the problem. A three-dimensional analysis of crustal shortening and crustal thickness shows that displacement of material towards the axis of the bend in the Central Andes has added a significant volume of crust not accounted for in previous comparisons. We find that present-day crustal thickness between 12°S and 25°S is accounted for (∼−10% to ∼+3%)with the same shortening estimates, and the same assumed initial crustal thickness as had previously led to the conclusion of a ∼25–35% deficit in shortening relative to volume of crustal material. We suggest that the present-day measured crustal thickness distribution may not match that predicted due to shortening, and substantial redistribution of crust may have occurred by both erosion and deposition at the surface and lower crustal flow in regions of the thermally weakened middle and lower crust.

Late Jurassic to Eocene evolution of the Cordilleran thrust belt and foreland basin system, western U.S.A.

Geochronological, structural, and sedimentological data provide the basis for a regional synthesis of the evolution of the Cordilleran retroarc thrust belt and foreland basin system in the western U.S.A. In this region, the Cordilleran orogenic belt became tectonically consolidated during Late Jurassic time (∼155 Ma) with the closure of marginal oceanic basins and accretion of fringing arcs along the western edge of the North American plate. Over the ensuing 100 Myr, contractile deformation propagated approximately 1000 kilometers eastward, culminating in the formation of the Laramide Rocky Mountain ranges. At the peak of its development, the retroarc side of the Cordillera was divided into five tectonomorphic zones, including from west to east the Luning-Fencemaker thrust belt; the central Nevada (or Eureka) thrust belt; a high-elevation plateau (the “Nevadaplano”); the topographically rugged Sevier fold-thrust belt; and the Laramide zone of intraforeland basement uplifts and basins. Mid-crustal rocks beneath the Nevadaplano experienced high-grade metamorphism and shortening during Late Jurassic and mid- to Late Cretaceous time, and the locus of major, upper crustal thrust faulting migrated sporadically eastward. By Late Cretaceous time, the middle crust beneath the Nevadaplano was experiencing decompression and cooling, perhaps in response to large-magnitude ductile extension and isostatic exhumation, concurrent with ongoing thrusting in the frontal Sevier belt. The tectonic history of the Sevier belt was remarkably consistent along strike of the orogenic belt, with emplacement of regional-scale Proterozoic and Paleozoic megathrust sheets during Early Cretaceous time and multiple, more closely spaced, Paleozoic and Mesozoic thrust sheets during Late Cretaceous–Paleocene time. Coeval with emplacement of the frontal thrust sheets, large structural culminations in Archean-Proterozoic crystalline basement developed along the basement step formed by Neoproterozoic rifting. A complex foreland basin system evolved in concert with the orogenic wedge. During its early and late history (∼155 - 110 Ma and ∼70 - 55 Ma) the basin was dominated by nonmarine deposition, whereas marine waters inundated the basin during its midlife (∼110 - 70 Ma). Late Jurassic basin development was controlled by both flexural and dynamic subsidence. From Early Cretaceous through early Late Cretaceous time the basin was dominated by flexural subsidence. From Late Cretaceous to mid-Cenozoic time the basin was increasingly partitioned by basement-involved Laramide structures. Linkages between Late Jurassic and Late Cretaceous Cordilleran arc-magmatism and westward underthrusting of North American continental lithosphere beneath the arc are not plainly demonstrable from the geological record in the Cordilleran thrust belt. A significant lag-time (∼20 Myr) between shortening and coeval underthrusting, on the one hand, and generation of arc melts, on the other, is required for any linkage to exist. However, inferred Late Jurassic lithospheric delamination may have provided a necessary precondition to allow relatively rapid Early Cretaceous continental underthrusting, which in turn could have catalyzed the Late Cretaceous arc flare-up.

