Isocon analysis of migmatization in the Front Range, Colorado, USA

Isocon analysis has been applied to five sets of leucosome, mafic selvages and immediately adjacent mesosome in the migmatites from a 15-m outcrop in the Colorado Front Range. The results show: (i) mafic selvages formed from the adjacent mesosome by loss of felsic components and therefore the mesosomes are indeed palaeosomes or protoliths; (ii) the leucosomes did not form in a closed system from the palaeosome (in which case the material lost from the palaeosome during selvage formation would become the leucosome). The observed volumes and compositions of leucosomes require that the present leucosome must contain some material in addition to the felsic components lost from the selvages. The materials that must be added are leucotonalitic to granitic in composition, varying greatly in K/(Na + Ca) ratio. The trend in leucosome composition can be reproduced by assuming that a metasomatic exchange, KNa + Ca, modified originally leucotonalitic leucosomes to more K-rich compositions. These leucosomes most likely formed by injection of silicate melts accompanied, or followed, by metasomatism. The trend of leucosome compositions in this study reflects the general trend in the leucosome compositions which have been published from other areas, indicating that the proposed mechanism can be applicable to other regional migmatites.     

Influence of snow upon rates of granodiorite weathering, Colorado Front Range, USA

Measurements of phenocryst relief in 1-m² quadrats placed randomly on southwest-facing granodiorite outcrops exposed by late Pleistocene deglaciation of a high-altitude Front Range col suggest that weathering is strongly influenced by snow. Depending upon local differences in snow-cover duration (inferred from differences in lichen cover), rates of bedrock-surface lowering vary from 0.0001 to 0.0013 mm/year. A quadratic polynomial best describes the relationship. Weathering is least effective beneath late-lying snowbanks that protect the rock from freeze-thaw and wetting-drying cycles for most of the year, and in winter snow-free sites where dryness and the binding effects of crustose lichens discourage granular disintegration. Weathering is most effective in areas of thin to moderate winter snow cover where meltout occurs early, moisture is available at critical seasons, and episodic lichen snowkill ensures that bare rock is exposed. If granite-weathering data are to be used for dating, measurements should be restricted to microenvironments with comparable snow-accumulation histories.     

Holocene alluvial stratigraphy and response to climate change in the Roaring River valley,Front Range,Colorado, USA

Stratigraphic analyses and radiocarbon geochronology of alluvial deposits exposed along the Roaring River, Colorado, lead to three principal conclusions: (1) the opinion that stream channels in the higher parts of the Front Range are relics of the Pleistocene and nonalluvial under the present climate, as argued in a water-rights trial USA v. Colorado, is untenable, (2) beds of clast-supported gravel alternate in vertical succession with beds of fine-grained sediment (sand, mud, and peat) in response to centennial-scale changes in snowmelt-driven peak discharges, and (3) alluvial strata provide information about Holocene climate history that complements the history provided by cirque moraines, periglacial deposits, and paleontological data. Most alluvial strata are of late Holocene age and record, among other things, that: (1) the largest peak flows since the end of the Pleistocene occurred during the late Holocene; (2) the occurrence of a mid- to late Holocene interval (~ 2450–1630(?) cal yr BP) of warmer climate, which is not clearly identified in palynological records; and (3) the Little Ice Age climate seems to have had little impact on stream channels, except perhaps for minor (~ 1 m) incision.     

Geologic and geomorphic controls of coal development in some Tertiary Rocky Mountain basins, USA

Previous investigations have not well defined the controls on the development of minable coals in fluvial environments. This study was undertaken to provide a clearer understanding of these controls, particularly in of the lower Tertiary coal-bearing deposits of the Raton and Powder River basins in the Rocky Mountain region of the United States. In this region, large amounts of coals accumulated in swamps formed in the flow-through fluvial systems that infilled these intermontane basins. Extrabasinal and intrabasinal tectonism partly controlled the stratigraphic and facies distributions of minable coal deposits. The regional accumulation of coals was favored by the rapid basin subsidence coupled with minimal uplift of the source area. During these events, coals developed in swamps associated with anastomosed and meandering fluvial systems and alluvial fans. The extensive and high rate of sediment input from these fluvial systems promoted the formation of ombrotrophic, raised swamps, which produced low ash and anomalously thick coals. The petrology and palynology of these coals, and the paleobotany of the associated sediments, suggest that ombrotrophic, raised swamps were common in the Powder River Basin, where the climate during the early Tertiary was paratropical. The paleoecology of these swamps is identical to that of the modern ombrotrophic, raised swamps of the Baram and Mahakam Rivers of Borneo.     

