Download Geology: Effect of subducting sea-floor roughness on fore

Effect of subducting sea-floor roughness on fore-arc kinematics,
Pacific coast, Costa Rica
Donald M. Fisher Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania 16802
Thomas W. Gardner Department of Geosciences, Trinity University, San Antonio, Texas 78212
Jeffrey S. Marshall
Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania 16802
Peter B. Sak
Marino Protti Observatorio Vulcanológico y Sismológico de Costa Rica, Universidad Nacional, Apartado 86-3000, Heredia, Costa Rica
ABSTRACT
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.
INTRODUCTION
It has long been recognized along convergent
margins that subduction of large bathymetric features, such as ridges, leads to broadly distributed
deformation in the fore arc (Gardner et al., 1992;
Chung and Kanamori, 1978). In this paper, we
examine the fore-arc response to subduction of
shorter-wavelength roughness elements such as
Figure 1. Two topographic data sets combined to form digital elevation model for Costa
Rica and Middle America Trench (viewed to the north). Offshore data from von Huene
et al. (1995). Onland data from National Imagery and Mapping Agency of U.S. Department of Defense (30 arc second grid). Symbols: EB—Esparza Block, OB—Orotina
Block, HB—Herradura Block, PB—Parrita Block, QB—Quepos Block, FC—Fila Costeña
Thrust Belt, CS—Cóbano Surface, RSB—rough-smooth boundary, FS—Fisher
Seamount, QP—Quepos Plateau, SL—sea level.
Geology; May 1998; v. 26; no. 5; p. 467–470; 3 figures.
seamounts. Along the Middle America Trench,
there is a relatively abrupt southeastward change
from subduction of smooth Cocos plate created
at the East Pacific Rise to rough crust created at
the Galápagos Rift system (Hey, 1977). Herein,
we describe features of the onland geology that
provide insights into how the basement and sedimentary rocks in the inner fore arc deform in
response to the incoming sea-floor roughness
southeast of the rough-smooth boundary. We use
marine and fluvial terraces to establish the distribution of Quaternary uplift along the Costa Rican
Pacific coast. These data are combined with
analyses of mesoscale fault populations to characterize the fore-arc kinematics. The pattern of
uplift and faulting is compared with the offshore
bathymetry to evaluate the relationship between
the seamount distribution on the Cocos plate and
both the kinematics and spacing of faults within
the inner fore arc.
OFFSHORE BATHYMETRY AND
STRUCTURE OF THE COSTA RICAN
FORE ARC
Offshore of Costa Rica, the major bathymetric
features on the Cocos plate are the Cocos Ridge,
the Quepos Plateau, the Fisher Seamount, and a
series of smaller seamount chains (von Huene
et al., 1995) (Fig. 1). The Cocos Ridge and adjacent seamount chains intersect the trench at high
angles and approximately parallel to the relativeplate-motion vector between the Cocos plate and
the overriding fore arc. Thus, the position of
incoming seamounts relative to the fore arc does
not vary greatly through time. To the northwest,
where smooth Cocos crust is subducting, the fore
arc marks the edge of the Caribbean plate. The
rough crust, however, is subducting beneath the
edge of the Panama block, a microplate that has
been independent of the Caribbean plate since
the late Tertiary or early Quaternary (Silver et al.,
1990). The position of the rough-smooth boundary along the trench may have migrated southeast
to its present position offshore of the tip of the
Península de Nicoya (von Huene et al., 1995;
Hinz et al., 1996).
The incoming seamount province has caused
considerable tectonic erosion in the offshore
part of the fore arc; the trench slope is scalloped
and indented around seamounts, and marginperpendicular scarps record seamount tracks
beneath the fore arc (Fig. 1; von Huene et al.,
467
1995). As these features indicate, the outer forearc kinematics are characterized by initial uplift
in response to the underthrusting of a seamount
followed by subsidence after the seamount is
subducted (e.g., von Huene and Lallemand,
1991; von Huene and Scholl, 1991; von Huene
et al., 1995). This deformation history leaves the
characteristic signature within the lower slope of
a scarp-bounded depression where a seamount
has passed beneath the margin (von Huene and
Scholl, 1991). In this paper, we describe the forearc response arcward of this scalloped or damaged
accretionary prism.
A prominent feature in seismic reflection
profiles of the fore arc is a rough, irregular reflector that represents the boundary between lowvelocity slope sediments and an underlying
higher-velocity wedge that extends out to the
lower slope (Stoffa et al., 1991; Shipley et al.,
1992; McIntosh et al., 1993; Hinz et al., 1996).
