Abstract
The Peruvian Andes have attained modern elevations of ~4000
m.
Many peaks along the drainage divide are in excess of 5500 m with
~2-3
km of vertical relief, and canyons cut into the western edge have
up to
3.5 km relief. The Rio Pativilica Canyon is cut into a
folded Mesozoic marine sequence intruded by granitic rocks of the
Coastal Batholith, which are overlain by Tertiary volcanics: this
canyon has relief in excess of ~3 km. Based on
geomorphological
evidence, two episodes of valley incision cut the Puna erosional
surface: 1) The Vallé stage, which is characterized by
broad
valleys with ~2000-2500 m of incision; and 2) the younger
Cañón stage, characterized by steep canyons with
~2000-2500 m of incision. The thermochronological record of
rocks
in the Canyon was evaluated to understand the timing of
incision.
Zircon fission-track (ZFT) and zircon U+Th/He (ZHe) ages from the
canyon are all nearly ~35 Ma except at the very top of the canyon
when
ages drop to ~25 Ma. These zircon cooling ages most likely
represent a late episode of cooling following the last phase of
plutonism in the Coastal Batholith.
Helium and fission track ages on apatite allow a detailed
understanding
of the timing of canyon incision. Here we integrate apatite
fission-track (AFT) and apatite U+Th/He (AHe) results with known
geomorphology. The Vallé stage surfaces are filled
locally
by 2000+ m of a Mio-Pliocene ignimbrite. This ignimbrite has ZFT
and
ZHe ages of ~5 Ma, so this phase of incision must predate these
ignimbrite ages. In a transect from the coast across the
piedmont
and up the modern Canyon, AHe ages decrease up valley from 30 to
5.5 Ma
and AFT ages decrease from 60 to 14 Ma. AFT/AHe and
AHe/Surface
age pairs allow estimates of erosion rates, and these data suggest
substantial increases in erosion rates at ~15 Ma and again at ~5-6
Ma.
Incision of the canyon is probably caused by either increase in
erosion
rates on a previously uplifted block, or progressive uplift of the
block and concurrent incision. Based on available evidence,
we
propose that uplift of the Peruvian Andes has occurred since ~15
Ma and
that the Rio Pativilca has responded by incising down as uplift
has
progressed. Increases in erosion rates are a result of
increased
uplift rates and occur at the start of the Vallé stage at
~15 Ma
when erosion rates increased from ~75 m/Myr to 150-200
m/Myr.
Erosion rates increase again at the start of the
Cañón
stage at ~5-6 Ma when rates increased to 300-400 m/Myr.
These
data suggest that uplift of this part of the Andes is slightly
younger
than uplift of the central Andes (i.e. Altiplano and Puna) farther
to
the south, and therefore these results support he idea of
northward
growth of the orogenic belt.
Montario, M.J., 2006. Thermochronological evidence for Neogene
incision of the Rio Pativilca Canyon, northern Peru.
Unpublished MSc. thesis, State University of New York at
Albany. 205 pp., + ix
University at Albany Science Library call number: SCIENCE
Oversize
(*) QE 40 Z899 2006 M66
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