Anne S. Meltzer

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.

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.

Transition from slab to slabless: Results from the 1993 Mendocino triple junction seismic experiment

Three seismic refraction-reflection profiles, part of the Mendocino triple junction seismicexperiment,allowustocompareandcontrastcrustanduppermantleoftheNorth American margin before and after it is modified by passage of the Mendocino triple junction. Upper crustal velocity models reveal an asymmetric Great Valley basin overlying Sierran or ophiolitic rocks at the latitude of Fort Bragg, California, and overlying Sierran or Klamath rocks near Redding, California. In addition, the upper crustal velocity structure indicates that Franciscan rocks underlie the Klamath terrane east of Eureka, California.TheFranciscancomplexis,onaverage,laterallyhomogeneousandisthickestinthe triple junction region. North of the triple junction, the Gorda slab can be traced 150 km inboardfromtheCascadiasubductionzone.Southofthetriplejunction,strongprecritical reflections indicate partial melt and/or metamorphicfluids at the base of the crust or in theuppermantle.BreaksinthesereflectionsarecorrelatedwiththeMaacamaandBartlett Springs faults, suggesting that these faults extend at least to the mantle. We interpret our datatoindicatetectonicthickeningoftheFranciscancomplexinresponsetopassageofthe Mendocino triple junction and an associated thinning of these rocks south of the triple junction due to assimilation into melt triggered by upwelling asthenosphere. The region of thickenedFranciscancomplexoverliesazoneofincreasedscattering,intrinsicattenuation, or both, resulting from mechanical mixing of lithologies and/or partial melt beneath the onshore projection of the Mendocino fracture zone. Our data reveal that we have crossed thesouthernedgeoftheGordaslabandthatthisedgeand/ortheoverlyingNorthAmerican crust may have fragmented because of the change in stress presented by the edge.

Nanga Parbat crustal anisotropy: Implications for interpretation of crustal velocity structure and shear-wave splitting

The Nanga Parbat- Haramosh massif repre- sents a unique exposure of mid-lower continental crust from beneath the Himalayan orogen. Seismic velocity measure- ments on a suite of quartzofeldspathic gneisses show up to 12.5% velocity anisotropy for compressional waves and up to 21% for shear waves. The degree of anisotropy is a func- tion of mica content and rock fabric strength. Over 30% of the samples have maximum compressional wave velocities of 6.4-6.5 km/s; velocities typically associated with more mafic lithologies. These results have implications for the inter- pretation of crustal velocity structure obtained from wide- angle seismic surveys where in situ velocity measurements are made from refracted or turning rays that potentially spend a substantial portion of their travel path propagat- ing in the foliation plane. Velocities determined from these surveys may overestimate mean velocities of crustal rocks with well-developed horizontal fabric. In addition, crustal anisotropy due to the development of pervasive rock fabric has the potential to be a significant contributing factor to shear-wave splitting observations.

Fluids in the lower crust following Mendocino triple junction migration: Active basaltic intrusion?

Geodynamic and plate tectonic models for the Mendocino triple junction, a fault-fault-trench triple junction in northwestern California, predict a slab-free zone south of the triple junction in which asthenospheric mantle upwells to the base of the crust. A variety of geological and geophysical data support this model, although fine-scale (<20 km) details of the lithospheric structure have been unknown previously. Seismic investigations in the onshore transform regime south of the Mendocino triple junction region reveal very strong short-offset reflections from the lower crust and at the crust-mantle boundary beneath the entire width of the Coast Range, particularly near Lake Pillsbury, California. Seismic analysis suggests that these reflections are from discrete zones of fluid. The reflector geometry implies that the source of the fluid is within the upper mantle. In this tectonic context it is likely that the fluids are largely partial melt, segregated from asthenospheric mantle upwelling into the slab-free zone. The tectonic setting and the location of Lake Pillsbury relative to the estimated position of the southern edge of the Gorda slab and the Clear Lake volcanic field suggest that volcanism may initiate in this region within the next 400 k.y.

The Mw 5.8 Mineral, Virginia, earthquake of August 2011 and aftershock sequence: constraints on earthquake source parameters and fault geometry

