GEOPHYSICAL AND GEOCHEMICAL MODELING OF THE STRUCTURE AND EVOLUTION OF THE EL VALLE VOLCANO, PANAMA

El Valle is the eastern most volcano in the Central American Volcanic Arc, and is characterized by two periods of volcanic activity, an old group from 10-5 Ma and a young group from 3-0 Ma. Both the old and young groups are calc-alkaline in nature. However, the old group is dominantly andesitic, has flat REE curves, low Sr-Y ratios. In contrast, the younger group is dominantly dacitic, hornblende bearing, has low HREE values, high Sr/Y ratios and overall exhibits adakitic characteristics. The transition between the two geochemical groups occurs gradually between 10-3 Ma, at which point the adakitic signature is fully developed. The modern El Valle volcanic edifice is dominated by voluminous young group eruptive products (e.g. the El Hato Ignimbrite). Geochemical observations on the El Valle volcanics were used to help distinguish between the various proposed petrogenetic / tectonic models in southern Panama (e.g. the existence of a slab window, oblique subduction, slab-melting or subduction erosion of Galapagos material), in conjunction with geophysical data.

El Valle is also characterized by a large 25-30 mgal negative Bouguer Gravity anomaly that is apparent in both surficial and satellite gravity measurements. Initial surficial data indicates Bouguer anomalies vary from 32 mgal at the edge of the volcano to -5 mgal in its central caldera. Field lithologic observations paired with the gravity measurements suggest that the negative gravity anomaly is due to a large low density batholith beneath the El Valle volcano, as well as low density surficial volcanics. Quantitative modeling of the gravity data produced multiple 2.5 dimensional models, transecting across the volcano, generating a postulated geometric shape of the subsurface feature causing the low gravity anomaly.