Shipley et al. (1992) proposed that the high-
velocity wedge consists of accreted Tertiary sedimentary rocks, whereas von Huene and Flüh
(1994) argued that this material was more likely
Cretaceous–early Tertiary basement rocks similar to onshore exposures of the Nicoya Complex
(see also Donnelly, 1994). Leg 170 of the Ocean
Drilling Program drilled through the upper-slope
sediments offshore of the Península de Nicoya
and encountered a breccia consisting of fragments similar to the Nicoya Complex within a
Miocene matrix (Kimura et al., 1997). This finding suggests that the scattered exposures of
Cretaceous–early Tertiary basement rocks found
along coastal peninsulas and headlands may
represent sections of a continuous fore-arc basement exposed within structural highs uplifted
along faults that offset the rough reflector. To
support this interpretation, we present the observation that the basement exposures coincide with
the locations of the highest Quaternary uplift
rates in the fore arc.
QUATERNARY GEOLOGY AND
STRUCTURE OF THE EXPOSED
COSTA RICAN FORE ARC
Trunk sections of major fluvial systems draining the Pacific side of the mountainous fore arc
typically follow active, margin-perpendicular
faults that bound a system of independent forearc blocks. Regionally, extensive marine and
fluvial terraces can be correlated along the Costa
Rican Pacific coast and across these faultbounded segments of the fore arc. Terrace correlations are variously constrained by radiometric
dating of terrace deposits (14C) and interbedded
volcanic rocks (K-Ar), and by soil chronosequences based on B-horizon development and
weathering-rind thicknesses on basalt clasts in
terrace gravels. The elevations of fluvial terraces
vary significantly along the coast with distinct,
measurable changes occurring across blockbounding, coast-orthogonal faults. Uplifted Holocene marine platforms show the same sense of
Figure 2. Central-southern Costa Rican Pacific
coast schematic cross sections (marginparallel) showing inner fore-arc fault blocks
and thrust belt, outer fore-arc peninsulas,
coastal rivers with basin asymmetry factors,
El Diablo fluvial terrace, Cóbano marine
terrace, late Quaternary marine-terrace uplift
rates, and offshore bathymetry. Upper plot:
Close-up of fault blocks within inner fore arc
and on southern edge of Península de Nicoya
within outer fore arc. Inner fore-arc blocks:
Ez—Esparza, O—Orotina, H—Herradura, Es—
Esterillos, P—Parrita, Q—Quepos. Outer forearc block: C—Cóbano. Solid squares—El
Diablo terrace outcrops (6 km inland); dashed
gray lines—projected El Diablo terrace surface; solid triangles—Cóbano marine terrace;
open triangles—Holocene marine-platform
uplift rates projected to expected elevation for
a 125 ka terrace (oxygen isotope stage 5e sealevel highstand). Arrows indicate sense of
vertical separation on block-bounding faults.
Shaded blocks expose Cretaceous basement
rocks (Nicoya Complex). Major coastal rivers
shown above plot (AF—drainage basin asymmetry factor: AF = 50 indicates symmetric
basin; AF < 50 indicates northwest-tilted block
with larger basin area southeast of trunk
river). Middle plot: Inner fore-arc fault blocks,
Fila Costeña thrust belt, and outer fore-arc
peninsulas (Nicoya and Osa). El Diablo terrace
elevations (from detailed plot above) shown
with range of late Quaternary coastal uplift
rates for southern tips of Nicoya and Osa
peninsulas (Marshall and Anderson, 1995;
Gardner et al., 1992). Uplift rate scale (right
side) corresponds with elevation scale (left
side) for a 125 ka surface. Lower plot: Marginparallel bathymetric profile across seamount
domain and Cocos Ridge showing position of
roughness elements with respect to fore arc.
468
GEOLOGY, May 1998
offset as fluvial terraces across block boundaries
and provide quantitative measurements of shortterm uplift rates within discrete fore-arc segments. In addition, the highest topography and
oldest rock exposures occur within the uplifted
blocks. Thus, continuous Cretaceous basement is
only exposed in structural highs with the fastest
uplift rates (Figs. 1 and 2).
Coast-orthogonal faults mark the boundaries
between seven major blocks along a 150 km
stretch of the central Costa Rican Pacific coast
(Fig. 1). Each block displays measurable northwest-down tilting. The highest measured uplift
rates along the Pacific coast of Costa Rica are
from the Península de Osa, directly inboard of the
axis of the incoming Cocos Ridge (Gardner et al.,
1992) (Fig. 2). The long-wavelength decrease in
elevation on the northwest flank of the ridge is
broadly consistent with the overall pattern of
coastal uplift, with lower uplift rates to the northwest and northwest tilting of individual blocks.
