A preliminary crustal model of the Oaxaca continental margin and subduction zone from magnetotelluric and gravity measurements
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Abstract
A preliminary model of the conducting layer associated with the continental margin and the Cocos Plate subduction zone along two transects normal to the southern Mexico continental margin is presented. The transects are located approximately from Oaxaca City to Puerto Escondido (W-transect), and to Puerto Angel (E-transect). The study is based on fourteen magnetotelluric (MT) soundings and on regional gravity measurements. Modelling of gravity data includes spectral analysis, 2 and 2.5-D Talwani-type models and several inversion schemes with constant and exponentially variable density contrasts. One dimensional inversion of MT data using a Marquardt SVD algorithm was performed on the rotationally invariant determin ant average apparent resistivities and phases after static shift was removed with reference to a site apparently devoid of galvanic distortion. A conductive layer of varying thickness was correlated with a seismic reflecting horizon under various MT sites of the western profile. These correlated soundings were used to constrain the inversion results of the remaining MT sites. We assume that the top of the electrically conductive horizon corresponds to the top of the Phanerozoic lower continental crust, which has an approximate dipping angle of 20° to 25° and is characterized by conductivities from 10 to 90 ohm-m. Although this boundary is in general plunging towards the continent there are several sites in both transects at which it flattens and even reverses its trend. This may be an artefact of the I-D inversion in zones where the data are 2 or 3-D. This is supported by relatively high induction vectors consistently pointing SWat these sites. The crust thickens towards the central portion of both MT transects, beneath the Sierra Madre del Sur. There is a good correspondence with gravity models derived from spectral analysis and estimates of the crust/mantle boundary. There is however an apparent depth difference (8 to 10 km) between the MT- and gravity-derived crust/mantle boundary. The dip of the subducting plate estimated from the gravity model is shallow, about 13 degrees. The topography of the Moho shows a correlation with major structural features at the surface.
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References
CHAKRIDI, R., M. CHOUTEAU and M. MARESCHAL, 1992. A simple technique for analysing and partly removing galvanic distortion from the magnetotelluric impedance tensor: Application to Abitibi and Kapuskasing data (Canada), Geophys. J. Int., 108, 917-929. DOI: https://doi.org/10.1111/j.1365-246X.1992.tb03480.x
COUCH, R. W. and G. V. BURBACH, 1985. Cross section, Southern Mexico, Guatemala and Costa Rica margin, In: J. W. Ladd and R. T. Burner (Eds), Middle America Trench off western Central America, Atlas 7, Ocean Margin Drilling Program, Regional Atlas Series: Marine Science International, Woods Hole, MA, Sheet 13.
COUCH, R. and S. WOODCOCK, 1981. Gravity and structure of the continental margins of southwestern Mexico and northwestern Guatemala. J. Geophys. Res., 86, 1829-1840. DOI: https://doi.org/10.1029/JB086iB03p01829
CROSS, T. A. and R. H. PILGER, 1982. Controls of subduction geometry, location of magmatic arcs, and tectonics of arc and back-arc regions., Geol. Soc. Am. Bull. 93, 545-562. DOI: https://doi.org/10.1130/0016-7606(1982)93<545:COSGLO>2.0.CO;2
