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McDannell, KT; Lewis, JC; Montero W.
IX Central American Geological Congress, 2008
Contemporary Shearing on the Tucurrique fault: Evidence for an
active pull-apart basin at Pejibaye, Costa Rica
Kalin T. McDannell, Jonathan C. Lewis, Walter Montero
Identifying active faults and estimating slip rates in central Costa Rica are critical to
understanding: (1) geological hazards; and (2) the upper plate architecture of the Middle
America Trench (MAT). The Tucurrique fault has been interpreted to be one of the more
active faults within a broad NW-striking zone of dextral shearing referred to as the Rio
Sucio – Atirro fault system. Here we provide geomorphic and geochronologic evidence
that a small intermontane basin near Pejibaye is an active pull-apart structure forming at a
releasing bend in the Tucurrique fault. Longitudinal profiles of the Pejibaye, Gato and
Tepemechin Rivers show upstream knick points that suggest structural control of stream
gradients. Below these knick points the rivers are sediment dominated and each
preserves well-developed terraces. Downstream of the confluence of these three rivers,
the trace of the Tucurrique fault is marked by the transition to a bedrock channel and
narrow bedrock canyon on the Pejibaye River.
Within the basin several observations suggest ongoing deformation. The course of
the Tepemechin River displays a prominent right turn upon entering the basin, after
which it flows along the foot of the steep slopes that mark the eastern margin of the basin.
Weakly preserved triangular facets occur on the slopes above the stream. We interpret
the river to be localized along an active normal fault. Comparable triangular facets occur
along the western margin of the basin above the Pejibaye River. In addition, terraces of
the Pejibaye River appear to display offset risers. Specifically the T2/T3 riser displays
~20 m of dextral offset and the T3/T4 riser displays ~30 m of dextral offset. These
offsets are aligned with a dextrally displaced bedrock spur to the south. Collectively the
offset features are interpreted to reflect a cross-basin fault. The ages of the offset terraces
have not yet been determined, however, wood found within a thick bentonite layer
(altered volcanic ash) exposed at the cutbank of the Tepemechin River where it turns
right yields a radiocarbon age of 4.02 ± 0.07 Ka. Wood collected from another bentonite
layer exposed upstream yields a radiocarbon age of 31.45 ± 0.55 Ka. These samples
come from the upstream portion of the pull-apart basin and suggest basin growth was
under way by Late Pleistocene time.
Our findings have two important implications. First they suggest that the slip rate on
the Tucurrique fault might be obtained with additional work to establish the geometry of
the active fault strands and the ages of offset geomorphic features. This is important
because the fault may direct a large component of contemporary dextral shear into the
more densely populated areas from Tucurrique westward into the Central Valley.
Obtaining slip rate estimates on active faults continues to be an important yet elusive
goal. Second, our results suggest that Late Pleistocene to Holocene eruptive products
from Turrialba Volcano occur SE of the volcano.
Reference
McDannell, K.T., Lewis, J.C., and Montero, W., 2008, Contemporary shearing on the
Tucurrique fault: Evidence for an active pull-apart basin at Pejibaye, Costa: IX Central
American Geological Congress, San José, Costa Rica, 2–4 June 2008: San José, Costa
Rica, Colegio de Geólogos de Costa Rica y Escuela Centroamericana de Geologı́a, p.
115–116.