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Contemporary shearing on the Tucurrique fault: Evidence for an active pull-apart basin at Pejibaye

Authors:

Abstract

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.
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.
... While the geochronology of offset geologic units is well established for the western part of the Central Costa Rica Deformed Belt (Valle Central and Pacifi c coast), precise age constraints are lacking for the eastern region, including the Atirro-Río Sucio fault system. Within the Atirro-Río Sucio system, isotopic ages are limited to a few scattered 40 Ar/ 39 Ar and radiocarbon dates on latest Pleistocene and Holocene units in the Irazú-Turrialba volcanic fi eld (e.g., Soto, 1988;Linkimer, 2003;Alvarado et al., 2004;Hidalgo et al., 2004;Pavanelli et al., 2004;Alvarado and Ganz, 2012) and the Pejibaye pull-apart basin (McDannell et al., 2008). Despite the scarcity of well-constrained ages for Atirro-Río Sucio structures, the relative ages of some individual faults can be inferred from landform expression, crosscutting relationships, and the distribution of modern seismicity. ...
... As slip migrated from the Omega fault (Fig. 3) to the Tucurrique fault, dextral motion through an abrupt right-stepping bend opened the Pejibaye pull-apart basin along the southern edge of the larger Irazú-Turrialba basin. Well-preserved fault scarps and offset late Pleistocene to Holocene stream terraces (31,000-4000 14 C yr B.P.) attest to continued growth of the Pejibaye pull-apart basin (McDannell et al., 2008). Ongoing dextral slip along the entire Atirro-Tucurrique fault segment generates occasional seismicity and has strongly affected the modern landscape by disrupting drainage patterns, displacing landforms, and offsetting geologic units (Figs. ...
Article
The Atirro-Río Sucio fault system forms a major northwest-trending strike-slip fault zone in east-central Costa Rica. We examined the kinematics and temporal evolution of this fault system through geomorphic, structural, and seismologic analysis. This 150-km-long strike-slip fault zone traverses the northern fl ank of the paleovolcanic Cordillera de Talamanca and extends northwestward into the active Cordillera Volcánica Central. Historical seismicity includes frequent minor swarms and occasional moderate-magnitude (M 5.0-6.5) damaging earthquakes. Field geomorphic evidence, fault kinematic data, and earthquake focal mechanisms are consistent in showing dextral slip along the mapped traces of northwest-striking faults. Continuity with other transcurrent faults in northwest Costa Rica indicates that the Atirro-Río Sucio fault system may form the southeastern end of a regional network of northwest-trending dextral faults that accommodate margin-parallel displacement of the Central American forearc sliver. The Atirro-Río Sucio fault system originates within the Central Costa Rica Deformed Belt inboard of the indenting Cocos Ridge. We infer that ridge collision drives lateral escape of crustal fragments northwestward along an array of dextral Central Costa Rica Deformed Belt faults including the major structures of the Atirro-Río Sucio fault system. This zone of arc-parallel extrusion thus represents the root of the Central American forearc sliver. Consistent with recent geodynamic models, we propose that northwestward sliver escape along the Atirro-Río Sucio faults is driven by rigid indentation of the aseismic Cocos Ridge into southern Costa Rica.
Thesis
Full-text available
Continental interiors are an underappreciated facet of plate tectonics due to the perception that they are often static over long timescales. Salient tectonic margins receive more attention, owing to their comparatively dynamic state during the creation and destruction of continents and ocean basins. I utilize low-temperature (U-Th)/He and 40Ar/39Ar thermochronology to address questions regarding the spatial and temporal thermal evolution, and by proxy, the exhumation and burial histories of these slowly-cooled terranes through deep time. Chapter One is focused on the topographic evolution of the Hangay Mountains of central Mongolia, where apatite (U-Th)/He data and thermal models suggest that the post-orogenic landscape experienced rapid relief loss of a few hundred meters in the mid-Mesozoic. The Hangay are now characterized by a relict landscape that has undergone slow exhumation on the order of ~10 m/Ma since the Cretaceous (~100 Ma), analogous to other old landscapes such as the Appalachians. The central Mongolian landscape remains in a state of topographic disequilibrium, while modest surface uplift since the Oligocene and recent glaciation have had little effect on erosion rates due to the fact that there has been minor tectonism and a very dry climate during the Cenozoic. Chapter Two confronts the problem of dispersed apatite (U-Th)/He cooling ages that often afflict slowly-cooled terranes, such as the Hangay Mountains. Conventional total- gas analysis offers little explanation or remedy for He age scatter that has been typically attributed to many factors, such as isotopic zonation, crystal lattice defects, and radiation damage. Unlike conventional analysis, the continuous ramped heating (CRH) technique exploits incremental 4He release during a continuous, controlled heating rate under static extraction line conditions. This approach allows the measurement of the cumulative gas released from apatite grains and assessment of the characteristic sigmoidal release curve shape as a means to distinguish between expected (radiogenic) and anomalous volume- diffusion behavior. Screening results for multiple apatite suites show that the CRH method can discriminate between the simple, smooth release of apatites exhibiting expected behavior and well-replicated ages, and grains that do not replicate well with more complicated 4He release patterns – and offers a means to correct these ages. Chapter Three is focused on understanding the assumed long-term stability of the southern Canadian Shield. Craton stability over billion-year timescales is often inferred due to the lack of geologic records to suggest otherwise. For the Proterozoic (2.5-0.54 Ga) there is little or no intermediate temperature thermal-history information for many locations, however K-feldspar 40Ar/39Ar MDD data and modeled thermal histories linked to published high- and low-temperature data from the Canadian Shield suggest the southern craton experienced unroofing delayed until ~1 Ga, coeval with the formation of the supercontinent Rodinia. K-feldspar data suggest a prolonged period of near- isothermal cooling of <0.5°C/Ma in the late Proterozoic where rocks were positioned at cratonic depths in the middle crust for up to ~500 million years at temperatures of ~150- 200°C and subsequently exhumed to the surface in the Neoproterozoic. Thermal history solutions and geophysical evidence of underplating and crustal thickening at the Mid- Continental Rift and adjacent regions suggest uplift and a previously unrecognized phase of cratonic unroofing that began in the Neoproterozoic, which ultimately contributed to the development of the Great Unconformity of North America.
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