Mass-balance and mass-transfer in migmatites from the Colorado Front Range

Metasomatic exchanges between the infiltrating fluids and wall rocks most likely initiated the formation of nine leucosomes in two large samples of the Precambrian biotite-quartz-feldspar migmatites from the east-central Colorado Front Range. The leucosomes, 2 to 20 mm thick and enclosed in mafic salvages 1 to 10 mm thick, are granitic to tonalitic in composition. Mass-balance calculations suggest that each leucosome formed by local introduction of mass. The net gains and losses calculated assuming that all such gains and losses were contained within the leucosome show that, in general, neither the gains nor the losses fit the composition of any silicate melt. It is more likely that the components were transported in a fluid. Recalculated on constant Al basis, the most significant relative mass transfers were gain of K and losses of Na and Mg by the rocks. The metasomatic reactions calculated are those for replacement of plagioclase by microcline and breakdown of biotite. The reactions must have been the cause of incipient migmatization. A mafic selvage formed from the paleosome by the loss of material whose composition is tonalitic to granodioritic varying systematically with the paleosome composition.It is proposed that an infiltrating fluid caused metasomatism and partial melting along its path and that the melt, segregated from the mafic residues, combined with the introduced material to form a leucosome. The degree of melting was controlled by the paleosome composition and by the net amount (but not the composition) of the introduced material. The cause of melting of the paleosome was most likely an increased due to the influx of H₂O from the water-rich fluid.The compositional range of the metamorphic solution in equilibrium with these rocks was calculated from available experimental data. The sample calculations show that such fluid could have been responsible for the reactions and mass transfers observed.     

Characteristics of landslides in western Colorado,USA

Mass movement can be activated by earthquakes, rapid snowmelt, or intense rainstorms in conjunction with gravity. Whereas mass movement plays a major role in the evolution of a hillslope by modifying slope morphology and transporting material from the slope to the valley, it is also a potential natural hazard. Determining the relationships of frequency and magnitude of landslides are fundamental to understanding the role of landslides in the study of landscape evolution, hazard assessment, and determination of the rate of hillslope denudation. We mapped 735 shallow and active landslides in the Paonia to McClure Pass area of western Colorado from aerial photographs and field surveys. The study area covers ～815 km². The frequency–magnitude relationships of the landslides illustrate the flux of debris by mass movement in the area. The comparison of the probability density of the landslides with the double Pareto curve, defined by power scaling for negative slope (α), power scaling for positive slope (β), and location of rollover (t), shows that α?=?1.1, β?=?1.9, and t?=?1,600 m² for areas of landslides and α?=?1.15, β?=?1.8, and t?=?1,900 m³ for volumes of landslides. The total area of landslides is 4.8?×?10⁶ m² and the total volume of the landslides is 1.4?×?10⁷ m³. The areas (A) and the volumes (V) of landslides are related by V?=?0.0254?×?A ^1.45. The frequency–magnitude analysis shows that landslides with areas ranging in size from 1,600 to 20,000 m² are the most hazardous landslides in the study area. These landslides are the most frequent and also do a significant amount of geomorphic work. We also developed a conceptual model of hillslope development to upland plateau driven by river incision, shallow landsliding, and deep-seated large landsliding. The gentle slope to flat upland plateau that dominated the Quaternary landscape of the study area was modified to the present steep and rugged topography by the combined action of fluvial incision and glacial processes in response to rock uplift, very-frequent shallow landsliding, and less-frequent deep-seated landsliding.     

