Small variations in gravity anomalies and topographic elevation observed in areas that have undergone highly variable amounts of upper crustal thinning can be satisfactorily explained by ductile flow of lower crustal material under the proper conditions. In this study we examine the boundary between the unextended Colorado Plateau and a strongly extended domain in the Basin and Range Province in the Lake Mead (Nevada) region. Bouguer gravity and topography data suggest that both present and preextensional variations in crustal thickness between the unextended and extended regions are small. Analytic channel flow models show that viscosities required for ductile flow in a lower crustal channel to reduce discontinuities in crustal thickness associated with variable amounts of extension are highly d•dent on the channel thickness and on the length scale of flow required. We test these hypotheses in the area around Lake Mead, Nevada, which is a transition zone between highly extended crust of the Basin and Range and the adjacent unextended Colorado Plateau.

The Adriatic region forms an intermediate continental foreland overthrust along its northeastern margin by the southwest vergent Dinaric thrust belt in Eocene-Oligocene time and along its southwestern margin by the northeast vergent Apennine thrust belt in Pliocene-Quaternary time. Orogenic activity within these thrust belts was accompanied by the development of two superposed foredeep basin systems of opposite polarity and different ages. Using well log, biostratigraphic, and seismostratigraphic data, the geometry of this composite basin system was reconstructed along three profiles at beginning of Quaternary, middle Pliocene, beginning Pliocene, and beginning Eocene time. Modeling of reconstructed geometries using a thin elastic sheet approximation yields a range of acceptable effective elastic plate thicknesses for the central Adriatic region of Te=5-10 km for Eocene-Oligocene flexure and Te= 10-15 km for Pliocene-Quaternary flexure (although an upper bound for Te could not be established on one of the three profiles). These results are consistent with a constant effective elastic plate thickness of Te=10 km for the Adriatic lithosphere and preclude the possibility that significant weakening of the Adriatic plate occurred between flexural events. Modeling of incremental deflections between Pliocene and Quaternary time gives results consistent with constant values of Te=10 km in the central Adriatic and Te=15 km in the northern Adriatic and Po Plain and shows little evidence for weakening of the plate during Pliocene-Quaternary time. Thus within the resolution of the data presented in this paper, there is little evidence for viscous relaxation of the lithosphere on time scales between about 2 and 50 m.y. Analysis of bending of an idealized lithosphere with a simple brittle-elasticductile theology, and a low to moderate thermal gradient suggests that the small values of Te observed within the Adriatic region can be readily understood as the result of bending of the lithosphere to unusually high curvature (4 x l0 -6 m -1) and do not require unusually high temperatures within the foreland lithosphere. The same theological model is also consistent with the absence of significant inelastic Paper number 93TC01935 0278-7407/94/93TC-01935510.00 yielding for at least 50 m.y. after the cessation of loading. An apparent unbending of the Adriatic lithosphere began in early Quaternary time, approximately coeval with the cessation of major thrusting within the Apennine thrust belt. The threedimensional pattern of Quaternary deflection makes it difficult to attribute this phenomenon to local depositional processes and suggests that unbending reflects a fundamental change in the subduction process in early Quaternary time. Our preferred interpretation is that unbending is the result of a diminution of forces acting on the subducted Adriatic lithosphere at mantle depths. The Dinaride and Apennine orogenic belts have formed in response to local accommodation of convergence between the Eurasian and African plates in Cenozoic time...

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Geochemical tracer data (i.e., 222Rn and four naturally occurring Ra isotopes), electromagnetic (EM) seepage meter results, and high-resolution, stationary electrical resistivity images were used to examine the bi-directional (i.e., submarine groundwater discharge and recharge) exchange of a coastal aquifer with seawater. Our study site for these experiments was Lynch Cove, the terminus of Hood Canal, WA, where fjord-like conditions dramatically limit water column circulation that can lead to recurring summer-time hypoxic events. In such a system a precise nutrient budget may be particularly sensitive to groundwater-derived nutrient loading. Shore-perpendicular time-series subsurface resistivity profiles show clear, decimeter-scale tidal modulation of the coastal aquifer in response to large, regional hydraulic gradients, hydrologically transmissive glacial terrain, and large (4-5 m) tidal amplitudes. A 5-day 222Rn time-series shows a strong inverse covariance between 222Rn activities (0.5-29 dpm L(-1)) and water level fluctuations, and provides compelling evidence for tidally modulated exchange of groundwater across the sediment/water interface. Mean Rn-derived submarine groundwater discharge (SGD) rates of 85 +/- 84 cm d(-1) agree closely in the timing and magnitude with EM seepage meter results that showed discharge during low tide and recharge during high tide events. To evaluate the importance of fresh versus saline SGD, Rn-derived SGD rates (as a proxy of total SGD) were compared to excess 226Ra-derived SGD rates (as a proxy for the saline contribution of SGD). The calculated SGD rates, which include a significant (>80%) component of recycled seawater, are used to estimate associated nutrient (NH4+, Si, PO4(3-), NO3 + NO2, TDN) loads to Lynch Cove. The dissolved inorganic nitrogen (DIN = NH4 + NO2 + NO3) SGD loading estimate of 5.9 x 10(4) mol d(-1) is 1-2 orders of magnitude larger than similar estimates derived from atmospheric deposition and surface water runoff, respectively.