Jeffrey S. Marshall

Effect of subducting sea-floor roughness on fore-arc kinematics, Pacific coast, Costa Rica

Fault kinematics and uplift in the Costa Rican fore arc of the Middle America convergent margin are controlled to a large extent by roughness on the subducting Cocos plate. Along the northwest flank of the incoming Cocos Ridge, seafloor is characterized by short wavelength roughness related to northeast-trending seamount chains. Onland projection of the rough subducting crust coincides with a system of active faults oriented at high angles to the margin that segment the fore-arc thrust belt and separate blocks with contrasting uplift rates. Trunk segments of Pacific slope fluvial systems typically follow these margin-perpendicular faults. Regionally developed marine and fluvial terraces are correlated between drainages and across faults along the Costa Rican Pacific coast. Terrace separations across block-bounding faults reveal a pattern of fore-arc uplift that coincides roughly with the distribution of incoming seamounts. Magnitude and distribution of Quaternary uplift along the Costa Rican Pacific coast suggests that, despite a thin incoming sediment pile, the inner fore arc shows an accumulation of mass—a characteristic that may be due to underplating of seamounts beneath the fore-arc high.

Central Costa Rica deformed belt: Kinematics of diffuse faulting across the western Panama block

Fault kinematics, seismicity, and geodetic data across central Costa Rica reveal a diffuse fault zone, here named the Central Costa Rica Deformed Belt (CCRDB). The CCRDB defines the western margin of the Panama block and links the North Panama Deformed Belt (NPDB) along the Caribbean coast with the Middle America Trench (MAT) along the Pacific coast. The junction of the CCRDB and the MAT coincides with an abrupt transition from smooth to rough crust on the subducting Cocos plate (rough-smooth boundary). Shallow subduction of rough, thickened oceanic crust associated with the Cocos Ridge shifts active shortening into the volcanic arc along faults of the CCRDB. Variable fault kinematics along this zone may reflect three combined deformation mechanisms: horizontal shortening and shear from oceanic ridge indentation, basal traction from shallow subduction, and localized block uplift from subducting seamount roughness. Within the forearc (domain 1), mesoscale faults express transtension where steep NE striking regional-scale faults intersect the Pacific coast. Across the volcanic arc (domain 2), mesoscale faults exhibit mostly sinistral and dextral slip on NE and NW striking conjugate faults, respectively. Approaching the NPDB in the back arc (domain 3), transcurrent faulting is modified by transpression and crustal thickening. Fault kinematics are consistent with earthquake focal mechanisms and Global Positioning System (GPS) measurements. Radiometric age constraints confirm that faulting postdates the late Neogene onset of shallow subduction. The ensuing deformation front has propagated northward into the volcanic arc to its present position along the seismically active CCRDB. Within the forearc, the effect of shallow subduction is overprinted by local uplift related to underthrusting seamounts.

Rough crust subduction, forearc kinematics, and Quaternary uplift rates, Costa Rican segment of the Middle American Trench

Orthogonal subduction of bathymetrically rough oceanic lithosphere along the northwestern flank of the Cocos Ridge imprints a distinctive style of deformation on the overriding Costa Rican forearc. We divide the Costa Rican forearc into three 100–160-km-long deformational domains based on the bathymetric roughness and thickness of the Cocos plate entering the Middle American Trench, the dip of the subducting plate, the variation in surface uplift rates of late Quaternary coastal deposits, and the orientations and types of faults deforming Paleogene and Neogene sedimentary rocks. In the ~100-km-long Nicoya domain, coastal deposits show localized surface uplift and arcward tilting above the downdip projections of the fossil trace of the Cocos-Nazca-Panama (CO-NZ-PA) triple junction and the Fisher seamount and ridge. In the ~120-km-long central Pacific forearc domain between the Nicoya Peninsula and Quepos, shallower (~60°) subduction of seamounts and plateaus is accompanied by trench-perpendicular late Quaternary normal faults. Steeply dipping, northeast-striking, margin-perpendicular faults accommodate differential uplift associated with seamount subduction. Uplift and faulting differ between the segments of the forearc facing subducting seamounts and ridges. Inner forearc uplift along the seamount-dominated segment is greatest inboard of the largest furrows across the lower slope. Localized uplift and arcward tilting of coastal deposits is present adjacent to subducting seamounts. In contrast, inboard of the underthrusting aseismic Cocos Ridge, along the ~160-km-long Fila Costena domain between Quepos and the Burica Peninsula, mesoscale fault populations record active shortening related to the ~100-km-long Fila Costena fold-and-thrust belt. The observed patterns of faulting and permanent uplift are best explained by crustal thickening. The uplifted terraces provide a first-order estimate of permanent strain along the forearc in Costa Rica. The permanent strain recorded by uplift of these Quaternary surfaces exceeds the predicted rebound of stored elastic strain released during subduction-zone earthquakes.