The noble gas geochemistry of natural CO2 gas reservoirs from the Colorado Plateau and Rocky Mountain provinces, USA

Identification of the source of CO₂ in natural reservoirs and development of physical models to account for the migration and interaction of this CO₂ with the groundwater is essential for developing a quantitative understanding of the long term storage potential of CO₂ in the subsurface. We present the results of 57 noble gas determinations in CO₂ rich fields (>82%) from three natural reservoirs to the east of the Colorado Plateau uplift province, USA (Bravo Dome, NM., Sheep Mountain, CO. and McCallum Dome, CO.), and from two reservoirs from within the uplift area (St. John’s Dome, AZ., and McElmo Dome, CO.). We demonstrate that all fields have CO₂/³He ratios consistent with a dominantly magmatic source. The most recent volcanics in the province date from 8 to 10 ka and are associated with the Bravo Dome field. The oldest magmatic activity dates from 42 to 70 Ma and is associated with the McElmo Dome field, located in the tectonically stable centre of the Colorado Plateau: CO₂ can be stored within the subsurface on a millennia timescale.The manner and extent of contact of the CO₂ phase with the groundwater system is a critical parameter in using these systems as natural analogues for geological storage of anthropogenic CO₂. We show that coherent fractionation of groundwater ²⁰Ne/³⁶Ar with crustal radiogenic noble gases (⁴He, ²¹Ne, ⁴⁰Ar) is explained by a two stage re-dissolution model: Stage 1: Magmatic CO₂ injection into the groundwater system strips dissolved air-derived noble gases (ASW) and accumulated crustal/radiogenic noble gas by CO₂/water phase partitioning. The CO₂ containing the groundwater stripped gases provides the first reservoir fluid charge. Subsequent charges of CO₂ provide no more ASW or crustal noble gases, and serve only to dilute the original ASW and crustal noble gas rich CO₂. Reservoir scale preservation of concentration gradients in ASW-derived noble gases thus provide CO₂ filling direction. This is seen in the Bravo Dome and St. John’s Dome fields. Stage 2: The noble gases re-dissolve into any available gas stripped groundwater. This is modeled as a Rayleigh distillation process and enables us to quantify for each sample: (1) the volume of groundwater originally ‘stripped’ on reservoir filling; and (2) the volume of groundwater involved in subsequent interaction. The original water volume that is gas stripped varies from as low as 0.0005 cm³ groundwater/cm³ gas (STP) in one Bravo Dome sample, to 2.56 cm³ groundwater/cm³ gas (STP) in a St. John’s Dome sample. Subsequent gas/groundwater equilibration varies within all fields, each showing a similar range, from zero to ∼100 cm³ water/cm³ gas (at reservoir pressure and temperature).     

龙门山冲断带北段前山带构造分带性及山前带变形特征

龙门山冲断带构造分带性是龙门山构造研究中的核心内容和关键科学问题.在野外实测地质大剖面以及路线地质调查的基础上,根据冲断带不同部位构造样式的不同,将龙门山冲断带北段前山带划分出3个构造亚带,即唐王寨-仰天窝飞来峰-滑覆体构造亚带、楼子坝-雁门坝-马角坝叠瓦冲断亚带、矿山梁-天井山-二廊庙冲断背斜亚带.各亚带具有不同构造变形特征:飞来峰-滑覆体构造亚带具有"推覆加滑覆"的构造叠加和演变特征;叠瓦冲断亚带中冲断构造十分发育,从北西至南东方向,不同系、同一系中不同组、同一组中不同段的地层依次由老向新逆冲,组成典型的叠瓦状构造样式,逆冲断裂向西呈铲式倾斜,并收敛于逆冲断裂系底板断层之上.构造岩片中褶皱构造十分发育,以断层传播褶皱为主,其轴迹走向和轴面产状与叠瓦冲断亚带中各条断层的总体产状几乎相同,均倒向前陆方向.冲断背斜亚带具有双重构造特点,在浅层表现为近直立的短轴背斜,构造样式以断层转折褶皱为主,深层由多个逆冲岩片叠置而成.下寺-金子山-厚坝山前带浅部层次中冲断稀少、规模较小、变形微弱,主要表现为向南东倾斜的单斜构造,深部层次发育有大型隐伏逆冲断裂.从前山带和山前带总体上具有变形强度向前陆方向有规律递减、卷入构造的地层逐渐变新、变形层次逐渐变浅的趋势,反映了冲断构造具有前展式特征.冲断构造是晚三叠世与新生代两期构造变形的产物:晚三叠世构造变形强烈,是龙门山前陆冲断带的主要形成期;新生代变形较弱,叠加和改造了晚三叠世的构造变形.     

