Abstract

Differing rates and styles of Quaternary deformation along the Costa Rican fore arc reflect segmentation of the trench corresponding with three contrasting domains of subducting sea floor offshore. Rapid upward flexure of the southern fore-arc segment results from the subduction of the buoyant Cocos Ridge, whereas moderate deformation along the central fore-arc segment reflects the subduction of buoyant seamounts. In contrast, the northern Costa Rican fore arc deforms in response to the subduction of relatively dense sea floor devoid of major bathymetric anomalies. Quaternary geomorphic evidence and earthquake oral histories from the Península de Nicoya, within the northern Costa Rican fore arc, demonstrate arcward tilting of the fore-arc crust, with discrete uplift events occurring during large subduction earthquakes.

Uplift rates calculated from the late Holocene Cabuya terrace, along the peninsula's trench-perpendicular southeastern coast, decrease systematically toward the arc, from 4.5 m/k.y. at Cabuya to 1.7 m/k.y. at Montezuma, 8 km to the northeast. An uplifted carbonate beachrock horizon (radiocarbon age: 4500–5200 yr B.P.), correlated between Cabuya and Montezuma, is tilted 0.1° downward toward the arc. Although Quaternary uplift is evident as far arcward as Tambor, 10 km northeast of Montezuma, uplifted terraces are absent between Tambor and the Golfo de Nicoya. A submerged archaeological site (radiocarbon age: 2500 yr B.P.), located along the Golfo de Nicoya coast 30 km northeast of Montezuma, demonstrates late Holocene subsidence of 0.5 m/k.y. These data indicate net late Holocene arcward tilting of the peninsula at an angular rotation rate of between 0.01° and 0.02°/k.y.

Oral histories describing the M 7.7 Nicoya subduction earthquake of 5 October 1950 provide evidence of seismic cycle deformation along the peninsula's southwestern coast between Puerto Carrillo and Nosara. Interviews with 48 residents show that coseismic uplift of at least 1 m affected this coastline, and that a significant fraction of this uplift has subsequently been reversed during four decades of gradual subsidence. The coseismic deformation pattern, estimated from a uniform slip dislocation model for the 1950 earthquake, is consistent with both geomorphic and oral history evidence. These observations suggest that seismic cycle deformation functions as an important mechanism of vertical tectonism within the Costa Rican fore arc. Arcward rotation and doming of the Península de Nicoya during the Quaternary may reflect repeated cycles of sudden coseismic deformation followed by gradual postseismic and/or interseismic crustal movement.

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