Landscape evolution within a retreating volcanic arc, Costa Rica, Central America

Subduction of hotspot-thickened seafloor profoundly affects convergent margin tectonics, strongly affecting upper plate structure, volcanism, and landscape evolution. In southern Central America, low-angle subduction of the Cocos Ridge and seamount domain largely controls landscape evolution in the volcanic arc. Field mapping, stratigraphic correlation, and 4 0 Ar/ 3 9 Ar geochronology for late Cenozoic volcanic rocks of central Costa Rica provide new insights into the geomorphic response of volcanic arc landscapes to changes in subduction parameters (slab thickness, roughness, dip). Late Neogene volcanism was focused primarily along the now-extinct Cordillera de Aguacate. Quaternary migration of the magmatic front shifted volcanism northeastward to the Caribbean slope, creating a new topographic divide and forming the Valle Central basin. Stream capture across the paleo-Aguacate divide led to drainage reversal toward the Pacific slope and deep incision of reorganized fluvial networks. Pleistocene caldera activity generated silicic ash flows that buried the Valle Central and descended the Tarcoles gorge to the Orotina debris fan at the coast. Growth of the modern Cordillera Central accentuated relief along the new divide, establishing the Valle Central as a Pacific slope drainage basin. Arc migration, relocation of the Pacific-Caribbean drainage divide, and formation of the Valle Central basin resulted from slab shallowing as irregular, hotspot-thickened crust entered the subduction zone. The geomorphic evolution of volcanic arc landscapes is thus highly sensitive to changes in subducting plate character.

Kinematics associated with late Cenozoic deformation in central Costa Rica: Western boundary of the Panama microplate

We present kinematic data for late Cenozoic deformation in central Costa Rica that marks the western margin of the Panama microplate (i.e., the Caribbean-Panama boundary). This boundary extends from the North Panama deformed belt, west through the Valle Central in Costa Rica, and then southwest along the East Nicoya Fracture Zone to intersect the Middle America Trench. Terrace correlation and basin asymmetry indicate a major change in tectonic evolution across the boundary, where three regional northeast-striking faults intersect the Pacific coast. Mesoscopic fault populations are consistent with transtension across these north-east-striking faults and with transpression within the east-trending Valle Central. This late Tertiary and Quaternary transcurrent deformation links the North Panama deformed belt to the east with the Middle America Trench to the west. Earthquake focal mechanisms are consistent with mesoscopic fault data, suggesting that fault populations characterize the present-day stress field. This deformation marks the western extent of the Panama microplate, a fragment of volcanic arc that separated from the Caribbean plate in the late Tertiary or early Quaternary and is currently advancing northward due to collisions with South America to the east and the indenting Cocos Ridge on the Cocos plate to the south.

Detailed Data Available for Recent Costa Rica Earthquake

On 5 September 2012 a magnitude 7.6 earthquake occurred beneath the Nicoya Peninsula of northwestern Costa Rica, rupturing the subduction zone between the Cocos and Caribbean plates. In most subduction zones the locus of seismic slip lies far offshore, making it difficult to infer interface seismogenic processes from on-shore observations. In contrast, the Nicoya Peninsula lies close to the trench (within 70 kilometers), allowing observations directly over the earthquake rupture zone.

Potential for Geologic Records of Coseismic Uplift and Megathrust Rupture along the Nicoya Peninsula, Costa Rica

ABSTRACT Spotila, J.A.; Marshall, J.; DePew, K.; Prince, P.S., and Kennedy, L., 2016. Potential for geologic records of coseismic uplift and megathrust rupture along the Nicoya Peninsula, Costa Rica. This study presents the first paleoseismic investigation of the Middle America subduction zone at the Nicoya Peninsula, Costa Rica. This megathrust has been intensely studied over the past decade using a range of geologic and geophysical techniques, and it experienced interplate rupture in 2012 (moment magnitude 7.6). Despite many factors that could hinder preservation of a paleoseismic record in this coastal environment, including complex deposition in a tropical mangrove setting, this reconnaissance identifies two sites where stratigraphic evidence may record relative sea-level changes associated with Holocene earthquakes. Although more work is required to better constrain the timing and nature of these events, this study suggests that the Tamarindo and Playa Carrillo estuaries contain possible paleoseismic records. At the main study site of Tamarindo, alternations between mud and peat below 1 m in depth may record relative sea-level change associated with multiple earthquakes between ∼5 and 8 ka. However, this site offers no paleoseismic evidence of late-Holocene earthquakes. Much younger stratigraphy occurs at a Playa Carrillo, where mud-peat alternations from 200 to 500 years ago could represent recent coseismic ground motions. The work presented here is limited to litho- and chronostratigraphy, however, and full interpretation of the relative sea-level histories of both sites will require quantitative, high-resolution biostratigraphic analysis. Although the results suggest paleoseismic records exist along the Nicoya Peninsula, they appear fragmentary and complex and ultimately may not provide a continuous, high-resolution paleoseismic record like those obtained at other subduction zones worldwide.