Fire-related hyperconcentrated and debris flows on Storm King Mountain, Glenwood Springs, Colorado, USA

 The South Canyon Fire of July 1994 burned 800 ha of vegetation on Storm King Mountain near Glenwood Springs, Colorado, USA. On the night of 1 September 1994, in response to torrential rains, debris flows inundated seven areas along a 5-km length of Interstate Highway 70. Mapping from aerial photographs, along with field observations and measurements, shows that the September rainstorm eroded unconsolidated, burned surficial soil from the hillsides, flushed dry-ravel deposits from the tributary channels, and transported loose, large material from the main channels. The hyperconcentrated flows and debris flows inundated 14 ha of Interstate Highway 70 with 70 000 m³ of material. Although the burned area was seeded in November 1994, the potential for continuing debris-flow activity remains. Incision and entrainment of channel alluvium, as well as erosion of loose material from the hillslopes could result in future debris- and hyperconcentrated-flow activity. Received: 15 October 1996 / Accepted: 25 June 1997     

The relationship between rock glacier and contributing area parameters in the Front Range of Colorado

Rock glaciers are an end unit of the coarse debris system that links frost weathering and rockfall to debris transport in mountainous environments. By examining topographic and climatic controls on creep, a better understanding of rock glacier formation and evolution could be obtained. A series of statistical tests were run comparing contributing area parameters (width, length, area, slope, headwall height, etc.) and rock glacier variables (width, length, area, thickness, slope, creep and temperature) in the Front Range of Colorado. Results showed that rock glacier width had the strongest correlation with contributing area width due to an abundance of lobate rock glaciers (r = 0.78). Rock glacier area and contributing area size were also related (r = 0.74), but are also a function of talus production factors. Mean surface velocity appears to be better correlated with thickness (r = 0.58) and length (r = 0.62), rather than slope (r = 0.33). Mean annual air temperature had a stronger exponential relationship with maximum velocities (r = 0.76) compared to mean annual velocity (r = 0.60). Front Range rock glacier velocities tend to decrease with warming, suggesting that the volume of ice and debris, rates of shear in plastic layers, or melt water may have a greater influence on deformation. Copyright © 2007 John Wiley & Sons, Ltd.     

Stability analysis of a horizontal coalbed methane well in the Rocky Mountain Front Ranges of southeast British Columbia, Canada

This study describes a wellbore stability analysis undertaken for a horizontal coalbed methane well in the Mist Mountain Formation, SE British Columbia, Canada. Three triaxial compression tests, with ultrasonic velocities, were conducted on 38-mm-diameter core plugs taken from a large, fresh block of Seam 7. Due to the small size of the tested samples, the laboratory-derived strength values were reduced to reflect the in-situ stress conditions considered relevant for a 156-mm-diameter horizontal well. The vertical stress gradient was calculated by integrating a bulk density log from an offset well. Horizontal maximum and minimum stresses were estimated from regional stress data, whereas formation pressure was estimated on the basis of a local hydrological study. The 2D elastoplastic STABView™ numerical modeling code was used to forecast horizontal wellbore stability. The sensitivity of the predicted yielded zone size was examined for varying linear and non-linear rock strength criteria, horizontal in-situ stresses, bottom-hole pressures, formation pressure, drilling depth, and wall-coating efficiency. Stability analysis was performed at bottom-hole pressures ranging from overbalanced to underbalanced in order to simulate the conditions expected during drilling and production. The effects of weak bedding planes and varying well trajectories were also investigated. When drilling at 650 m depth under underbalanced to slightly overbalanced conditions, a high probability of getting the drill pipe stuck was predicted. STABView™ showed that, if the 38-mm-diameter core plug strengths were used directly for forecasting purposes, the predicted yielded zone would be almost 20% overgauge when drilling at balanced conditions. When peak cohesion of coal was reduced by 50% to reflect the conditions expected along weak intervals of a horizontal wellbore, the predicted enlarged borehole was almost 85% overgauge under the same drilling conditions. The most unstable trajectory has N–S azimuth, which is perpendicular to the maximum horizontal stress in the study area. STABView™ indicates that a production liner should be inserted immediately following the drilling of the horizontal well in this structurally complex geologic setting since pressure drawdown and reservoir pressure depletion would undoubtedly lead to enlarged yielded zones and collapse of the wellbore over time.     

