HONGN Fernando Daniel
capítulos de libros
A compilation of space-time distribution of Cenozoic volcanism in the Central Adean arc, pattern recognition and a conservative assessment of tectonic significance
The Andes - Active Subduction Orogeny
Lugar: Berlin; Año: 2006; p. 29 - 43
The coincidence of late Tertiary shortening and crustal thickening with vigorous arc and back-arc volcanic activity in the Central Andes has long invited speculation about a causal relationship between magmatism and deformation. In aid of understanding this and related issues, this paper and attached data files present a new compilation of radiometric ages, location and lithology of over 1200 Cenozoic volcanic features of the southern Central Andes (18°-28°S). Synoptic maps comparing the distribution of arc activity with zones of active shortening at different age intervals demonstrate that the two processes are unrelated on a regional scale. This is confirmed by a more detailed comparison of shortening rate and frequency of volcanism from 40 Ma to present at latitude 21°S. Shortening begins earlier than magmatism in the plateau region and with time, deformation concentrates on the eastern edge of the plateau whereas magmatism narrows to the western edge where the arc is active today. Spatial density analysis of the late Miocene and younger volcanism shows local a clustering of activity where the arc is crossed by major NW-SE lineament zones, confirming the influence of these structures in enhancing magma ascent. Age-frequency plots of arc activity from 18° to 28°S show that the onset of volcanism was similar everywhere (30±2 Ma) but the main phase of activity shifted from north to south with time. Whereas 50% of arc activity north of 22°S accumulated by 14 Ma and 90% by 4 Ma, the southern segments reached these levels only at about 8Ma and 2 Ma, respectively. This southward shift may be related to the late development of uplift and plateau formation in the southern Puna relative to the Altiplano region but this cannot be tested with available data. There is no evidence from the age-frequency plots that subduction of the Juan Fernandez ridge under the study area significantly affected the arc productivity. The complexity of arc distribution patterns with time and the changes in these patterns along the arc suggest that subduction geometry (slab dip) is a contributing factor but not the only cause. Comparison of young volcanic arc location and width with independent estimates of present-day slab dip and upper mantle temperature show a poor correlation. We conclude that the factors contributing to the real distribution of surface volcanism in this geodynamic setting are too complex and variable to justify inferring past conditions of subduction from the distribution of arc magmatism alone.