Macrogeomorphic evolution of the post-Triassic Appalachian mountains determined by deconvolution of the offshore basin sedimentary record

A perplexing macrogeomorphic problem is the persistence of topography in mountain ranges that were initially formed by orogenic events hundreds of millions of years old. In this paper, we deconvolve the post-Triassic macrogeomorphic history of a portion of one of these ranges, the central and northern Appalachians, using a well-documented offshore isopach sedimentary record of the US Atlantic margin. Topography is an important signature of tectonic, eustatic and/or geomorphic processes that produces changes in the erodible thickness of the crust (ETC). We define ETC as the total thickness of crust that would have to be consumed by erosion to reduce the mean elevation of a landscape to sea level. We use the term ‘source flux’, designated by ν˙, to describe the rate of change in ETC attributed to deep-seated geological processes such as crustal thickening, crustal extension, magmatic intrusions or dynamic flow in the mantle. In a mountain belt, the rate of change of mean elevation with respect to a base level, designated by ? ′, can be represented as ? ′ = c(ν˙ ? k _d z ′ ?; ? _c ) ?& hairsp;l˙ , where k _d is a proportionality constant relating the mean mechanical erosion rate to mean elevation, ? _c is the mean chemcial erosion rate, c  is a compensation ratio (held constant for Airy isostasy at 0.21) and l˙  is the rate of eustatic sea-level change. This equation describes the sum of constructive source terms, two destructive erosion terms and a eustatic sea-level term. We use this simple linear equation to develop a landscape evolution model based on the concept of a unit response function. The unit response function is analogous to a unit hydrograph which describes the relationship between input (rainfall) and output (discharge) in a hydrological system. In our case, we solve for the general relationship between the source flux into the mountain belt and the erosional flux out of the belt. Offshore sediment volumes are used to determine the erosional flux. Drainage basin area is treated as either a constant (pinned drainage divide) or as increasing through time (migrating drainage divide). We use a third-order polynomial fit to a global sea-level curve to account for long-term eustatically driven changes in ETC and in drainage basin area. Chemical erosion is treated as a constant fixed at 5 m Myr^?1. We consider two end-member models. The first is a ‘tectonic’ model in which the source flux is allowed to vary while k _d is assumed to be constant over geological time and equal to its mean Pleistocene value of about 0.07 Myr^?1. The second is an ‘erodibility’ model in which k _d is allowed to vary, reflecting changes in climatic, climatic variables and rock-type erodibility, while the source flux is held constant at zero. The ‘tectonic’ model reveals four important increases in the source flux, ranging from 200 to 2000 m Myr^?1 that occur over short (<10 Myr) time spans, followed by a protracted period (>25 Myr) where ν˙ drops below zero to values of ?1000 to ?6000 m Myr^?1. The ‘erodibility’ model produces a topography that decays in a step-like fashion from an initial mean elevation that ranges between ～1800 and 2300 m. These models cannot unequivocally distinguish the relative importance of tectonic vs. climatic processes in the macrogeomorphic evolution of the post-rift Appalachians, but they do provide some first-order quantitative prediction about these two end-member options. In light of existing stratigraphic, geological and thermochronological data, we favour the tectonic model because most of the events correlate well in time and form with known syn- and post-rift magmatic events. Nevertheless, the most recent episode of increased sediment flux to the offshore basins during the Miocene remains difficult to explain by either model. Limited evidence suggests that this event may reflect asthenospheric flow-driven uplift and the development of dynamically supported topography at a time when mechanical erosion rates were increasing in response to a cooling terrestrial climate.     

