Construction and degradation of a broad volcanic massif: The Vicuña pampa volcanic complex, southern central Andes, NW Argentina

The Vicuña Pampa volcanic complex,
at the SE edge of the arid Puna Plateau of
the Central Andes, records the interplay
between volcanic construction and degradational
processes. The low-sloping Vicuña
Pampa volcanic complex, with a 1200-mdeep,
southeastward-
opening depression,
was previously interpreted as a collapse caldera
based on morphological considerations.
However, characteristic features associated
with collapse calderas do not exist, and close
inspection instead suggests that the Vicuña
Pampa volcanic complex is a strongly eroded,
broad, massif-type composite volcano of
mainly basaltic to trachyandesitic composition.
Construction of the Vicuña Pampa volcanic
complex occurred during two distinct
cycles separated by the development of the
depression. The first and main cycle took
place at ca. 12 Ma and was dominated by
lava flows and subordinate scoria cones and
domes. The second cycle, possibly late Mio-cene in age, affected the SW portion of the
depression with the emplacement of domes.
We interpret the central depression as the
result of a possible sector collapse and subsequent
intense fluvial erosion during middle
to late Miocene time, facilitated by faulting,
steepened topography, and wetter climate
conditions compared to today. We estimate
that ~65% of the initial edifice of ~240 km3
was degraded. The efficiency of degradation
processes for removing mass from the Vicuña
Pampa volcanic complex is surprising, considering
that today the region is arid, and
the stream channels within the complex are
predominantly transport limited, forming
a series of coalesced, aggraded alluvial fans
and eolian infill. Hence, the Vicuña Pampa
volcanic complex records the effects of past
degradation efficiency that differs substantially
from that of today.