Neotectonic, seismicity and seismotectonic data allow to define the Central Costa Rica Deformed Belt (CDCCR) as a wide and diffuse recent fault system located between the Caribbean plate and the Panama microplate, transverse to the central region of Costa Rica, and located between the Middle America Trench (Pacific side) and the North Panama Deformed Belt (Caribbean side). In the central pacific forearc, the fault system combines sinistral strike slip and normal slip. In the inner arc there is: 1) An external thrust fault system, with a predominantly WNW trend, and 2) An inner fault system including northwest dextral strike slip faults and east-northeast to northeast sinistral strike slip faults. In the backarc, the fault system joins the North Panama Deformed Belt, which shows an external northwest thrust and fold system and an inner northwest fault system combining dextral strike slip and reverse faulting. The neotectonic faults which define the CDCCR are related to the collision of the Cocos Ridge with southern Costa Rica. The tectonic indentation caused by this collision produces the tectonic displacement to the east and north of the Panama microplate relative to the Caribbean Plate. This tectonic process produces normal slip combined with sinistral strike slip faulting in the western pacific rear side of the diffuse limit, causing the development of marine basins transverse to the forearc. On the other hand, in the leading side of the limit, an external thrust and fold system has been developed between the inner arc and the backarc. The predominantly strike slip faulting located inside the inner arc allows translational as well as rotational movements between the diverse tectonic blocks of the plate boundary zone. RESUMEN:Datos neotectónicos, de sismicidad y de sismotectónica son usados para definir el Cinturón Deformado del Centro de Costa Rica (CDCCR) como un sistema de fallas reciente, difuso y ancho localizado entre la placa Caribe y la microplaca de Panamá, el cual es transversal a la región central de Costa Rica y localizado entre la fosa Mesoamericana (lado pacífico) y el Cinturón Deformado del Norte de Panamá (sector caribe). En el antearco pacífico central, el sistema de fallas combina desplazamientos de rumbo siniestral y normal. En el arco interno existe: 1) Un sistema externo de fallas inversas, que tiene rumbo predominante WNW en la región norte del país y 2) Un sistema interno de fallas que está constituído principalmente por fallas de desplazamiento de rumbo de tipos dextrales de dirección NW y siniestrales con rumbo entre ENE y NE. En la región trasarco, el sistema de fallas del CDCCR se une con el Cinturón Deformado del Norte de Panamá, el cual incluye un sistema externo de fallamiento inverso y plegamiento de rumbo noroeste y otro interno de desplazamiento de rumbo dextral combinado con desplazamiento inverso de rumbo noroeste. Las fallas neotectónicas que constituyen el CDCCR son originadas por la colisión del levantamiento del Coco con el sur de Costa Rica. La indentación tectó...

No abstract

Long sublinear ridges and related scarps located at the base of large volcanic structures are frequently interpreted as normal faults associated with extensional regional stress. In contrast, the ridges bordering the Central Costa Rica volcanic range (CCRVR) are the topographic expression of hanging wall asymmetric angular anticlines overlying low‐angle thrust faults at the base of the range. These faults formed by gravitational failure and slumping of the flanks of the range due to the weight of the volcanic edifices and were perhaps triggered by the intrusion of magma over the past 20,000 years. The thrust fault underlying Alajuela ridge rises from a depth of ∼700 m at a dip of ∼15° over a distance of a few kilometers. It ramps up at ∼600 m depth on a ∼40° slope and terminates in the axis of the forward syncline, ∼400 m below the surface. The hanging wall of this fault has been thrusted about 200 m away from the volcanic range axis. Thrusting on the lower flanks of the CCRVR is compensated by extension on the range axis with formation of summit grabens. Shallow seismicity in the volcanic range suggests that thrusting is still active. We postulate that at Kilauea volcano, Hawaii, the gravity‐ and magma‐induced slumping of the southern flank on low‐angle thrust faults generates hanging wall anticlines similar to those observed in Costa Rica. These anticlines are hypothesized to occur along the base of the volcano, where the thrust faults ramp up toward the sea bottom. Ridges and scarps between 2000 and 5000 m below sea level are interpreted as the topographic expression of these folds. We further suggest that the scarps of the CCRVR and Kilauea are valid scaled terrestrial analogs of the perimeter scarp of the Martian volcano Olympus Mons. We suggest that the crust below Olympus Mons has failed under the load of the volcano, triggering the radial slumping of the flanks of the volcano on basal thrusts. The thrusting would have, in turn, formed the anticlinal ridges and scarps that surround the edifice. The thrusting seems to be preferential toward the northwest and southeast, and a northeast‐southwest rift zone seems to have developed, perpendicular to the regional topographic gradient. The thrust faults may extend all the way to the base of the Martian crust (about 40 km), and they may have been active until almost the end of the volcanic activity. We suggest that gravitational failure and slumping of the flanks of volcanoes is a process common to most large volcanic edifices. In the CCRVR and at Kilauea this slumping of the flanks is a slow intermittent process, but it could evolve to rapid massive avalanching leading to catastrophic eruptions. Thus monitoring of uplift and displacement of the folds related to the slump tectonics could become an additional effective method for mitigating volcanic hazards.