Speculation on the timing and nature of Late Pleistocene hunter-gatherer colonization of the Tibetan Plateau

Hunter-gatherer populations in greater north- east Asia experienced dramatic range expansions during the early Upper Paleolithic (4522 ka) and the late Upper Pa- leolithic (1810 ka), both of which led to intensive occupa- tions of cold desert environments including the Mongolian Gobi and northwest China. Range contractions under the cold, arid extremes of the Last Glacial Maximum (LGM, 22 18 ka) may have entailed widespread population extirpa- tions. The high elevation Qinghai-Tibetan Plateau is signifi- cantly more extreme in both climate and environment than either the Gobi or the Siberian taiga forests, and provides an ideal setting to test fundamental models of human biogeog- raphy in the context of regional population fluctuations. The area is presently occupied primarily by nomadic pastoralists, but it is clear that these complex middle Holocene (<6 ka) economic adaptations were not a necessary prerequisite for successful colonization of the high elevation Plateau. Ex- ploratory field-work in 20002001 has established that Up- per Paleolithic hunter-gatherers were present on the Qing- hai-Tibetan Plateau by at least 12 ka and possibly much ear- lier. A speculative model for the colonization process is de- veloped and preliminary archaeological data in support of the model are presented.

Landscape development in western and central Dronning Maud Land, East Antarctica

Large-scale bedrock morphology and relief of two key areas, the Jutulsessen Nunatak and the Jutulstraumen ice stream are used to discuss glascial history and landscape development in western and central Dronning Maud Land, Antarctica. Two main landform components were identified: well-defined summit plateau surfaces and a typical alpine glacial landscape. The flat, high-elevation plateau surfaces previously were part of one or several continuous regional planation surfaces. In western Dronning Maud Land, overlying cover rocks of late Palaeozoic age show that the planation surface(s) existed in the early Permian, prior to the break-up of Gondwana. A well-develoment escarpment, a mega landform typical for passive continental margins, bounds the palaeosurface remnants to the north for a distance of at least 700 km. The Cenozoic glacial landscape, incised in the palaeosurface and escarpment, is exemplified by Jutulsessen Nunatak, where a c. 1.2 km deep glacial valley system is developed. However, the prominent Penck-Jutul Trough represents some of the deepest dissection of the palaeosurface. This originally tectonic feature is today occupied by the Jutulstraumen ice stream. New topographic data show that the bed of the Penck-Jutul Trough is situated 1.9±1.1 km below sea level, and that the total landscape relief is at least 4.2 km. Today's relief is a result of several processes, including tectonic faulting, subaerial weathering, fluvial erosion, and glacial erosion. It is probable that erosion by ice streams has deepened the tectonic troughs of Dronning Maud Land since the onset of ice sheet glaciation in the Oligocene, and continues today. An attempt is made to identify major events in the long-term landscape development of Dronning Maud Land, since the break-up of the Gondwana continent.

Long-term rates of denudation in the Dry Valleys, Transantarctic Mountains, southern Victoria Land, Antarctica based on in-situ-produced cosmogenic [formula omitted

Concentrations of cosmogenic 21 Ne ( 21 Ne c ) measured in quartz have been used to estimate long-term rates of denudation for contrasting landscape components in the Dry Valleys area of the Transantarctic Mountains, southern Victoria Land, Antarctica. Samples of Beacon Supergroup sandstones and granitic basement were collected from two contrasting landscape elements—low-relief, high-elevation surfaces and rectilinear slopes—to assess variations in rates of denudation with topographic position. The sample sites for rectilinear slopes were selected because of proximity to 40 Ar/ 39 Ar-dated lavas and ash-avalanche deposits. All 21 Ne/ 20 Ne ratios are significantly greater than the atmospheric value, and concentrations of 21 Ne c were calculated from the measured 21 Ne values assuming an atmospheric composition for the trapped component. Apparent exposure ages calculated from the concentrations of 21 Ne c , assuming no denudation since exposure, range from 3.78–4.66 Ma for samples from the high-elevation plateau surface, and 0.61–2.48 Ma for samples from the rectilinear slopes. Exposure ages for 3 He c were 2 to 42 times lower than those derived from the abundances of 21 Ne c because of preferential diffusive loss of 3 He c from the quartz lattice; concentrations of 3 He c were, therefore, not used in the calculation of rates of denudation. We interpret the abundances of 21 Ne c as reflecting variations in the rates of denudation rather than exposure age in view of independent evidence for prolonged exposure (>15 Ma) of bedrock surfaces at the sample sites. Calculated maximum rates of denudation range from 0.26–1.02 m Ma −1 for the rectilinear slopes, down to only 0.133–0.164 m Ma −1 for the high-elevation surface sites. These rates are comparable to other estimates of denudation rates for the Dry Valleys derived from analyses of cosmogenic isotopes, but are around two orders of magnitude lower than the long-term mean rate over the past ∼50 Ma estimated from fission-track thermochronology. Combined with the preservation of volcanic deposits dating back to the mid-Miocene, these 21 Ne c data demonstrate that only minimal modification of the landscape has occurred in the Dry Valleys over at least the past ∼15 Ma. This significant conclusion supports the view that the East Antarctic Ice Sheet has been essentially stable over this period rather than experiencing major fluctuations as late as the Pliocene, as has previously been suggested.

