Analysis of flexural isostasy of the northern Andes

The mode in which a lithosphere plate supports overlying topography is greatly driven by the strength of the plate. By analyzing the geophysical signature of lithosphere flexure, in the space and spectral domains, the strength of the plates that support the north Andean mountains and adjacent basins, and the topography of Kenya was investigated. In addition, the effect of windowing on elastic thickness estimates obtained via the coherence method was evaluated. ^ The coherence between the topography and Bouguer gravity spectra of northern South America suggests that the average elastic thickness of the lithosphere is 30 km. Although lateral variations were not resolved by the coherence implementation, these became apparent by modeling the foreland stratigraphy of the Llanos, Barinas and Maracaibo sub-Andean basins. Flexural models reveal a zone of lithosphere weakness beneath the eastern flank of the Eastern Cordillera and western flank of the Venezuelan Andes. The gravity anomaly calculated from these models is consistent with the observed Bouguer gravity anomaly. This zone of weakness appears to separate the strong, old Guyana shield lithosphere from the weaker and probably younger Andean lithosphere. The zone of weakness may correspond to a Paleozoic feature at the western margin of cratonic South America, or a Mesozoic rift arm that weakened the proto-Andean lithosphere. ^ Using synthetic data as well as the northern South America topography and gravity, this study demonstrates that lithosphere strength calculated from the coherence of mirrored data may overestimate the true lithosphere strength. As a result, many lithosphere plates may be weaker than currently thought. In light of this observation, gravity and topography data from Kenya were reevaluated using multitaper spectral techniques. The elastic thickness of this plate, currently undergoing rifting, was estimated at 7 to 8 km, a factor of 2 less than previously estimated. These estimates suggest that despite intense fracturing and sustained tensile stresses, continental lithosphere plates undergoing rifting are able to retain some strength. ^

Isotopic studies of the Guerrero composite terrane, west-central Mexico: Implications for provenance of crustal rocks and genesis of ore metals

A variety of world-class mineral deposits occur in Mesozoic and Tertiary rocks of the Guerrero terrane. New Pb isotope analyses of various crustal units and ores from distinct subterranes of the Guerrero terrane are presented to trace metal sources in these deposits and infer source reservoirs. New Sr and Nd isotope results are provided to gain insight into the provenance of the crustal rocks from the Guerrero terrane. Triassic schist samples from the Arteaga Complex and Triassic-Jurassic phyllite and slate samples from the Tejupilco metamorphic suite contain radiogenic Pb (206Pb/204Pb = 18.701–19.256) relative to bulk earth models. Cretaceous sedimentary rocks of the Zihuatanejo Sequence are more radiogenic (206Pb/204Pb = 18.763–19.437) than samples from the Huetamo Sequence (206Pb/204Pb = 18.630–18.998). Tertiary intrusive rocks from La Verde, Inguaran, La Esmeralda, and El Malacate plot to the right of the average Pb crust evolution curve of Stacey and Kramers (206Pb/204Pb = 18.705–19.033). Ores from the La Verde and La Esmeralda porphyry copper deposits yield isotopic ratios (206Pb/204Pb = 18.678–18.723) that are generally less radiogenic than the host igneous rocks, but plot within the field defined by the sedimentary rocks from the Huetamo Sequence. Tertiary intrusive rocks from the Zimapan and La Negra districts in the Sierra Madre terrane plot above and to the right of the Stacey-Kramers reference line (206Pb/204Pb = 18.804–18.972). Lead isotope ratios of ore minerals from the Zimapan and La Negra skarn mines ( 206Pb/204Pb = 18.775–18.975) resemble those of the associated igneous rocks, implying a magmatic Pb input in the skarn deposits. New Sr and Nd isotope data on metamorphic rocks (87Sr/ 86Sr = 0.707757–0.726494 and 143Nd/144 Nd = 0.512109–0.512653) suggest that the basement of the Guerrero terrane originated from sources that had been derived from an old cratonic area. The narrow ranges and generally low 87Sr/86Sr ratios (0.704860–0.705755) and 143Nd/144Nd values (0.512765–0.512772) above that of bulk earth for igneous rocks from Inguaran, El Malacate, and La Esmeralda suggest a relatively low degree of crustal contamination. However, the isotopic values for the La Verde site (87Sr/86Sr = 0.708784 and 143Nd/144Nd = 0.512640) may indicate the involvement of a more evolved crustal component.