Crustal structure of the Tehuantepec Ridge and adjacent continental margins of southwestern Mexico and western Guatemala

Graduation date: 1976A free-air gravity anomaly map of the area between 10°-17°N and\ud 90°-101°W shows distinctive positive and negative anomalies which\ud parallel the Tehuatepec Ridge. The positive anomaly approximately\ud overlies the topographic expression of the ridge. On the wide continental\ud shelf southeast of the Gulf of Tehuantepec a positive gravity\ud anomaly with an amplitude in excess of +100 mgal parallels the coast\ud for most of its length and turns abruptly inland at its northern end.\ud A crustal and subcrustal cross section constrained by gravity,\ud magnetic, seismic reflection and seismic refraction data and oriented\ud normal to the Guatemala continental margin indicates the positive\ud shelf anomaly is primarily the result of a relatively shallow\ud 2.62 g/cm³ density block which may be continuous with and genetically\ud similar to the rocks of the Nicoya Complex on the Nicoya Peninsula,\ud Costa Rica. A crustal and subcrustal cross section normal to the\ud continental margin off southern Mexico northwest of the Tehuantepec\ud Ridge shows a shelf structure which is very different from the continental\ud margin off Guatemala. This suggests different tectonic\ud interactions at the convergent plate boundaries on opposite sides of\ud the ridge.\ud The Tehuantepec Ridge, in a crustal and subcrustal cross section\ud oriented normal to its trend, is shown to occur at the juncture of\ud two oceanic crusts of different structure and age. Southeast of the\ud ridge in the Guatemala Basin the crust is about 9.5 km thick and\ud northwest of the ridge the crust is about 12 km thick. Hence the\ud Tehuantepec Ridge is essentially a fracture zone, but because its\ud orientation is oblique to present plate motions and it is aseismic, it\ud is concluded to be a relic fracture zone