No abstract

On April 22, 1991, a large earthquake (M w = 7.7) occurred along the Caribbean coast of Costa Rica and western Panama. The rupture area of the fault mapped from the aftershocks is 45x85 km 2. The distribution of aftershocks and the local geological record suggest that faulting occurred on a blind thrust sheet that shallows toward the northeast. Uplift of the Caribbean coast ranging from 1.5 m near Puerto Lim6n and decreasing gradually toward the southeast was observed along the Caribbean. Northwest of Puerto Lim6n no significant coastal uplift was observed. This observation agrees with the aftershock data suggesting the rupture did not extend to the northwest of this location. The Lim6n earthquake also triggered aftershocks on secondary faults in the crust. These events are apparently associated with a family of imbricate thrust and strike-slip faults that lie in the eastern piedmont of the Talamanca Cordillera. The historical seismicity indicates that the Caribbean coast has been the site of several historical earthquakes with magnitudes greater than 7.0. On April 26, 1916, another earthquake (M s = 6.9) took place in the same region. Summing the scalar seismic moment release along the Caribbean coast, the average rate of slip is approximately 0.8 cm/yr, compared with a value of 0.4 to 0.8 cm/yr along the Pacific subduction zone, depending on the estimated width of the seismogenic zone. Therefore a large fraction of the relative plate motion between the Cocos and Caribbean plates (9.8 cm/yr) appears to be taken up by crustal deformation in the back arc. The tectonic regime in the area appears to be dominated by the collision of the buoyant Cocos ridge with the subduction zone. The absence of a Wadati-Benioff zone where the Cocos ridge collides with the trench suggests the slab does not subduct beneath the Osa Peninsula; this is supported by the Pliocene gap of volcanism present in Costa Rica. Thus the predicted relative motion between the Cocos and Caribbean plates appears to be absorbed by a low rate of seismic moment release in the forearc and by a broad zone of Paper number 94TC02546. 0278-7407/95/94TC-02546510.00 active crustal shortening and underthrusting in the back arc. This type of tectonic deformation resembles more a collisional regime than a typical subduction zone environment. 20-40-ß ß ß ß 24, Apr 22, Apr ß ß ß ß ß ß e ß ß A B gm , 0 20 40 60 80 Km 60, 1993. Bowland, C.L., Seismic stratigraphy and structure of the western Colombian basin, Caribbean Sea, M.S. thesis, 248 pp., Univ. of Tex., 1984. Brune, J., Seismic moment, seismicity, and rate of slip along major fault zones, J. Geophys. Res., 68, 777-784, 1968. Camacho, E., E1 tsunami del 22 de abril de 1991 en Bocas del Toro, Panamfi, Rev. Geol. Am. Cent., Spec. Vol., 61-64, 1994. Camacho, E. and V. Viquez, Historical and instrumental seismicity of the Caribbean region of Panama, U.S. Geol. Surv. Prof Pap., in press, 1994. Chinn, D.S., and B.L. Isacks, Accurate source depths and focal mechanisms of shallow earthquakes in western South America, Tec...