P. O. Koons

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.

Two-sided orogen: Collision and erosion from the sandbox to the Southern Alps, New Zealand

In continental collision zones, the indentor is of approximately the same vertical dimension as the material being deformed, thereby allowing material to move over the indentor and out of the deforming orogen. This simple constraint on the height of the indentor differs from that imposed by the bulldozer model of wedge deformation and results in the formation of an orogen with two opposite-dipping topographic slopes. These two wedges are mechanically coupled, and the mechanics are strongly influenced by the erosion conditions imposed on the wedge. Where the growing mountains perturb prevailing, moisture-laden winds, the erosion pattern is highly asymmetric, erosional work being concentrated at the toe of the steep wedge facing the indentor (the inboard wedge). Concentration of erosional energy results in concentration of mechanical energy in the same region and produces the distinctive geological and geophysical patterns associated with continental collision. These include rapid uplift and exposure of deep crustal assemblages adjacent to the indentor accompanied by high crustal heat flow and anomalously low seismicity. The outcrop pattern along the inboard wedge evolves from that of a crustal cross section currently exposed in the Southern Alps to one dominated by repetitive nappe structures consisting predominantly of lower crustal units, as in the western Alps of Europe. The other wedge (the outboard wedge) undergoes relatively little erosion, is broad with a more gentle surface slope, and maintains an outcrop pattern consisting of upper crustal material.

The obliquely-convergent plate boundary in the South Island of New Zealand: implications for ancient collision zones

Abstract The Alpine Fault of New Zealand forms the western boundary of a zone of distributed deformation formed by the oblique convergence of continental crust belonging to the Pacific and Australian plates. Structural and geodetic data from the Alpine Fault show that a large proportion of the total plate displacement is accommodated by rapid oblique slip on the fault. The remaining displacement is distributed over a 200 km wide zone to the east. The collision may be modelled as a two-sided deforming orogen, with the partitioning of deformation being controlled by erosional differences between the narrow high-strain inboard and broad low-strain outboard sides. In ancient collision zones, little evidence may remain of the nature and amount of displacement on the inboard side. Partitioning of deformation among pre-existing structures complicates interpretation of the outboard zone. Radiometric ages may post-date collision by several tens of millions of years and indicate slow isostatic uplift and unroofing. Collision ages, if preserved, may be recognized by high uplift rates calculated from muscovite-biotite pairs.

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.

Geodynamics of the southeastern Tibetan Plateau from seismic anisotropy and geodesy

Ongoing plate convergence between India and Eurasia provides a natural laboratory for studying the dynamics of continental collision, a fi rst-order process in the evolution of continents, regional climate, and natural hazards. In southeastern Tibet, the fast directions of seismic anisotropy determined using shear-wave splitting analysis correlate with the surfi cial geology including major sutures and shear zones and with the surface strain derived from the global positioning system velocity fi eld. These observations are consistent with a clockwise rotation of material around the eastern Himalayan syntaxis and suggest coherent distributed lithospheric deformation beneath much of southeastern Tibet. At the southeastern edge of the Tibetan Plateau we observe a sharp transition in mantle anisotropy with a change in fast directions to a consistent E-W direction and a clockwise rotation of the surface velocity, surface strain fi eld, and fault network toward Burma. Around the eastern Himalayan syntaxis, the coincidence between structural crustal features, surface strain, and mantle anisotropy suggests that the deformation in the lithosphere is mechanically coupled across the crust-mantle interface and that the lower crust is suffi ciently strong to transmit stress. At the southeastern margin of the plateau in Yunnan province, a change in orientation between mantle anisotropy and surface strain suggests a change in the relationship between crustal and mantle deformation. Lateral variations in boundary conditions and rheological properties of the lithosphere play an important role in the geodynamic evolution of the Himalayan orogen and Tibetan Plateau and require the development of three-dimensional models that incorporate lateral heterogeneity.

Some thermal and mechanical consequences of rapid uplift: an example from the Southern Alps, New Zealand

The purpose of the invention is to produce a plated wire array mat which is simpler to fabricate, permits higher packing densities, and reduces the required strap drive currents over the present array mat techniques. The structure permits the positioning of ferrite keeper material in closely adjacent and fully surrounded relationship to the plated wire so as to minimize straying magnetic fields, thereby allowing higher packing densities and lower drive currents to be used. The fabrication technique utilizes highly precise photographic techniques to build a simple tunnel structure array utilizing photolithic layers, exposure, and wash thereof to achieve the desired structural relationship.