Late pleistocene glacial equilibrium-line altitudes in the Colorado Front Range: A comparison of methods

Six methods for approximating late Pleistocene (Pinedale) equilibrium-line altitudes (ELAs) are compared for rapidity of data collection and error (RMSE) from first-order trend surfaces, using the Colorado Front Range. Trend surfaces computed from rapidly applied techniques, such as glaciation threshold, median altitude of small reconstructed glaciers, and altitude of lowest cirque floors have relatively high RMSEs (97–186 m) because they are subjectively derived and are based on small glaciers sensitive to microclimatic variability. Surfaces computed for accumulation-area ratios (AARs) and toe-to-headwall altitude ratios (THARs) of large reconstructed glaciers show that an AAR of 0.65 and a THAR of 0.40 have the lowest RMSEs (about 80 m) and provide the same mean ELA estimate (about 3160 m) as that of the more subjectively derived maximum altitudes of Pinedale lateral moraines (RMSE = 149 m). Second-order trend surfaces demonstrate low ELAs in the latitudinal center of the Front Range, perhaps due to higher winter accumulation there. The mountains do not presently reach the ELA for large glaciers, and small Front Range cirque glaciers are not comparable to small glaciers existing during Pinedale time. Therefore, Pleistocene ELA depression and consequent temperature depression cannot reliably be ascertained from the calculated ELA surfaces.     

Aluminum chemistry: Fractionation,speciation, and mineral equilibria of soil interstitial waters of an alpine watershed,Front Range,Colorado

Soil interstitial waters and minerals were collected and analyzed to evaluate the influence of acid deposition on Al chemistry in the soil environment of the Green Lakes Valley Front Range, Colorado. The soil solutions were subjected to a series of batch Al experiments followed by computer modeling to separate the labile from the nonlabile Al, and to estimate the activity of Al³⁺. The Al solubility in the interstitial waters is complex and is controlled by organic solutes, H₄SiO₄, and pH. The pH and concentrations of SO²⁻₄ do not correlate with Al concentrations. The chemical equilibria of Al are controlled by amorphous aluminosilicate Al(OH)_3(1−x) SiO_2x. Studies of mineralogy and soil water chemistry provide a useful combination to evaluate and predict the chemical processes of a soil environment.     

Relative dating of Late Quaternary deposits using cluster and discriminant analysis, Audubon Cirque, Mt. Audubon, Colorado Front Range

Holocene and Late Quaternary talus, lobate rock glaciers, and moraines within Audubon Cirque, Colorado Front Range, were assigned reltive ages using the following age-dependent criteria: fresh-weathered ratio, pitting, weathering rind development, angularity, and surface, oxidation of boulders, together with lichen cover and largest lichen diameter. Principal Component scores, yielding four major groups of deposits (relative age units C, R, E and V, from youngest to oldest). Tentative correlation with other Colorado Late Quaternary sequences suggests that unit C is of Gannett Peak age (100–300 years B. P.), unit R of Audubon age (950–1850 years B. P.) unit E of Early Neoglacial age (3000–5000 years B. P.), and unit V of Late Pinedale age (about 10,000 Years B. P.). Correlation is problematic due to differences in operationsl definitions of relative dating parametrs between workers, and because climatic and lithologic variations between areas may confound the date.     

Evaluation of groundwater withdrawal from a mountain watershed, Colorado, USA

The purpose of this study is to evaluate the groundwater-withdrawal potential of the Fraser River watershed, a mountainous drainage system in north-central Colorado. Laboratory tests, field investigations, and numerical modeling are conducted to present a quantitative understanding of the watershed’s groundwater-flow system. Aquifer hydraulic conductivity values obtained from aquifer tests range from 1E?5 to 1E?3 m/s. Groundwater withdrawal is concentrated in channel-fill deposits of the Troublesome Formation within the Fraser basin. A steady state groundwater-flow model of the Fraser River watershed is developed and calibrated using 24 observation wells in the Fraser River valley and estimated baseflow of the Fraser River. Modeling results suggest that surface recharge is the major source of groundwater in the watershed. Groundwater exits the watershed through evapotranspiration and discharge to rivers. Transient groundwater-flow modeling evaluates future withdrawal scenarios using the hydraulic head distribution from the steady state model as the initial condition. Drawdown within Troublesome Formation aquifers from the current pumping schedule approaches 2 m. When the daily pumping rate is doubled, drawdown approaches 4 m. The radius of influence is hundreds of meters to 1 km. Pumping wells withdraw approximately 2 and 15 % of groundwater flowing through the well field for hydraulic conductivity of 1E?3 and 1E?5 m/s, respectively. This study suggests that the groundwater system at the Fraser Valley could sustain current and future withdrawals, given that the current recharge condition is maintained.     