We suggest, however, that the shorter-wavelength
roughness related to seamounts superimposes
local variability on this overall uplift pattern.
Below we report data used to evaluate the vertical
separation across block-bounding faults, the tilting within blocks, and the Holocene uplift rates
from northwest to southeast along a 150 km
stretch of the central Costa Rican Pacific coast.
The thickest and most regionally extensive
aggradational fluvial terrace, the El Diablo terrace
(Fisher et al., 1994), occurs near the coast along
most major Pacific slope rivers (Fig. 2). Aggradational terraces commonly develop within the
lower reaches of coastal rivers in response to rising base level during major sea-level highstands
(e.g., Merritts et al., 1994). Dated stratigraphic
relationships, modern elevation, and extrapolation
of Holocene uplift rates indicate that the El Diablo
terrace may record the last major Quaternary
sea-level highstand at oxygen isotope stage 5e
(ca. 125 ka). El Diablo gravels overlie a welded
tuff dated at 240 ka (K-Ar) near the Río Tárcoles
(Fisher et al., 1994). An estuarine clay horizon at
the base of the El Diablo gravels near Quepos
yields abundant wood that is in excess of
46 020 yr B.P. Furthermore, Holocene ages for
elevated wave-cut benches have been obtained on
two blocks that preserve the El Diablo terrace.
The Holocene uplift rate obtained for these blocks
is broadly consistent with the elevation of the
El Diablo terrace if these gravels were deposited
during the highstand at oxygen isotope stage 5e.
In this paper, we use abrupt first-order variations
in the elevation of the El Diablo terrace with
respect to modern river elevation to characterize
relative uplift between fault-bounded blocks. We
recognize that these elevation differences may
also include second-order effects related to differences in river gradient and position of the coastline through time.
Northwest of the rough-smooth boundary,
fluvial systems are symmetric with equal drainage
GEOLOGY, May 1998
areas on both sides of the trunk segment (Fig. 2).
However, all rivers to the southeast of the Río
Naranjo show asymmetric drainage basins as
northeast-trending faults allow northwest tilting
about rotation axes parallel to the trunk streams.
This northwest down tilting is generally consistent with the regional decrease in uplift rates
along the coast northwest from the Cocos Ridge
(Gardner et al., 1992).
Just to the southeast of this change in drainage
basin morphology, a fault offsets the El Diablo
terrace along the Río Barranca with a stratigraphic separation of ~30 m measured 11 km
upstream from the coast (Fig. 1) (Fisher et al.,
1994). The Esparza block has been uplifted faster
than the Puntarenas lowlands to the northwest.
Wood collected from beneath a colluvial wedge
on a raised wave-cut bench at Punta Carballo
yields a radiocarbon age of ca. 3 ka, which indicates an uplift rate of 1–1.5 m/k.y. Extrapolation
of this Holocene uplift rate to oxygen isotope
stage 5e at ca. 125 ka would give an elevation for
the El Diablo terrace of between 125 and 190 m.
The observed elevation for the El Diablo terrace
of 140 m above river level, 6 km upstream from
the coast, falls within that range (Fig. 2).
Along the Río Jesús María, a steep, northeaststriking fault separates the Esparza block from
the Orotina block (Fig. 1). The contact between
Miocene sedimentary rocks and an overlying
lahar is offset across the Jesús María fault, with a
stratigraphic separation of ~120 m. The El Diablo
terrace on the Orotina block is projected along its
longitudinal profile to an elevation of 90 m above
the river 6 km inland from the coast (Fig. 2).
A steep, northeast-striking fault along the Río
Tárcoles juxtaposes Miocene sedimentary rocks
along the southeast margin of the Orotina block,
and Cretaceous pillow basalts of the Nicoya
Complex on the Herradura block (Fig. 1). The
Herradura block coincides with the highest
topography in the fore arc, and the exposed basement has been stripped of most of its Tertiary and
Quaternary cover. Another margin-perpendicular
fault separates the Herradura block from the
Esterillos block to the southeast, a block that also
exposes Cretaceous–early Tertiary rocks but has
undergone less uplift, so it preserves Miocene
sedimentary cover and the El Diablo terrace
(Fig. 1). The El Diablo terrace is found at an
elevation of 170 m above river level 6 km inland
on the Esterillos block. The western end of the
low-lying Parrita block has an uplift rate an order
of magnitude slower than the Herradura or
Esterillos blocks on the basis of the 20–40 m
elevation above river level for the El Diablo
terrace 6 km inland. The Parrita, Esterillos,
Orotina, and Esparza blocks all have basin asymmetries and an inferred tilt on the El Diablo surface that indicate down-to-the-northwest tilting.
