OROGENIC CRUSTAL ARCHITECTURE AND SHORTENING DISTRIBUTION: INSIGHTS FROM NORTHERN PATAGONIA AND DISCUSSION OF THE ANDEAN-TYPE MOUNTAIN BUILDING STYLE (Invited Presentation)

Even though the different nature of the lower plate in convergent margins -oceanic or continental- defines the formation of subduction or collisional orogens, several common parameters have been invoked as first-order controls over these types of mountain belts (e.g., plates velocities, continental strength, mantle/lower crust flow, climate). Moreover, considering that crustal shortening and thickening accommodates the deformation of orogenic systems, both types of mountain systems possess different crustal configuration and surficial morphology. Within this context, it is noteworthy that the long-term behavior of the different components of mountain chains has been more thoroughly analyzed in collisional belts through numerical and analogue modeling and the structural study of natural examples.

This issue is clearly exemplified in the Andes, the archetype of subduction orogeny. Here, segmentation of the mountain belt has been traditionally addressed as a function of its contrasted along-strike topography and lateral development, crustal shortening-thickening values, and structural styles. However, jointed across-strike analyses involving both the forearc (prowedge) and retroarc (retrowedge) are scarce, leaving crucial parameters such as shortening distribution somehow ignored. This factor is important when considering opposed models of Andean orogenic vergence, i.e., with overall trenchward or cratonward tectonic transport.

In this contribution I discuss this problem through new data of the north Patagonian margin. Particularly, new field work and seismic data (borehole-controlled 2D seismic lines and basement isobaths) provide a structural and stratigraphic characterization of the forearc that is integrated with the retroarc structure. The tectonic controlling factors of the Neogene mountain building stage are analyzed with respect to the pro-and retrowedges differential growth, and the role of a major west-directed (trenchward) crustal fault is discussed within the continental-scale configuration of the Andes. According to shortening distribution between the orogenic wedges, the Andean orogenic architecture is compared with type-examples of collisional orogens, and the driving mechanisms are discussed.