Resistivity cross section through the Juan de Fuca Subduction System and its tectonic implications

Abstract

A resistivity cross section to depths exceeding 200 km has been derived from magnetotelluric observations along a profile near latitude 45øN from the Juan de Fuca spreading center, across the coastal subduction complex, the High Cascades volcanic arc, and into the back-arc Deschutes Basin region. In this two-dimensional interpretation, emphasis was placed on data approximating the transverse magnetic mode since these data are relatively robust to common departures from the two-dimensional assumption. The vertical magnetic field, however, has been very valuable in defining structure of the offshore sediments, of the oceanic asthenosphere and below the arc volcanics of the Westem and High Cascades. The transverse electric data on land suffer a variety of three-dimensional effects, making their interpretation very difficult. In contrast, the greater uniformity of upper crustal conditions on the seafloor allowed a good fit to both modes of the impedance plus the vertical magnetic field at least down to 104 s' Important components of . While a number of aspects of the subsurface resistivity can be deduced straight from inspection of the data, computer simulation of the observations helps to quantify more rigorously the permissible or required structures. In this paper, we derive a two-dimensional resistivity model, with a presumed northsouth strike, by trial-and-error fitting of our MT measurements with a finite element forward modeling algorithm. Constraints on model geometry from independent geological or geophysical investigations have been incorporated where justified. Our paper is divided into three major sections. First, we explain the approach toward two-dimensional modeling. On land, the transverse magnetic (TM) impedance functions are emphasized because theory and experience show that they are more robust to plausible three-dimensional effects in the region than are the vertical magnetic field or, especially, the transverse electric (TE) impedance. Upper crustal structure on the seafloor appears to be rhuch simpler than on land, however, and both modes of impedance plus the vertical field are fit fairly well. Second, the model cross section is described and its goodness of fit to the data demonstrated. Model uniqueness is investigated by perturbing certain features of the model and assessing the increase in misfit.

H2O and Magmatism in Island Arc Mantle Inferred from Seismic Anelasticity and Heat Flow Data.

“…A lower crustal conductor beneath the aseismic front to the west in northeastern Japan has suggested the presence of H2O (e.g., Utada, 1987). Beneath the Oregon coast and Vancouver Island in North America, a high conductivity layer on the top of the descending Juan de Fuca plate has been observed, and lends itself to the inference of H2O-charged sediments that are partially subducted with the oceanic crust (e.g., Wannamaker et al, 1989;Young and Kitchen, 1989;Kurtz et al, 1990). A similar conductive layer has also been proposed in the Japan arc subduction zone by Utada (1987 …”

Two-Dimensional Electrical Conductivity Structure across the Southern Coastline of Australia.

“…The experiment transect intersects the rifted marginal Otway basin, which is a few kilometres wide and deep (Williams and Corliss, 1982;Finlayson et al, 1994). The effect of brinesaturated sediments on the coast effect at subduction zones has been discussed by Kurtz et al (1986Kurtz et al ( , 1990 and Wannamaker et al (1989), but has not been observed at passive margins. For example, the models of White et al (1990) and Kellett et al (1991) for southeastern Australia gave a good fit of the observed data without the presence of sediment beneath the continental shelf and slope.…”

“…Thermal structure and chemical composition may lead to lateral variations in conductivity across the coastlines. At active continental margins in northwestern USA (Kurtz et al, 1986(Kurtz et al, , 1990Wannamaker et al, 1989) and Japan (Ogawa et al,1986) the subduction of fluids gives rise to a very marked lateral variation in upper mantle conductivity across the margin, but at passive margins there is less obvious reason for such a difference. In Fig.…”

“…This model is compared to that determined by White et al (1990) and Kellett et al (1991) for the passive southeastern Australian margin, and more generally with 2D coast effect studies in different tectonic settings of western North America (e.g. Delaurier et al, 1983;Kurtz et al, 1986Kurtz et al, , 1990Wannamaker et al, 1989) and Japan (Ogawa et al, 1986). Figure 1 shows the deployment location of the four seafloor magnetometers (ROBE 1-4) and one land based magnetometer (ROBE 6) in the experiment.…”

The CAFE Experiment : a joint seismic and MT investigation of the Cascadia Subduction System

“…We can additionally utilize the tipper function, which relates the horizontal magnetic field information to the vertical magnetic field. The relationship between these parameters can be expressed as in equation We generated models that inverted both the TM and TE modes, with error floors of 5% and 15% respectively, and a set of models using only the TM mode, which has been shown to be less influenced by, and therefore more robust to the effects of 3D structure (Wannamaker et al, 1989). The tipper function was included in both sets of inversions.…”