D. Craw

Erosion, Himalayan geodynamics, and the geomorphology of metamorphism

Is erosion important to the structural and petrological evolution of mountain belts? The nature of active metamorphic massifs colocated with deep gorges in the syntaxes at each end of the Himalayan range, together with the magnitude of erosional fluxes that occur in these regions, leads us to concur with suggestions that erosion plays an integral role in collisional dynamics. At multiple scales, erosion exerts an influence on a par with such fundamental phenomena as crustal thickening and extensional collapse. Erosion can mediate the development and distribution of both deformation and metamorphic facies, accommodate crustal convergence, and locally instigate high-grade metamorphism and melting. INTRODUCTION Geologists have long recognized the interplay between erosional unloading and passive isostatic response, but the past two decades have seen a new focus on the role of surface processes in active tectonic environments. Erosions influence on structural evolution has been examined at a variety of spatial scales (e.g., Pavlis et al., 1997; Norris and Cooper, 1997; Hallet and Molnar, 2001). Thermal modeling yielded the fundamental result that variations in the timing and rate of erosion influence the thermal and hence metamorphic evolution of thickened crust (e.g., England and Thompson, 1984). Geodynamical models now link the mechanical and thermal evolution of orogens to lateral variations in erosion rate and magnitude and show how erosion can exert a strong control on particle paths through an orogen and thus on the surface expression of metamorphic facies (Koons, 1990; Beaumont et al., 1992; Willet et al., 1993). To further explore interactions between surface and lithospheric processes during orogeny, three-dimensional geodynamic models have been developed to explain particular patterns of crustal deformation and metamorphic exposures (e.g., Koons, 1994; Royden et al., 1997; see below). The general conclusion is that erosion can be a significant agent in active tectonic systems, particularly at larger spatial scales, and that interpretation of mountain belts past and present requires consideration of erosion (e.g., Hoffman and Grotzinger, 1993). The issue is complex, because, as pointed out by Molnar and England (1990), records of unroofing that have traditionally been viewed as evidence for tectonic activity, such as sedimentation or radiometric cooling ages, could in fact document erosion events driven by climate. Further, it can be argued that tectonics can force a climate response (e.g., Raymo and Ruddiman, 1992), and vice versa. Thus, to get beyond a “chicken and egg” controversy, we need to study specific processes, in specific settings, and look for feedback relationships between erosion and tectonism (e.g., Brozovic, et al., 1997). With their high elevations, great relief, and highly active surface and tectonic processes, the eastern and western syntaxial terminations of the Himalayan chain offer an opportunity to examine questions about the interplay between erosion and tectonics in the context of the India-Asia collision. In this article, we hope to stimulate debate by offering our conclusions and speculations about the role of erosion during collisional orogenesis, from a perspective grounded in the Himalayan syntaxes. In particular, we draw on results obtained from multidisciplinary study of the Nanga Parbat massif in the western syntaxis (Fig. 1), as well as preliminary work that has been done at the Namche Barwa massif in the eastern syntaxis. Erosion, Himalayan Geodynamics, and the Geomorphology of Metamorphism Figure 1. View to south of Nanga Parbat and central Nanga Parbat massif. Indus River in foreground passes base of massif in middle distance, more than 7 km below summit of Nanga Parbat itself.

Crustal reworking at Nanga Parbat, Pakistan: Metamorphic consequences of thermal‐mechanical coupling facilitated by erosion

Within the syntaxial bends of the India-Asia collision the Himalaya terminate abruptly in a pair of metamorphic massifs. Nanga Parbat in the west and Namche Barwa in the east are actively deforming antiformal domes which expose Quaternary metamorphic rocks and granites. The massifs are transected by major Himalayan rivers (Indus and Tsangpo) and are loci of deep and rapid exhumation. On the basis of velocity and attenuation tomography and microseismic, magnetotelluric, geochronological, petrological, structural, and geomorphic data we have collected at Nanga Parbat we propose a model in which this intense metamorphic and structural reworking of crustal lithosphere is a consequence of strain focusing caused by significant erosion within deep gorges cut by the Indus and Tsangpo as these rivers turn sharply toward the foreland and exit their host syntaxes. The localization of this phenomenon at the terminations of the Himalayan arc owes its origin to both regional and local feedbacks between erosion and tectonics.