Potential radionuclide transport pathways from seafloor dumpsites: Kamchatka region of the North Pacific Ocean

Encapsulated nuclear waste materials, dumped by Russia, are present at two deepwater seafloor locations in the offshore north-west Pacific Ocean, south-east of the Kamchatka Peninsula. This paper assesses potential pathways by which these wastes might, if released from their containers, disperse away from the dumpsites and through the surrounding ocean. A review of large-scale ocean circulation theory and of field and model results suggests that mean abyssal currents are north-eastward to eastward from the dumpsite locations and would advect leaking materials toward the north-eastern Pacific. Results of advective and diffusive horizontal plume transport models are consistent with this sense of flow. Trajectory speeds are, however, subject to considerable uncertainty. Our results suggest that as little as 5 years or as long as 100 years might be required for material to be transported from the dump sites to the north-east Pacific. Dilution by 4 or 5 orders of magnitude is predicted during this transit. Vertical mixing or upwelling are necessary in order to transport contaminants upward from north-east Pacific abyssal waters to the near-surface layers before they can potentially impact productive coastal regions, such as those off Alaska. Information concerning such upwelling mechanisms is inadequate for estimation of rates or to identify geographical areas that might be at risk.     

Precursory seismic quiescence: A preliminary assessment of the hypothesis

Numerous cases of precursory seismic quiescence have been reported in recent years. Some investigators have interpreted these observations as evidence that seismic quiescence is a somewhat reliable precursor to moderate or large earthquakes. However, because failures of the pattern to predict earthquakes may not, in general, be reported, and because numerous earthquakes are not preceded by quiescence, the validity and reliability of the quiescence precursor have not been established.We have analyzed the seismicity rate prior to, and in the source region of, 37 shallow earthquakes (M 5.3–7.0) in central California and Japan for patterns of rate fluctuation, especially precursory quiescence. Nonuniformity in rate for these pre-mainshock sequences is relatively high, and numerous intervals with significant (p<0.10) extrema in rate are observed in some of the sequences. In other sequences, however, the rate remains within normal limits up to the time of the mainshock. Overall, in terms of an observational basis for intermediate-term earthquake prediction, no evidence is found in the cases studied for a systematic, widespread or reliable pattern of quiescence prior to the mainshocks.In earthquake sequences comprising full seismic cycles for 5 sets of (M 3.7–5.1) repeat earthquakes on the San Andreas fault near Bear Valley, California, the seismicity rates are found to be uniform. A composite of the estimated rate fluctuations for the sequences, normalized to the length of the seismic cycle, reveals a weak pattern of a low rate in the first third of the cycle, and a high rate in the last few months. While these observations are qualitative, they may represent weak expressions of physical processes occurring in the source region over the seismic cycle.Re-examination of seismicity rate fluctuations in volumes along the creeping section of the San Andreas fault specified by Wyss and Burford (1985) qualitatively confirms the existence of low-rate intervals in volumes 361, 386, 382, 372 and 401. However, only the quiescence in volume 386 is found by the present study to be statistically significant.     

An example of trondhjemite genesis by means of alkali metasomatism: Rockford Granite,Alabama Appalachians

A model for trondhjemite genesis is proposed where granite is transformed to trondhjemite via infiltration by a Na-rich metamorphic fluid. The Rockford Granite of the Northern Alabama Piedmont serves as the case example for this process and is characterized as a synmetamorphic, peraluminous trondhjemite-granite suite. The major process operative in the conversion of granite to trondhjemite involves cation exchange of Na for K in the feldspar and mica phases through a volatile fluid medium. Whole-rock ¹⁸O values for the trondhjemites are negatively correlated with atomic proportion K/Na ratio indicating a partial reequilibration of the altered granitoids with a Na- and¹⁸O-rich metamorphically derived fluid. Biotite decomposition to an Al-epidote-paragonitic muscovite-secondary quartz assemblage is also associated with the sodium metasomatism, as are apatite replacement by Al-epidote and secondary zircon crystallization. Rare albitization of primary magmatic plagioclase and discontinuous grossularite reaction rim growth on magmatic garnet are present in the trondhjemites indicating the mobility of Ca during alkali metasomatism. The replacement of magmatic phases by me tasomatic phases exemplifies the chemical changes produced during infiltration metasomatism where the trondhjemites are depleted in P₂O₅, Th, Rb, U, K₂O, V, Sn, F, MgO, Pb, TiO₂, FeO^* and Li and enriched in CaO, Na₂O, Zr and Sr relative to the granites. Other elements, such as Cr, MnO, Cu, Zn, Co, Ba, SiO₂, Ni, Al₂O₃, are shown to be relatively immobile during the metasomatism. The infiltration metasomatism probably occurred during prograde regional metamorphism, when a discrete fluid phase was produced in the surrounding amphibolite-grade metasediments. Foliation planes in the granite apparently served as conduits for fluid flow with reaction-enhanced permeability accompanying the 8% molar volume reduction during Na-for-K exchange in the feldspars. A source for the Na and Sr in the metamorphic fluid may have been paragonitic muscovite in the metasedimentary country rocks. Rubidium and K were probably retained in metasedimentary biotite. The Silent Lake pluton in southeastern Ontario is a possible analogue to the alkali metasomatic processes affecting the Rockford Granite.     