北大巴山滔河镁铁质岩中钛闪石的厘定及指示意义

滔河岩体位于北大巴山地区岚皋县,滔河镁铁质岩的岩石类型与区域镁铁质岩石类型一致,主要为辉绿岩、辉长岩、辉长辉绿岩及富含钛闪石的闪长岩,各相邻镁铁质岩呈明显的渐变过渡状态,指示为同期岩浆作用的产物。闪长岩内部角闪石呈细长条状、针状巨晶产出,粒径为1~3 cm不等,自形程度较高,具有明显的高铝(Al=1.93~2.06)、高钛(Ti O2=4.54%~5.50%)特征,其种属为钛闪石。钛闪石结晶温度为1 062~1 129℃,形成深度为19.11~22.64 km,氧逸度变化范围为-0.36~-0.01,说明岩浆来源较深且上侵速度较快。钛闪石的存在指示岩体的源区在部分熔融前发生了明显的地幔交代作用,地幔交代和岩体形成都与俯冲消减作用存在内在的成因联系。岩体具有明显的深源特征、与俯冲消减作用有关及区域岩体的地壳混染特征,共同指示滔河岩体可能形成于与俯冲相关的弧后拉张环境。     

Magmatic modification of the uppermost mantle beneath the Basin and Range to Colorado Plateau Transition Zone; Evidence from xenoliths, Wikieup, Arizona

Upper mantle xenoliths from Wikieup, AZ, provide abundant evidence for magmatic modification of the uppermost mantle beneath the Transition Zone between the Colorado Plateau and the southern Basin and Range province. Upper mantle lithologies in this xenolith suite are represented by spinel peridotite, wehrlite, plagioclase peridotite, and Al-augite group pyroxenites. Isotopic data for these xenoliths yield relatively uniform values and suggest a common petrogenesis. Al-augite-bearing gabbro and pyroxenite xenoliths from this locality are interpreted to have formed by crystal fractionation processes from parent alkali basalts similar to the Wikieup host basalt. Mineral and whole rock compositions show consistent trends of increasing incompatible element contents (Fe, Al, Ca, Na, K, LIL, and LREE), and decreasing compatible element contents (Mg, Cr, Ni) from spinel peridotite to wehrlite to plagioclase peridotite to the host basalt composition. These compositional trends are interpreted as resulting from varying degrees of magma-mantle wall rock interaction as ascending mafic magmas infiltrated upper mantle peridotite. Small degrees of melt infiltration resulted in slightly modified spinel peridotite compositions while moderate degrees metasomatized spinel peridotite to wehrlite, and the highest degrees metasomatized it to plagioclase peridotite. Whole rock compositions and clinopyroxene, plagioclase, and whole rock isotopic data suggest that the infiltrating magmas were the same as those from which the gabbros and pyroxenites crystallized, and that they were alkalic in composition, similar to the Wikieup host alkali olivine basalts. Relatively uniform ¹⁴³Nd/¹⁴⁴Nd for the mineral separates and whole rocks in spite of the significantly wide range in their ¹⁴⁷Sm/¹⁴⁴Nd (0.71–0.23 in clinopyroxene) suggests that the Wikieup xenoliths including gabbro, pyroxenite, peridotite, wehrlite, and plagioclase peridotite, are all relatively young rocks formed or metasomatized by a relatively recent magmatic episode. Received: 21 May 1996 / Accepted: 23 December 1996     

横过西昆仑和塔里木结合带的山隆盆降机制动力学模拟

最近的地震探测表明,在西昆仑和塔里木结合带有岩石圈根存在.据此,本文提出地壳均衡与岩石圈根拖曳共同作用导致"山隆盆降"的动力学机制假说.利用新近中英合作完成的有限元粘弹塑构造数值模拟技术FEVPLIB,模拟研究了青藏高原西部横过西昆仑和塔里木结合带剖面的这一动力学演化过程.这种模式既能解释高原隆升,又能解释边缘沉积盆地的成因.模拟结果表明,两个大陆碰撞到一起,当岩石圈根一旦形成以后,造山水平挤压力主要来源于岩石圈根的向下拖曳,而印度板块向北挤压沦为次要因素.