Topographic development of the Southern Alps recorded by the isotopic composition of authigenic clay minerals, South Island, New Zealand

Abstract The Southern Alps are developing as a consequence of oblique collision between the Pacific and Australian plates. The Southern Alps lie on the west side of the South Island of New Zealand and create a massive rain shadow where greater than 12 m/year of rain falls on the west coast and semiarid conditions exist to the east. The rain-out effect across the mountains causes precipitation west of the Southern Alps to have δD and δ 18 O values averaging −30‰ and −5.5‰, whereas precipitation in the rain shadow to the east is isotopically lighter (δD=−72‰ and δ 18 O =−9.8‰). Such large differences in the isotopic composition of precipitation would not have existed prior to the development of significant topography. We have examined the topographic evolution of the Southern Alps using oxygen isotope analyses of authigenic kaolinites formed in the rain shadow to the east of the mountains between the Cretaceous (low topography) and the Pleistocene. Changes in the isotopic composition of authigenic clay minerals forming in equilibrium with meteoric water in the stratigraphic sequence record the development of Southern Alps topography and the resultant rain shadow. Our oxygen isotope analyses of authigenic kaolinites show a 5–6‰ decrease in the early Pliocene, from ∼18.2‰ in older rocks, to ∼12.3‰ in younger rocks. In addition, smectite is abundant in all samples from the Late Miocene to Recent, but is conspicuously absent in most older rocks, suggesting a change to a generally drier climate roughly coincident with the isotopic shift in kaolinites. This method may be useful in unraveling timing of development of mountain belts elsewhere in the world.

Environmental mobility of antimony around mesothermal stibnite deposits, New South Wales, Australia and southern New Zealand

Antimony (Sb) occurs principally in stibnite (Sb2S3) in mesothermal vein deposits hosted in low-grade metamorphic belts of eastern Australia and southern New Zealand. Stibnite is commonly associated with gold. Many deposits have been mined historically, with one large deposit, at Hillgrove, New South Wales, being mined recently. Natural outcrops in the relatively rugged terrains are oxidised under humid to semiarid conditions and stibnite transforms to oxides including valentinite, senarmontite, stibiconite, and rarely cervantite. Oxidation of stibnite and associated arsenopyrite and pyrite causes local acidification; however, acid is readily neutralised by carbonates in mineralised zones and host rocks, and associated waters are near neutral (pH 6–8.5). Stibnite dissolves readily in moderately oxidised waters as SbO3−, in conjunction with antimony oxide formation. Experimental stibnite oxidation yielded up to 37 ppm dissolved Sb, and mine and tailings dam waters at Hillgrove have up to 55 ppm dissolved Sb. Natural stibnite can contain >5000 ppm As in solid solution. Dissolution of stibnite, arsenopyrite and arsenian pyrite releases arsenic, and resultant dissolved As concentrations are up to 3.6 ppm (experimental) and up to 7.2 ppm (mine and tailings dam seepages). Mine and tailings discharge waters have elevated Sb and As where they emerge, but attenuation occurs by deposition of the metals onto amorphous iron oxyhydroxides which can contain >10% each of Sb and As. Historic disposal of mineralised waste rock material into the stream system at Hillgrove has caused strong contamination of stream sediments with Sb and As. Equilibration of stream water with contaminated stream sediment, as well as additions from erosion of natural outcrops and mine and tailings dam seepages, has led to the main drainage system (Bakers Creek) containing strongly contaminated water (up to 1.8 mg/l Sb and 0.3 mg/l As) for 20 km until its junction with the Macleay River. Environmentally high values of dissolved Sb (and As), are inevitable in waters associated with mesothermal stibnite (–gold) deposits.

Environmental arsenopyrite stability and dissolution: theory, experiment, and field observations

Arsenopyrite has traditionally been considered to be chemically unstable in the surficial environment. However, field evidence demonstrates that arsenopyrite does not readily decompose under water-saturated near-surface conditions. Arsenopyrite occurs in the heavy mineral suite of a variety of thin sediments in southern New Zealand, with no evidence for chemical corrosion. These sediments have been water-saturated but within metres of the surface for 10, 60, 28000, and >2 million years. These observations of long-term arsenopyrite stability are in accord with geochemical predictions based on recent experimentally determined thermodynamic data for arsenopyrite. Calculations using the new thermodynamic data suggest that arsenopyrite has similar Eh–pH stability range to pyrite, except under acid conditions (pH<4) where arsenopyrite should transform to realgar or orpiment. Calculated As concentrations in equilibrium with the pyrite–arsenopyrite assemblage in the surficial environment are 0.01–0.1 ppm, similar to groundwater in arsenopyrite-bearing rocks in this study (0.005–0.3 ppm), and these waters may be in chemical equilibrium. Arsenopyrite decomposes in oxidised waters to yield up to 1100 ppm dissolved As in the laboratory and up to 400 ppm dissolved As in mine processing waters. These concentrations of dissolved As are several orders of magnitude lower than equilibrium solubility of arsenopyrite because of kinetic effects and development of protective oxide coatings on arsenopyrite grains. Mine tailings containing arsenopyrite should be chemically stable during longterm storage provided they are kept water-saturated and moderately reduced. D 2003 Elsevier Science B.V. All rights reserved.