Pollutant transport in a shallow unconfined aquifer in Perry,Ohio

The groundwater system in northern Perry Township in Lake County, Ohio, is a shallow, unconfined aquifer consisting of periglacial lake beach deposits and less permeable lacustrine plain deposits. Groundwater flow is generally toward Lake Erie from south to north and is controlled by the top of the Ashtabula Till, but strong, local variations are caused by northward flowing streams During the study period, water levels in most wells exhibited a seasonal fluctuation of less than 0 3 m from their mean values. The areal distributions of chloride and nitrate concentrations indicate that road salt runoff easily infiltrates the aquifer and that nitrate may be sourced from fertilizer application. Ground-water flow and solute transport models indicate that in excess of 27 years are required to obtain chemical steady-state under hydrologic steady-state conditions. The simulations also demonstrate that nitrate loading must occur in more than one cultivated field in order to obtain the observed wide-spread nitrate distribution.     

Fe-Mn partitioning between garnet and ilmenite: experimental calibration and applications

A new mineralogic geothermometer based on the partitioning of Fe and Mn between garnet and ilmenite has been calibrated by reversal experiments in the P-T range 600–900° C, 2 and 5 kbars and for fO₂=QFM. The results constitute a sensitive geothermometer applicable over a broad range of composition and conditions. Garnetilmenite thermometry has advantages relative to existing geothermometers because of its accurate calibration, marked temperature sensitivity and the chemical and structural simplicity of the crystalline solutions involved. Application to natural assemblages reveals that the garnet-ilmenite geothermometer yields temperatures that agree well with other estimates. The reactivity of, and relatively rapid Fe-Mn diffusion in ilmenite may lead to retrograde resetting of high temperature partition values, but these factors may be useful for estimating rock cooling rates. Analysis of the experimental data indicates minor positive deviations from ideality for Fe-Mn garnets and ilmenites. Absolute magnitudes of interaction parameters (W _AB) derived from a regression analysis are subject to considerable uncertainty. The partition coefficient is, however, strongly dependent on the difference between solution parameters. These differences are well constrained with a magnitude of W _FeMn ^ilm –W _FeMn ^gar 300 cal mol^–1. The accuracy and applicability of garnet-ilmenite thermometry will improve with the availability of better thermodynamic data for garnet crystalline solutions.Abbreviations and symbols used in text R universal gas constant (cal/mol/°K) - T absolute temperature (°K or °C) - P pressure (kbars) - V ⁰ volume change of reaction (1) - H _{1, T} ⁰ standard state enthalpy change of reaction (1) at 1 bar and the T of interest, in cal/mole - S _T ⁰ entropy change of reaction (1) at T of interest, in cal/mole/°K - G _P,T ⁰ standard free energy change of reaction (1) at the T and P of interest, in cal/mole - distribution coefficient for Fe-Mn partitioning between garnet and ilmenite - K apparent equilibrium coefficient for reaction (1) - _i ^j activity of component i in phase j - W _A-B binary A-B interaction (Margules) parameter - gar garnet - ilm ilmenite - biot biotite - ol olivine - opx orthopyroxene