3.8. The Andean Analogue: Insights from active mountain building processes
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| Fig. 72. Geophysical framework of the Central Andes: a) surface topography; b) depth to lithosphere-asthenosphere boundary; c) depth to continental Moho discontinuity (after Tassara, 2005). Black rectangles show location of the transect given in Fig. 73. |
To this date, the question of why and how the Andean plateau-type orogen formed with crustal thickening at the leading edge of western South America remains one of the hotly debated issues in geodynamics. During the Cenozoic, the Altiplano and Puna plateaux of the Central Andes (average elevation some 4 km, with an extent of 400 x 2000 km) developed during continuous subduction of the oceanic Nazca plate in a convergent continental margin setting – a situation that is unique along the 60,000 km of convergent margins around the globe. The key challenge is to understand why this plateau developed only along the central portion of the South American leading edge, as well as why and how this feature developed only during the Cenozoic, although the cycle of Andean subduction had been ongoing since at least the Jurassic. Moreover, it would appear that this style of orogeny has only rarely occurred during the Earth’s history, another example probably being the Cretaceous North American Laramides (Fig. 72).
Deep geophysical data across the Central Andes between 20°S and 24°S (ANCORP-Working Group, 2003) (ANCORP'96 and associated geophysical studies) indicate the widespread presence of partial melts or metamorphic fluids at mid-crustal level under the plateau between its bounding Cordilleras. From structural balancing studies, these fluids or melts are associated with decoupling of upper crustal shortening and lower crustal thickening. Based on similar indications from the distribution of magmatism it has been argued that upper plate weakening resulting from widespread heating and partial melting may have been the key to widespread shortening behind the volcanic arc (Isacks, 1988; Allmendinger et al., 1997). In addition, changes in plate convergence are generally considered to have been responsible for tuning the changes in the upper plate system. While the available wealth of geophysical data would seem to lend support to the role of melts and fluids in upper plate orogeny, the sensitivity of the observed elastic, thermal, and conductivity properties to fluids may overemphasize their role (Fig. 73).
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| Fig. 73. Geophysical transect through the Central Andes (for location see Fig. 72). a) Automatic line drawing of depth migrated ANCORP reflection data, including onshore wide angle and receiver function results (Yuan et al., 2000), merged with results of the offshore CINCA experiment and its onshore recording (Patzwahl et al., 1999). Thick black lines denote reflector elements. Reflector elements from below the western and eastern Andean plateau margins after Wigger et al. (1994). The grey line at 65-75 km below the Andean plateau represents the Moho discontinuity as inferred from receiver function data (Yuan et al., 2000). b) Suggested interpretation of seismic observations including an alternative interpretation of the relation between slab geometry and seismicity. Isotherms after Springer (1999) are only included for the fore-arc domain due to unconstrained convective heat transfer below the Andean plateau (after ANCORP-Working Group, 2003). |
Another feature unique to the Central Andes is the complete preservation of syn-tectonic volcanics and sediments throughout the orogen and at its margins allowing spatial and temporal reconstruction of the deformation and uplift history (see Elger et al., 2005; Oncken et al., 2006 for details). Accordingly, analysis shows that the difference between the upper plate velocity and the oceanic slab rollback velocity is crucial in determining the amount and rate of shortening and surface uplift as well as their lateral variability at the leading edge of the upper plate. This first order control is tuned by factors affecting the strength balance between the South American upper plate lithosphere and its interface with the subducting Nazca lower plate. These include variations in trench-ward sediment flux affecting plate interface coupling and slab rollback. Ultimately, the location of the Central Andes in the global southern hemisphere arid belt plays a key role by allowing the rise and lateral spread of a high plateau (e.g. Lamb and Davis, 2003; Elger et al., 2005; Oncken et al., 2006). The combination of these parameters was highly uncommon during the Phanerozoic, leading to very few plateau style orogens at convergent margins, such as the Cenozoic Central Andes in South America or the Laramide North American Cordillera. This combination was never realized in Europe in its entirety, but some elements are apparent, as for example during the Variscan Orogeny.
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| Fig. 74. Cartoon showing main factors affecting the development of orogens at convergent ocean-continent plate margins (Courtesy O. Oncken). |
Although the Andes and the Alps, as a typical representative of a young European orogen, are extremely diverse in terms of size, internal architecture, completion of a Wilson cycle, etc., their direct comparison indicates substantial differences (e.g. Schmid et al., 1996). Early Alpine deformation, including subsequent collision, was essentially focused on the fore-arc domain of the Adriatic plate (although no real arc was present with exception of the Colli Euganei). The Alps have virtually grown by continuous material addition through mainly basal accretion to the former fore-arc system where all topography evolution was focused. In contrast, nearly all of the Andean deformation and uplift has been confined to the back-arc domain (or the arc domain in the south). Only very little deformation has affected the South American fore-arc during Cenozoic plateau building with very diverse styles from southern Peru to southern Chile. Mechanisms responsible for these features are gradually emerging from various ongoing research initiatives and may provide clues for understanding the pre-collisional evolution of South European convergent plate margins. The obvious key is the kinematic response of the fore-arc to the above variations resulting from trench fill evolution and the ratio between upper plate motion and slab/hinge rollback. These affect fore-arc material addition or destruction leading to tectonic accretion or subduction erosion with resultant vertical motions of the fore-arc system as well as to various internal kinematics (c.f. Heuret and Lallemand, 2005). Spatial and temporal variability of these processes at the South American margin indicates the very delicate, partly self-controlled balance of the interaction of several of the above processes. All of these observations underscore the role of the Cenozoic climatic evolution of the Andean margin and the influence of its N-S extent through various climate zones (Fig. 74).
Recent observations of the kinematic behaviour on very short time scales from satellite-based techniques (GPS, INSAR), seismology, and neotectonics show that strain accumulation is partly related to the style of seismicity. At convergent plate margins the extent and degree of seismic coupling play a major role in force transmission, as well as in the mechanisms generating great intraplate earthquakes. Despite the key role of the coupling zone for plate tectonics, the processes that shape it and its relation to surface deformation are poorly understood. Based on observations of transients at various time scales, fore-arc systems may tend to be close to self-organized criticality, reacting in a complex mode with highly complex kinematics from surface to depth in different stages of the seismic cycle. These kinematic variations and the related transients are poorly understood but probably a future key to understanding strain accumulation in the brittle crust. Hence, in the present day Andean fore-arc as well as in the South European fore-arc systems we may be facing a complex system of coupled processes responsible for deformation and surface response that primarily include the interaction of the climatically controlled trench fill evolution, the upper plate structural heterogeneity and various transients related to the seismic cycle as well as to changes in accretion mode. These may – in conjunction with other aspects – ultimately be the cause for differences between the fore-arc orogens of South European plate margins and the Andean back-arc orogen.