SURFACE UPLIFT OF THE WESTERN SLOPE OF THE PUNA SEGMENT OF THE CENTRAL ANDEAN PLATEAU

One might expect different timing and magnitudes of surface uplift of the Altiplano and Puna segments of the Central Andean plateau because their times and magnitudes of upper crustal shortening differ. However, recent work reveals only a limited correlation of surface uplift of the Altiplano to local or orogen-wide upper crustal shortening. To identify other causes of uplift of orogens, such as deep crustal flow or loss and mantle processes, independent measures of surface uplift are needed, as we describe here. Originally sub-horizontal Neogene depositional surfaces of known age along the western Andean slope may serve as markers of regional tilting and uplift of the plateau. We measured long-wavelength (6-8 km) dips of those surfaces, in areas without local-scale folds, west of the northern Puna. In the Atacama Salar basin (23-24ºS) immediately west of a ~30-km-wide, 5º W-facing slope that accompanies a 2000 m increase in relief, a horizon (~5-10 Ma years old) which was nearly horizontal when formed (salt pan facies) now dips westward 2.0º, 3.9º and 2.5º (3 E-W trending seismic lines spanning ~50 km N-S distance). An underlying horizon, ~ middle Miocene and unknown facies, dips 2.3º, 5.3º, 5.9º on the same profiles. Correction for differential compaction yields minimum long-wavelength tectonic rotations of 0.5º, 3.5º, and 1.2º (~10-5 Ma horizon) and ~1º, 5º, and 4.6º (~middle Miocene horizon). To the northwest in an equal position relative to the western Andean slope, at ~22º30'S, Calama basin seismic data reveal ~2.3º west dip of a 19 Ma unit whose salt pan facies indicate near horizontal deposition. Corrected for compaction, the tectonic rotation exceeded 2ºW. Extrapolated to the 30 km width of the western mountain front, these rotations marginal to the western slope may explain ~250-1800 m of relief generation since 10 Ma and 500-2600 m since the middle Miocene at 23-24ºS, and 2000 m since ~19 Ma at 22º30'S. In magnitude, long-wavelength rotation is a plausible explanation for all the current long-wavelength relief of the Puna, without local surface-breaking or upper crustal blind faults. Nester et al. (this meeting) show similar magnitudes and timing at 21-22ºS (Altiplano west slope). The similarity suggests that mantle and lower crustal dynamics are as important as upper crustal shortening as causes of surface uplift.