EMSLAB group, The, 1988, The EMS LAB Electromagnetic sounding experiment, EOS (Trans. A. Geophys. Union), 69, 7, 89. DOI: https://doi.org/10.1029/88EO00060
GOUGH, D. I., 1986. Magnetoteluric evidence for subduction of seafloor sediments, Nature, 321, 566. DOI: https://doi.org/10.1038/321566a0
HYNDMAN, R. D. and P. M. SHEARER, 1989. Water in the lower continental crust: modelling magnetotelluric and seismic reflection results, Geophys. J. Int., 98, 343-365. DOI: https://doi.org/10.1111/j.1365-246X.1989.tb03357.x
KURTS, R. D., J. M. DeLAURIE and J. C. GUPTA, 1986. A Magnetotelluric sounding across Vancouver Island sees the subducting Juan de Fuca Plate, Nature. 312, 596-599. DOI: https://doi.org/10.1038/321596a0
LOMNITZ, C. 1982. Direct evidence of a subducted plate under southern Mexico. Nature, 269, 235-238. DOI: https://doi.org/10.1038/296235a0
MOORE, J. C., J. S. WATKINS, T. H. SHIPLEY, K. C. McMILLEN, S. B. BACHMAN and N. LUNDBERG, 1982. Geology and tectonic evolution of a juvenile accretionary terrane along a truncated convergent margin: Synthesis of results from the Leg 66 of the Deep Sea Drilling Project, southern Mexico. Geol. Soc. Am. Bull. 93, 847-861. DOI: https://doi.org/10.1130/0016-7606(1982)93<847:GATEOA>2.0.CO;2
NAVA, F. A., V. R. TOLEDO and C. LOMNITZ, 1985. Plate waves and the 1980 Huajapan de Leon, Mexico earthquake. Tectonophysics. 112, 463-492. DOI: https://doi.org/10.1016/0040-1951(85)90191-X
NAVA, F. A. et al., 1988. Structure of the Middle America trench in Oaxaca, Mexico. Tectonophysics. 154, 241-251. DOI: https://doi.org/10.1016/0040-1951(88)90106-0
ORTEGA-GUTIERREZ, F., 1990. Ocean-Continent Transect H3. Geol. Soc. Am. Chart, Scale 1: 1500,000.
RATSCHBACHER, L., U. RILLER, M. MESCHEDE, U. HERRMANN and W. FRISCH., 1991. Second look at suspect terranes in southern Mexico. Geology, 19, 1233-1236. DOI: https://doi.org/10.1130/0091-7613(1991)019<1233:SLASTI>2.3.CO;2
ROSS, D. and G. SHOR, 1965. Reflection profiles across the Middle America Trench. J. Geophys. Res. 70, 5551-5572. DOI: https://doi.org/10.1029/JZ070i022p05551
SHANKLAND T. J. and M. E. ANDER, 1983. Electrical conductivity temperatures and fluids in the lower crust, J. Geophys. Res. 88, 9475-9484. DOI: https://doi.org/10.1029/JB088iB11p09475
SHANKLAND, T. J., 1989. A case of two conductors, Nature, 340, 102. DOI: https://doi.org/10.1038/340102a0
SHOR, G. G., JR. and R. L. FISHER, 1961. MiddIe America Trench: Seismic refraction studies. Geol. Soc. Am. Bull., 72, 721-730. DOI: https://doi.org/10.1130/0016-7606(1961)72[721:MATSS]2.0.CO;2
TALWANI, M., J. L. WORZEL, and M. LANDISMAN, 1959. Rapid gravity computations for two-dimensional bodies with application to the Mendocino submarine fracture zone. J. Geophys. Res., 64, 49-59. DOI: https://doi.org/10.1029/JZ064i001p00049
URRUTIA-FUCUGAUCHI, J., 1984. On the tectonic evolution of Mexico: paleomagnetic constraints. In: Plate Reconstruction from Paleozoic Paleomagnetism, AGU. Geodynamics Series, 12, 29-47. DOI: https://doi.org/10.1029/GD012p0029
VALDES, C. M., W. D. MOONEY, S. K. SINGH, R. P. MEYER, C. LOMNITZ, J. H. LUETGERT, C. E. HELSLEY, B. T. R. LEWIS and M. MENA., 1986. Crustal structure of Oaxaca, Mexico, from seismic refraction measurements. Bull. Seismol. Soc. Am. 76, 547-563.
WANNAMAKER et al. 1989. Resistivity cross section through the Juan de Fuca subduction system and its tectonic implications, J. Geophys. Res., 94. 14, 127-14, 144. DOI: https://doi.org/10.1029/JB094iB10p14127