The Mw 5.8 earthquake of 23 August 2011 (17:51:04 UTC) (moment, M0 5:7 × 10 17 N·m) occurred near Mineral, Virginia, within the central Virginia seis- mic zone and was felt by more people than any other earthquake in United States history. The U.S. Geological Survey (USGS) received 148,638 felt reports from 31 states and 4 Canadian provinces. The USGS PAGER system estimates as many as 120,000 people were exposed to shaking intensity levels of IV and greater, with approximately 10,000 exposed to shaking as high as intensity VIII. Both regional and teleseismic moment tensor solutions characterize the earthquake as a northeast- striking reverse fault that nucleated at a depth of approximately 7 2 km. The distri- bution of reported macroseismic intensities is roughly ten times the area of a similarly sized earthquake in the western United States (Horton and Williams, 2012). Near- source and far-field damage reports, which extend as far away as Washington, D.C., (135 km away) and Baltimore, Maryland, (200 km away) are consistent with an earthquake of this size and depth in the eastern United States (EUS). Within the first few days following the earthquake, several government and aca- demic institutions installed 36 portable seismograph stations in the epicentral region, making this among the best-recorded aftershock sequences in the EUS. Based on modeling of these data, we provide a detailed description of the source parameters of the mainshock and analysis of the subsequent aftershock sequence for defining the fault geometry, area of rupture, and observations of the aftershock sequence mag- nitude-frequency and temporal distribution. The observed slope of the magnitude- frequency curve or b-value for the aftershock sequence is consistent with previous EUS studies (b 0:75), suggesting that most of the accumulated strain was released by the mainshock. The aftershocks define a rupture that extends between approxi- mately 2-8 km in depth and 8-10 km along the strike of the fault plane. Best-fit modeling of the geometry of the aftershock sequence defines a rupture plane that strikes N36°E and dips to the east-southeast at 49.5°. Moment tensor solutions of the mainshock and larger aftershocks are consistent with the distribution of aftershock locations, both indicating reverse slip along a northeast-southwest striking southeast- dipping fault plane.

Seismic characterization of an active metamorphic massif, Nanga Parbat, Pakistan Himalaya

Earthquakes recorded by a dense seismic array at Nanga Parbat, Pakistan, provide new insight into synorogenic metamorphism and mass flow during mountain building. Micro- seismicity beneath the massif drops off sharply with depth and defines a shallow transition between brittle failure and ductile flow. The base of seismicity bows upward, mapping a thermal boundary with 3 km of structural relief over a lateral distance of 12 km. Anom- alously low seismic velocities are observed at the core of the massif and extend to depth through the crust. The main locus of seismicity and low velocities correlates with a region of high topography, rapid exhumation, high geothermal gradients, young metamorphic and igneous ages, and crustal fluid flow. We suggest a genetic link between these phenom- ena in which hot rocks, rapidly advected from depth, are pervasively modified at relatively shallow levels in the crust.

Proposed project would give unprecedented look under North America

An unprecedented examination of the Earths deep interior and investigation across a broad range of scales of the structure of the North American continent and the processes that formed it would be among the undertakings of a proposed 10-year Earth Science project called USArray. Now in the planning and development stage, the project would permit a three-dimensional (3-D) systematic investigation of North America, improving the resolution of lithospheric images by an order of magnitude. For the Earth sciences, the project would be the seismological equivalent of the Hubble space telescope. A number of factors suggest that North America is particularly suited for this project, including the states of current knowledge and technology, the availability of a sophisticated infrastructure, organization in the seismological community scientific economy, and widespread scientific interest.

Deep crustal reflection profiling offshore southern central California

Deep crustal reflection profiling offshore southern central California shows that the offshore Santa Maria Basin and adjacent shelf and slope are underlain by oceanic crust which underthrusts the margin. On common-midpoint (CMP) sections the top of the oceanic crust appears as a strong coherent reflection occurring at 6 s two-way time (twt). The reflection can be traced continuously more than 50 km from the deep ocean basin eastward beneath the continental shelf. Depth conversion places the top of oceanic crust at 6–16 km depth with a dip of 8° toward the coast. Beneath the Santa Lucia High the crustal reflection breaks up into several east dipping segments. Above the oceanic crust an accretionary wedge increases in thickness from 3 km at the base of the slope to over 12 km beneath the Santa Lucia High. The Neogene sedimentary section above the accretionary prism exhibits relatively little deformation, implying that most of the accretion occurred during pre-Neogene convergence between the North American plate and oceanic plates located to the west. The offshore Santa Maria Basin exhibits at least two periods of Neogene deformation: an early extensional phase, accompanied by strike-slip faulting, occurred in the lower to middle Miocene followed by compression and basin inversion in the upper Miocene-lower Pliocene. The development of structures in the basin began in the northwest and is progressively younger to the southeast. Faults identified in the offshore sedimentary column offset acoustic basement but are not imaged in the Franciscan basement material below 2–3 s twt and do not extend to, or offset, the deep reflections. We suggest that the oceanic crust served as a detachment surface above which the shallow deformation occurred. Neogene sediments in the offshore basins show only 1–3 km of shortening. This implies that shortening, due to Neogene oblique plate motion between the Pacific and North American plates, extends east of the offshore Santa Maria Basin and is distributed across the entire transform margin.