Geology and Architecture of a Continental Arc in Flare-up Mode: Observations and Insights From a Crustal Cross - Section in Sierras Valle Fertil-La Huerta.

OTAMENDI, J.E., BANIK, T., BERGANTZ, G., DUCEA, M. Y STAIR, K.,

San Francisco, California, EE.UU.

Congreso; American Geophysical Union Fall Meeting 2008; 2008

New understanding of the mechanisms and rates by whichmagmatic arcs are constructed requires geological control from crustalcross-sections. Previously characterized arc sections are generally from islandarcs, and may reflect relatively low-flux conditions potentially quitedifferent than those associated with the large scale volcanism and rapidformation of pluton-flooded, mid-to-upper crust such as occurs in thePuna-Altiplano region today. However the deepest levels of the early OrdovicianFamatinian arc crust provides a window into the ´plumbing´ of a continental arcin flare-up mode and is exposed in an approximately 140km by 40 kmcross-section in the Sierras Valle Fertil-La Huerta in northwestern Argentina.The Valle Fertil portion of the arc grades continuously from regional-scalecalc-alkaline metaluminous I-type granitoids (granodiorite into deeperheterogeneous tonalities) downward into a complex arrangement of large maficintrusions (both cumulate and non-cumulate) and partially melted supracrustalpackages composed largely of siliciclastic sediments. The region ischaracterized by granulite grade conditions formed broadly synchronously withigneous plutonic activity spanning up to 20 Ma, with a portion of the sectionrecording 5.2-7.1 kbar and temperatures of 770-840°C.Peak metamorphic conditions produced thermal gradients of around 36° per km andthere was little pressure change upon cooling producing a very flat ccw P-Tloop. No resolvable differences in peak metamorphic conditions from localitiesalong strike 70 kmapart were detected, indicating that the arc is exposed at the same paleo-depthand the average crustal thickness was more than 20-25km. It has also been proposed that this orogen formed ina margin without continent-continent collision and hence no terrane accretion.Thus the lower crustal section exposed in the Sierras Valle Fertil-La Huertamay be an analog for the Cenozoic high plateau of the Central Andes. This is an especially attractive idea as the large maficbodies and heterogeneous tonalities might be an analog for the attenuatingmid-crustal low velocity layer imaged in Puna-Altiplano region today. Ourpresentation will present work in progress on our efforts to refine theregional architecture of this arc with a special emphasis on applications tocontinental arcs in flare- up mode. <!-- /* Style Definitions */ p.MsoNormal, li.MsoNormal, div.MsoNormal {mso-style-parent:""; margin:0cm; margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:12.0pt; font-family:"Times New Roman"; mso-fareast-font-family:"Times New Roman"; mso-ansi-language:ES; mso-fareast-language:ES;} @page Section1 {size:595.3pt 841.9pt; margin:70.85pt 37.3pt 70.85pt 63.0pt; mso-header-margin:35.4pt; mso-footer-margin:35.4pt; mso-paper-source:0;} div.Section1 {page:Section1;} -->