Jeffrey S.
Marshall
Ph.D.
Dissertation Abstract
Marshall, J.S., 2000, Active
tectonics and Quaternary landscape evolution across the western Panama block,
Costa Rica, Central America, Pennsylvania
State University, 304 p.
Active
tectonism across the central Pacific coast and volcanic arc of Costa Rica,
Central America marks the western margin of the Panama block. This deformation
is the product of subhorizontal subduction of rough, thickened oceanic
lithosphere associated with the Cocos Ridge and seamount domain on the Cocos
plate. This investigation examines three primary aspects of this active
tectonism: 1. the fault kinematics along a diffuse deformation front across the
volcanic arc; 2. the Quaternary stratigraphy and landscape evolution associated
with volcanic arc retreat; and 3. the geomorphic expression of spatially
variable rock uplift within the coastal zone.
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 from the fore arc 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 fore arc (domain 1), mesoscale
faults show 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 historic earthquake focal mechanisms and
recent Global Positioning System (GPS) measurements. Isotopic 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 fore arc,
the effect of shallow subduction is overprinted by local uplift related to
underthrusting seamounts.
The complex architecture
of the central Costa Rican volcanic arc preserves an intricate history of
landscape evolution influenced by the changing character of the Middle America
subduction system. Stratigraphic correlations supported by the first 40Ar/39Ar
isotopic dating of late Cenozoic volcanic rocks of central Costa Rica provide
new age constraints on the tectonic evolution of the western Panama block.
Quaternary arc migration coincided with a period of vigorous eruptive activity
as the Cordillera de Aguacate became extinct and volcanism shifted northeastward
to the Cordillera Central. The newly formed Valle Central basin located between
the extinct Aguacate range and the actively evolving Cordillera Central rapidly
filled with a thick Quaternary sequence of basal andesite lavas and pyroclastic
flows (Qv4: Fm. Intracañon);
overlying ignimbrites (Qv3: Fm.
Avalancha Ardiente), and upper basaltic-andesite lavas, tephras, and lahars (Qv1: Poás Group). Middle Pleistocene
drainage capture carved the Tárcoles gorge through the Aguacate range, linking
the Valle Central with the Pacific slope. Ash flows and volcaniclastic debris
(Qv2: Fm. Orotina) descending the
gorge were deposited across the Orotina debris fan on the Pacific coastal
piedmont. A network of resistant ridges, composed of welded tuff ("snake
flow") overlying fluvial gravels, preserve meandering river channels
across a framework of lower Pleistocene lahar deposits (Fm. Tivives). The
results of field mapping, stratigraphic correlation, and 40Ar/39Ar
step-heating analyses on 22 volcanic samples provide important new insights
into the late Cenozoic landscape evolution of the central Costa Rica volcanic
arc. The isotopic ages for Aguacate Group lava flows (5.5-1.9 Ma) confirm late
Neogene activity in the Aguacate-Tilarán arc. Dating of Orotina fan lahar deposits
(Fm. Tivives), formerly mapped as Mio-Pliocene, yields ages of 1.7-1.1 Ma,
placing this unit in the lower Pleistocene. The transition from the Valle
Central Qv4 lavas (Fm. Intracañon)
to explosive Qv3 ignimbrites (Fm.
Avalancha) occurred without major interruption at approximately 0.3 Ma. The Qv2 "snake flow" welded tuff on
the Orotina fan (Fm. Orotina) represents a distal lobe of the Qv3 middle ignimbrite member (Electriona) in
the Valle Central. This unit gives six isotopic ages in the Valle Central and
on the Orotina debris fan ranging from 0.4-0.3 Ma. In general, the 40Ar/39Ar
results clearly illustrate the pitfalls of prior K/Ar dating and emphasize the
need to reevaluate Costa Rica's late Cenozoic volcanic geochronology. Since the
early Pliocene, the volcanic arc of central Costa Rica has migrated away from
the Middle America Trench at a rate of 10-20 km/Ma. Volcanic arc retreat
reflects the shallowing of subduction and enhanced outer arc tectonic erosion
as rough, hotspot-thickened oceanic crust (Cocos Ridge and seamount domain)
propagated down the subduction zone.
The
Pacific coast of southern Central America is ideally suited for the study of
active tectonics using fluvial and marine terraces. Profound changes in the
character of the Middle America subduction zone (e.g., crustal age, slab dip,
and roughness) produce marked contrasts in coastal tectonism along a 500 km
trend from central Nicaragua to southern Costa Rica. Correlation of late
Quaternary terraces along this trend provides a framework for evaluating
differences in the rates and style of subaerial fore-arc deformation. Detailed
field study of fluvial terraces developed along the Orotina-Esparza coastal
piedmont of central Costa Rica establishes a critical link for terrace
correlation across the rough-smooth boundary (RSB). Three terrace groups (I,
II, and III) are identified and correlated along the coast based on isotopic
age constraints, relative surface distribution, characteristics of gravel
deposits, and soil development. The regionally extensive El Diablo terrace
(Group I) consists of a massive, deeply weathered alluvial-fill prism (up to 50
m thick) that forms an extensive piedmont upland along most Pacific coast
drainages. Intermediate terraces (Group II), inset below the El Diablo upland,
consist of stratified, moderately weathered gravel deposits exposed above the
lower reaches of river profiles. Low elevation surfaces (Group III) adjacent to
active flood plains are formed on stratified sand and gravel deposits with
poorly developed soils. Along this tectonic coastline, base level fluctuations
are controlled by the interaction between eustatic sea level and spatially
variable rock uplift. Terrace formation is modeled as a product of aggradation
during interglacial sea level maxima, with terrace groups (I-III) linked to
major high stands (e.g., oxygen isotope stages 7, 5, 3, and 1). Climatically
induced shifts in basin hydrology may amplify or dampen this signal. The number
of terraces within each group (I-III) varies between drainages with respect to
local rock uplift. Where uplift rates are low along the Pacific slope of the
Cordillera de Tilarán (north of RSB), each terrace group is represented by a
single surface. Coastal blocks with moderate uplift rates onshore of the seamount
domain (south of RSB) preserve a maximum number of terraces reflecting better
resolution of sub-stage high stands (e.g., 5a and 5e). Where uplift rates are
highest (e.g., Herradura block and Península de Osa), older terraces (Group I)
are poorly represented, having reached elevations above an optimum zone of
terrace preservation. Estimated age ranges for the terrace groups, constrained
by isotopic analyses (40Ar/39Ar and 14C)
and by sea level curve correlations, allow for the evaluation of rates of active
faulting and coastal uplift along the Orotina-Esparza coastal piedmont.
Vertical separation rates across coast orthogonal block bounding faults range
from 0.1-0.7 m/k.y., and coastal uplift rates range from 0.7-1.7 m/k.y. for the
Esparza block and 0.4-1.5 m/k.y. for the Orotina block. Differences in coastal
uplift rates across block-bounding faults are attributed to the sub-horizontal
subduction of pronounced short wavelength roughness associated with seamounts.