26 research outputs found
Tectono-stratigraphic response of the Sandino Forearc Basin (N-Costa Rica and W-Nicaragua) to episodes of rough crust and oblique subduction
The southern Central American active margin is a world-class site where past and
present subduction processes have been extensively studied. Tectonic erosion/accretion
and oblique/orthogonal subduction are thought to alternate in space and time
along the Middle American Trench. These processes may cause various responses
in the upper plate, such as uplift/subsidence, deformation, and volcanic arc migration/
shut-off. We present an updated stratigraphic framework of the Late Cretaceousâ
Cenozoic Sandino Forearc Basin (SFB) which provides evidence of
sedimentary response to tectonic events. Since its inception, the basin was predominantly
filled with deep-water volcaniclastic deposits. In contrast, shallow-water
deposits appeared episodically in the basin record and are considered as tectonic
event markers. The SFB stretches for about 300 km and varies in thickness from
5 km (southern part) to about 16 km (northern part). The drastic, along-basin, thickness
variation appears to be the result of (1) differential tectonic evolutions and (2)
differential rates of sediment supply. (1) The northern SFB did not experience major
tectonic events. In contrast, the reduced thickness of the southern SFB (5 km) is the
result of at least four uplift phases related to the collision/accretion of bathymetric
reliefs on the incoming plate: (i) the accretion of a buoyant oceanic plateau (Nicoya
Complex) during the middle Campanian; (ii) the collision of an oceanic plateau (?)
during the late DanianâSelandian; (iii) the collision/accretion of seamounts during
the late Eoceneâearly Oligocene; (iv) the collision of seamounts and ridges during
the PlioceneâHolocene. (2) The northwestward thickening of the SFB may have
been enhanced by high sediment supply in the Fonseca Gulf area which reflects
sourcing from wide, high relief drainage basins. In contrast, sedimentary input has
possibly been lower along the southern SFB, due to the proximity of the narrow,
lowland isthmus of southern Central America. Moreover, two phases of strongly
oblique subduction affected the margin, producing strike-slip faulting in the forearc
basin: (1) prior to the Farallon Plate breakup, an Oligocene transpressional phase
caused deformation and uplift of the basin depocenter, triggering shallowing-upward
of the Nicaraguan Isthmus in the central and northern SFB; (2) a PleistoceneâHolocene transtensional phase drives the NW-directed motion of a forearc sliver
and reactivation of the graben-bounding faults of the late Neogene Nicaraguan
Depression. We discuss arguments in favour of a Pliocene development of the
Nicaraguan Depression and propose that the Nicaraguan Isthmus, which is the
apparent rift shoulder of the depression, represents a structure inherited from the Oligocene
transpressional phase
Composition of fossil manganese nodules from Costa Rica
Horizons of several types of Upper Jurassic to Lower Cretaceous manganese nodules occur locally in sequences of radiolarian cherts within the Nicoya Ophiolite Complex (NW Costa Rica). Field studies, X-ray diffraction analysis, petrographic, chemical and experimental studies give evidence of a sedimentary, early diagenetic origin of the nodules, in contrast to earlier suggestions. Smooth, discoidal, compact and very dense nodules with diameters of some mm to 9 cm dominate. They are characterized by braunite, hollandite, pyrolusite and quartz as well as 39-61% Mn, 0.9-1.6% Fe, 5-26% SiO2, 1.3-1.9% Al2O3, 1.5-3.0% Ba, 460-5400 ppm Cu, 85-340 ppm Ni and 40-130 ppm Co, among others. It is suggested that the original mineralogy (todorokite?) was altered during thermometamorphic (braunite) and hydrothermal (hollandite. pyrolusite) events. Petrographic similarities between the fossil nodules and modern deep-sea nodules are striking. Using standard hydrothermal techniques in an experimental study it is shown that under special conditions, braunite can be produced from modern nodule material
Earthâs magnetic field strength and the Cretaceous Normal Superchron: New data from Costa Rica
Constraining the longâterm variability and average of the Earthâs magnetic field strength is fundamental to understanding the characteristics and behavior of the geomagnetic field. Questions remain about the strength of the average field, and the relationship between strength and reversal frequency, due to the dispersion of data from key time intervals. Here, we focus on the Cretaceous Normal Superchron (CNS; 121â84 Ma), during which there were no reversals. We present new intensity results from 41 submarine basaltic glass (SBG) sites collected on the Nicoya Peninsula and MurciĂšlago Islands, Costa Rica. New and revised 40Ar/39Ar and biostratigraphic age constraints from previous studies indicate ages from 141 to 65 Ma. One site with an age of 135.1 ± 1.5 Ma (2Ï) gave a reliable intensity result of 34 ± 8 ”T (equivalent to a virtual axial dipole moment, VADM, value of 88 ± 20 ZAm2), three sites from 121 to 112 Ma, spanning the onset of the CNS, vary from 21 ± 1 to 34 ± 4 ”T (53 ± 3 to 87 ± 10 ZAm2). These results from the CNS are all higher than the longâterm average of âŒ42 ZAm2 and data from Suhongtu, Mongolia (46â53 ZAm2) and are similar to the Troodos Ophiolite, Cyprus (81 ZAm2, reinterpreted in this study). Together with the reinterpreted data, the new Costa Rica results suggest that the strength of the geomagnetic field was approximately the same both before and after the onset of the CNS. Therefore, the data do not support a strict correlation between polarity interval length and the strength of the magnetic field