Uplifting the Cordillera Blanca: Elevation increases during extensional deformation along the crest of the Peruvian Andes
Abstract
The Cordillera Blanca Detachment Fault is a major extensional structure cutting the northern Peruvian Andes. Uplift of the footwall block is responsible for creating the highest topography in the central Andes. Based on field mapping, analysis of DEM's and new geochronology, we show that the post Miocene increase in the mean elevation of the footwall is driven predominantly by tectonic forcings, but with a significant contribution from the flexural response to glacial unloading. We have obtained ages using 10Be on moraines and fluvial terraces offset by fault motion at four locations spanning ~110 km along the fault. Although all dated features are similar in age (~14 to ~21 kyr), total vertical offset increases monotonically from ~15 m in the south to ~105 in the north. Likewise, vertical offset rates increase from 0.59 ± 0.2 mm/yr (at Quebrada Tuco) to 5.1 ± 0.8 mm/yr (at Quebrada Huaytapallana). Several lines of evidence suggest that the fault motion over at least the last ~5 Myr has been primarily restricted to footwall uplift. First, the Cordillera Blanca Detachment Fault has dissected a formerly continuous, northward extension of the Puna Plateau, which has resulted in displacing this surface along the crest of the Cordillera Blanca up to ~2000 m above the elevation of the regional remnants of this formerly continuous plateau. Second, a well- preserved, range-parallel paleovalley lies within the hanging wall supradetachment Callejon de Huaylas basin, and contains the Rio Fortaleza ignimbrite (dated at ~6-8 Myr). The long profile of this paleovalley parallels the profile of the modern river bed (Rio Santa) for ~60 km, suggesting little of the down-to-the north tilting of the hanging wall that would be expected if hanging wall motion accommodated significant fault offset. Finally, the fault offset rates are strongly correlated to both maximum elevation and relief within the footwall. We can use the relief below the offset Puna Plateau to calculate the magnitude of glacial unloading and thus, the flexurally-driven uplift of the footwall produced by glacial incision. We have estimated this effect by determining the missing mass below a reconstructed plateau surface along three 30 km x 250 km range normal topographic swaths. The missing mass distribution was input into a one-dimensional flexural model, yielding estimates of ~500 m to ~1000 m of flexurally-driven uplift, with an increase in the total uplift from south to north. The flexurally-driven uplift accounts for no more than 40% of the total uplift of the range since onset of motion along the Cordillera Blanca Detachment Fault. Thus, the post Miocene increases in relief and mean elevation along the Cordillera Blanca Detachment Fault require a more deep seated tectonic forcing.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2006
- Bibcode:
- 2006AGUFM.T23G..03H
- Keywords:
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- 1824 Geomorphology: general (1625);
- 4918 Cosmogenic isotopes (1150);
- 8175 Tectonics and landscape evolution;
- 8177 Tectonics and climatic interactions