A Revision of Blepharoneuron (Poaceae: Eragrostideae)

Blepharoneuron is a North American genus of higher elevations that ranges from Colorado to southern Mexico. It is characterized by spikelets with a single floret, a hairy lemma and palea, and a base chromosome number of x = 8. As revised, it includes two species, B. shepherdii, an annual with lemmas 1.3-1.9 mm long and B. tricholepis, a perennial with lemmas 2-3.8 mm long. Blepharoneuron shepherdii represents a transfer from Muhlenbergia. Historically, Blepharoneuron Nash, s. str., contained a single species, B. tricholepis (Torrey) Nash, that ranged from Colorado to central Mexico. It differs from other eragrostoid genera in having a single floret per spikelet with a lemma that is hairy on the midnerve and margins, a palea that is villous between the nerves, and a base chromosome number of x = 8 (Peterson 1989). Blepharoneuron tricholepis or pine dropseed is a valuable range grass and is often the dominant grass in high elevation plateaus throughout the southern Rocky Mountains. While working on the revision of the annual species of Muhlenbergia Schreb. (Peterson and Annable, in press) and collecting in southwestern Chihuahua, we recognized the morphological similarities between M. shepherdii (Vasey) Swallen and B. tricholepis. These two taxa share the following features: three-nerved lemmas with silky hairs located on the midnerve and margins, grayish-green spikelets with subequal glumes less than or equal to the floret, erect to nodding or reflexed spikelets on capillary pedicels that are granular just below the spikelet, paleas that are villous between the nerves, strongly ribbed leaf blades, and a chromosome number of n = 8 (Peterson 1988). In western Chihuahua and northwestern Durango these two species are sympatric and sometimes occur in mixed populations. Individuals of M. shepherdii are usually found in dense populations occupying an area of less than 1/3 hectare, whereas individuals of B. tricholepis are more widely spaced and can occupy many square kilometers. There are four unawned annual species of Muhlenbergia [M. annua (Vasey) Swallen, M. fragilis Swallen, M. minutissima (Steud.) Swallen, and M. sinuosa Swallen] that share the characteristically hairy floral bracts and granular capillary pedicels, but none of these four species have grayish-green spikelets nor a chromosome number of n = 8 (Peterson 1988). The present revision is based on field work, combined with laboratory and herbarium studies of morphology and chromosome numbers. These studies suggest that M. shepherdii is more closely related to B. tricholepis than to other species of Muhlenbergia and belongs in Blepharoneuron, and will be treated as such throughout this paper. MATERIALS AND METHODS The external morphology of the study group was examined both in the field and on herbarium specimens. A complete set of voucher specimens has been deposited in US and duplicates have been distributed to various herbaria (see specimen citations). The specimens cited of B. tricholepis were selected to portray the geographic range and a more complete list is available upon request, whereas all known collections of B. shepherdii were cited. For the anatomical studies field-collected leaf blades from the mid-culm region were fixed in FAA. After dehydration in an increasing ethanol series and infiltration with xylene, the leaves were embedded in paraffin, sectioned at 10 ,um thickness, and stained with safranin/fast green (Berlyn and Miksche 1976). Samples were examined and photographed on a Olympus BH2 photomicroscope using Kodak Technical Pan film. Anatomical descriptions were completed following the procedure for standardizing comparative leaf anatomy as outlined by Ellis (1976). For purposes of comparison and standardization, primary, 10 vascular bundles (first order) are defined as those containing large metaxylem vessels on either side of the protoxylem elements and usually associated with scleren-