The wavelength of the uplift profile along the
central Pacific coast is similar to that of the shortwavelength bathymetry of the subducting plate.
The Esparza, Herradura, and Quepos blocks
show anomalously high uplift rates and have a
trench-parallel spacing and a width comparable
to incoming seamounts (Fig. 2). Inboard of the
incoming Fisher Seamount at the tip of the Península de Nicoya, a Holocene wave-cut bench
records uplift rates of 1.7 to 4.5 m/k.y. (Marshall
and Anderson, 1995) and the Pleistocene Cóbano
terrace, which may represent oxygen isotope
stage 5e highstand at 125 ka (Mora, 1985), is
found at an elevation of 180–200 m (Fig. 2). Late
Figure 3. Geologic map of Costa Rica showing fault plane solutions and kinematic axes (Marrett
and Allmendinger, 1990) within Miocene sediments (Ms) and Quaternary fluvial deposits (Qt).
Solid circles—P axes, open squares—T axes, PFZ—Panama fracture zone. Inset shows tectonic
map of Central America.
469
Quaternary surfaces record rapid uplift and arcward tilting, possibly in response to incoming
seamounts (Marshall and Anderson, 1995). The
magnitude and distribution of uplift along the
Pacific Coast of Costa Rica suggests that, in contrast to behavior near the toe of the wedge and
despite a thin incoming sediment pile, there is net
accumulation of mass. A comparison of uplift
patterns and offshore bathymetry raises the possibility that incoming seamounts are underplating
beneath the fore-arc high.
Regional faults that lie along major trunk
streams (Barranca, Jesús María, Tárcoles, and
Parrita) are steep and strike northeast, roughly
parallel to both the Cocos Ridge and the relativemotion vector between the Cocos plate and the
overriding Panama block. These faults segment
the fore arc and separate blocks with different uplift rates. The largest faults (along the Río
Tárcoles and Río Parrita) juxtapose Cretaceous–
early Tertiary basement with both shallow
marine–deltaic sedimentary rocks of late Tertiary
age, and Quaternary fluvial and volcanic units.
Vertical offsets along major faults over the past
100 k.y. range from tens of meters to several kilometers (total potential relief on the rough surface
inferred from the absence of any slope cover on
basement blocks).
Mesoscale faults are concentrated near
regional faults and are observed in all units from
Miocene sedimentary rocks to gravels of late
Quaternary age (Fig. 3). The fault kinematics are
also similar in all units: Steep faults at a high
angle to the margin display normal, strike, and
oblique slip. Shortening axes based on the method
of Marrett and Allmendinger (1990) are typically
vertical or north trending, whereas extension axes
are typically east trending. Thus, mesoscale faults
display orientations, strain axes, and recent
kinematics that are consistent with the vertical
separations inferred from stratigraphic correlations across regional faults. Focal mechanisms for
recent seismic events are also consistent with the
regional fault orientations and kinematics (e.g.,
Güendel et al., 1989), suggesting that the deformation on observed faults is ongoing.
CONCLUSIONS
The distribution of coastal uplift along the
Costa Rican fore arc is broadly consistent with
long-wavelength roughness related to the incoming Cocos Ridge (Gardner et al., 1992) (Fig. 2).
Superimposed on this pattern is more local uplift
related to incoming seamounts. Relative longterm and short-term uplift rates determined from
fluvial and marine terraces vary significantly
between independent blocks. The corresponding
uplift profile along the margin is thus corrugated,
and the distribution of uplifted blocks is roughly
similar to the small-wavelength roughness on the
incoming Cocos plate.
Faults that bound fore-arc blocks are always at
high angles to the margin and display strike,
470
normal, and oblique slip. We find that the Mesozoic basement rocks are restricted to blocks with
higher short-term and long-term uplift rates. The
boundary between basement rocks and slope sediments that is observed in offshore seismic lines is
presumably eroded off these fore-arc highs. These
observations are consistent with a continuous
basement wedge that has undergone laterally variable uplift in response to small-wavelength
roughness on the subducting plate. This roughness results in segmentation of the fore-arc basement, a consequence that may limit the lateral
propagation of faults during earthquakes and prevent large subduction events (Protti, 1991). Thus,
fore-arc kinematics and seismicity along thinly
sedimented margins may largely depend on
roughness of the subducting crust.
ACKNOWLEDGMENTS
This research was funded by National Science
Foundation grants EAR-9214832 and EAR-9526955.
We thank R. von Huene for providing us with the
GEOMAR bathymetry data offshore of Costa Rica, and
D. Pope for invaluable assistance with Figure 1. We
also appreciate helpful reviews of this manuscript by
E. Silver and R. Bürgmann.
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GEOLOGY, May 1998