Metamorphogenic Au‐W veins and regional tectonics: Mineralisation throughout the uplift history of the Haast Schist, New Zealand

Abstract Gold‐scheelite (Au‐W) vein mineralisation in the Mesozoic Haast Schist (Otago, Alpine, and Marlborough Schist belts) resulted from metamorphic dewatering and fluid channelling periodically throughout the >100 Ma tectonic history of the schist belt. The earliest metal mobilisation occurred during synmetamorphic vein formation and was both accompanied and succeeded by ductile deformation. Structural style of deformation within vein zones became progressively more brittle as mineralisation accompanied Mesozoic‐Tertiary isostatic and extension‐related uplift of the Otago Schist. Rapid tectonic uplift in the Late Cenozoic resulted in mineralisation in the narrow belt of Alpine Schist along the Alpine Fault. The wide range of structural styles of vein systems seen in the Haast Schist represents different erosion levels exposed in tectonic zones. Geothermal gradients during Otago Schist mineralisation appear to have been close to normal, whereas mineralisation in the Southern Alps took place under anoma...

Lithologic variations in Otago Schist, Mt Aspiring area, northwest Otago, New Zealand

Abstract Psammitic schist, 2 types of pelitic schist (grey and porphyroblastic), and 4 types of greenschist (metavolcanic rock — light, spotted, foliated, and epidote rich) are recognisable on outcrop scale in textural zone 4 of the Otago Schist, northwest Otago, New Zealand. Thin horizons of metachert, marble, and ultramafic rock are commonly associated with greenschist. Broad units with 1 predominant rock type (greenschist, psammite, grey pelite, or porphyroblastic pelite) are mappable on a regional scale. These units also contain most or all of the above rock types and have poorly defined boundaries. The degree of original Stratigraphic continuity within and between these broad units is unknown. The studied area can be subdivided into 2 lithologic associations, the “Aspiring association” and the eastern “Wanaka association”, separated by a north-trending, poorly defined but lithologically gradational boundary. The Aspiring association is made up predominantly of pelitic rock types with considerable qua...

Redefinition and interpretation of late Miocene‐Pleistocene terrestrial stratigraphy, Central Otago, New Zealand

Abstract The stratigraphic succession in eastern Central Otago consists of Eocene quartzose fluvial sediments and middle Tertiary marine strata (Onekakara Group), early‐middle Miocene quartzose fluvial sediments and lake deposits (Manuherikia Group), late Miocene‐Pliocene immature sandstones and conglomerates, and Quaternary terrace and fan gravels. Published literature contains at least 20 different approaches for subdivision of this succession. The late Miocene‐Pliocene conglomerates were formed during the rise of fault‐bounded greywacke and semischist mountain ranges. Conspicuous conglomerates in the upper part of this succession are widely referred to as Maori Bottom Formation, but that name was originally applied by miners to a locally auriferous erosion surface beneath Quaternary terrace and fan gravels in Otago. In addition, the term is culturally offensive. We propose the name Hawkdun Group for the late Miocene‐Pliocene succession of tectonically generated sediments in the Maniototo, Ida, and Manu...

Stable isotopic evidence for mixing between metamorphic fluids and surface-derived waters during recent uplift of the Southern Alps, New Zealand

Recent studies have shown that the Southern Alps of New Zealand have active hydrothermal systems driven by tectonic uplift. These studies have concentrated on the rapidly exhumed rocks immediately adjacent to the Alpine Fault. We present new stable isotopic evidence that shows that fluid flow and fluid mixing processes thought to be restricted to rocks near the Alpine Fault also occurred in the low-uplift rate region of the Southern Alps orogen during the Kaikouran orogeny (10 Ma to present). Low δ18Ocalcite values of post-metamorphic veins in the eastern Southern Alps indicate that meteoric waters have penetrated to hot, midcrustal levels (350–300°C, >5 km depth) and mixed with metamorphic fluids in areas far removed from the Alpine Fault. In addition, the isotopic values of calcites precipitated in active faults in the MacKenzie basin define a trend of increasing δ18O, decreasing δ13C and decreasing temperature and form an isotopic mixing line between fault and vein calcites crosscutting metamorphic rocks and authigenic calcites precipitated in MacKenzie basin sediments. The isotope data indicate a second phase of mixing between a modified metamorphic fluid and a surficial diagenetic fluid at shallow depths. We suggest that mixed metamorphic–meteoric hydrothermal systems have developed in the east side of the Southern Alps during uplift and that active faults have played a critical role in transporting metamorphic fluids outward from the mountain front and into the adjacent intermontane basins.

Structure of schist in the Mt Aspiring region, northwestern Otago, New Zealand

Abstract Five phases of deformation are recognised in the schists of the Mt Aspiring area, northwest Otago, New Zealand.The first two phases resulted in isocIinal ductile folding and macroscopic nappe formation during greenschist facies metamorphism. The third phase, which occurred during waning metamorphic conditions, resulted in tight folding, with east-west overthrusting of macroSCOPIC nappes.Ouring this third phase, earlyformed fold axes in high-strain zones were rotated towards the stretching or translation direction. The fourth deformation phase is characterised by chevron folds (on all scales) with north-trending axes and associated west-dipping thrust faults.The Moonlight Fault, a major regional feature.is associated with this phase of deformation. The fifth phase of deformation involved mesoscopic conjugate sets of crenulations with east- and southeasttrending axes. Steeply dipping, fold axial surface joints developed in these crenulations are commonly intruded by lamprophyric dikes or associated...