Climatic and tectonic controls on chemical weathering in the New Zealand Southern Alps

Abstract Climatic and tectonic controls on the relative abundance of solutes in streams draining the New Zealand Southern Alps were investigated by analyzing the elemental and Sr isotope geochemistry of stream waters, bedload sediment, and hydrothermal calcite veins. The average relative molar abundance of major cations and Si in all stream waters follows the order Ca2+ (50%) > Si (22%) > Na+ (17%) > Mg2+ (6%) > K+ (5%). For major anions, the relative molar abundance is HCO3− (89%) > SO42− (7%) > Cl− (4%). Weathering reactions involving plagioclase and volumetrically small amounts of hydrothermal calcite define the ionic chemistry of stream waters, but nearly all streams have a carbonate-dominated Ca2+ and HCO3− mass-balance. Stream water Ca/Sr and 87Sr/86Sr ratios vary from 0.173 to 0.439 μmol/nmol and from 0.7078 to 0.7114, respectively. Consistent with the ionic budget, these ratios lie solely within the range of values measured for bedload carbonate (Ca/Sr = 0.178 to 0.886 μmol/nmol; 87Sr/86Sr = 0.7081 to 0.7118) and hydrothermal calcite veins (Ca/Sr = 0.491 to 3.33 μmol/nmol; 87Sr/86Sr = 0.7076 to 0.7097). Streams draining regions in the Southern Alps with high rates of physical erosion induced by rapid tectonic uplift and an extremely wet climate contain ∼10% more Ca2+ and ∼30% more Sr2+ from carbonate weathering compared to streams draining regions in drier, more stable landscapes. Similarly, streams draining glaciated watersheds contain ∼25% more Sr2+ from carbonate weathering compared to streams draining non-glaciated watersheds. The highest abundance of carbonate-derived solutes in the most physically active regions of the Southern Alps is attributed to the tectonic exhumation and mechanical denudation of metamorphic bedrock, which contains trace amounts of calcite estimated to weather ∼350 times faster than plagioclase in this environment. In contrast, regions in the Southern Alps experiencing lower rates of uplift and erosion have a greater abundance of silicate- versus carbonate-derived cations. These findings highlight a strong coupling between physical controls on landscape development and sources of solutes to stream waters. Using the Southern Alps as a model for assessing the role of active tectonics in geochemical cycles, this study suggests that rapid mountain uplift results in an enhanced influence of carbonate weathering on the dissolved ion composition delivered to seawater.

Influence of exhumation on the structural evolution of transpressional plate boundaries: An example from the Southern Alps, New Zealand

Concentration of erosional activity along transpressional plate boundaries can significantly alter the pattern of mechanical behavior through the influence of exhumation on crustal strength. Three-dimensional numerical modeling of an obliquely convergent orogen shows that a single oblique plate-bounding structure is stable if asymmetric erosion patterns, such as those observed in orographic mountain belts, pertain, and if Earth9s crust has a strong-on-weak rheology. In early stages of oblique convergence of an initially laterally homogeneous material, lateral (boundary-parallel) strain is accommodated along a near vertical structure and convergent (boundary-normal) strain is concentrated on structures dipping at moderate angles into the orogen. Exhumation of deep crustal material along the convergent structure results in thermal weakening along this dipping structure. When the upper crust beneath the orogen is significantly weakened by exhumation, lateral strain abandons the vertical structure and shifts to the dipping structure, combining with the convergent strain to form a single oblique fault that accommodates the plate motion in the upper crust, as is the case along the Alpine fault, New Zealand. The process of thermal thinning is controlled by advection and occurs on time frames of ∼1–2 m.y. The two components of strain remain separate in the lower crust. During active convergence, exhumation of lower crustal material occurs only along those structures accommodating convergent strain. Consequently, material exhumed from lower regions of ductile deformation, as is the case along the Alpine fault, contains lineations that indicate a greater component of convergence than predicted from the total plate motion. Postorogenic exhumation of the roots of an oblique plate boundary will expose two parallel shear zones, one dominantly convergent and one dominantly strike slip. Widely reported orogen-parallel transport in the late stages of ancient oblique convergence may represent not a change in plate vector, but the exhumation of the lateral transport zone.

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.

Evolution of fluid driving forces and composition within collisional orogens

Subaerial collisional mountain belts have a predictable asymmetric two-sided wedge geometry. The major driving forces for crustal fluid flow in this framework arc thermal and topographic gradients. It is possible to predict both a time-averaged fluid state and the evolution of the fluid regime during orogenesis, with three distinct fluid flow regimes: the outboard toe where material is incorporated into the orogen, a transitional zone of concentrated deformation, and a zone of rapid exhumation along the inboard slope adjacent to the indentor. As rocks move into the outboard wedge, relatively low temperature, compaction and head driven fluid flow dominates. Mid-crustal material undergoes disequilibrium dehydration as rock-fluid packets move into the transition zone, producing a dominantly aqueous ‘metamorphic fluid’. In the inboard region, rapid uplift produces geothermal gradients in excess of 80°C/km which encourage vigorous free convection. Advecting fluids evolve through immiscibility towards increasing CO2 content during uplift of the rock-fluid packets. Mixing of thermally driven and head driven fluids occurs at depths of 5–10 km. The Southern Alps of New Zealand, the European Alps and the Himalaya represent variants of this orogenic-hydrothermal system.