Download Chapter 9 Convergent margin tectonics: A marine perspective

Chapter 9
Convergent margin tectonics:
A marine perspective
CÉSAR R. RANERO, ROLAND VON HUENE, WILHELM WEINREBE AND
UDO BARCKHAUSEN
“These geological factors are important, but
they only represent one fact of a complex set
of relations that, although clearly related to
ocean plate subduction, have yet to be
investigated in adequate detail.” G.P.
Woollard: The Status of Geophysical
Research in Latin America. In: Geophysics
in the Americas, 1977.
9.1
INTRODUCTION
The Central American convergent margin is a classic representative of the “Pacific
Margin” in the nomenclature of Gutenberg and Richter [1] and early plate tectonics. It
extends from the Gulf of Tehuantepec to Panama (Fig. 9.1). The dominant
morphological feature is the Middle America trench (MAT) that was named by
Heacock and Worzel [2], and runs from the Riviera fracture zone offshore Mexico to
the Cocos ridge offshore south Costa Rica. Earthquakes clearly define a WadatiBenioff zone of landward dipping seismicity or a subduction zone where Cocos plate
and the older Farallón plate subducts beneath the Caribbean plate. Hypotheses
regarding the MAT contributed to the evolution of concepts in the geosciences over the
past 75 years.
The history of marine geoscience research along Central America margin (CAM)
and the MAT is intimately interwoven with the evolution of concepts regarding
convergent plate boundaries. Geophysical data tested with scientific ocean drilling
along the CAM was significant in modifying ideas in the plate tectonic paradigm about
convergent plate boundaries. This chapter is introduced with a narrative history of this
evolution and then concentrates on the insights from modern bathymetry, potential field
data, and improved seismic reflection information. A review of the marine geological
and geophysical studies carried out along the convergent margin of Middle America
shows that the area’s unique features have motivated research over the past 50 years
that influenced the understanding of convergent plate boundary evolution.
Interpretations of processes that shaped the CAM have been markedly modified as
techniques and quality of data improved. Proposed in early models was a steady growth
of the continent by long-term accretionary processes through which material from the
incoming oceanic plate was tectonically transferred to the overriding plate.
Subsequently, as the accretionary hypothesis was tested with scientific drilling, it was
realized that the margin is non-accretionary. In the last ~10 years a wealth of new,
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higher resolution data, have led to a consensus that the continental margin has been
tectonically eroded during the Neogene–Quaternary periods (~23 Ma) and that a large
mass of the overriding plate has been removed and probably recycled to the mantle.
Modern data show the close correlation between the character of the incoming oceanic
plate and the recent (~5 Ma) tectonic evolution of the arc-forearc system. Forearc
tectonics, submarine sliding, arc magmatism and interplate seismicity differ in
segments and those segments parallel segmentation of the oceanic plate subducting
along the MAT. This understanding helps in assessments of the risk from natural
hazards.
9.2
ACCRETION VERSUS NON-ACCRETION
9.2.1 Early studies offshore Guatemala
Investigations offshore Central America between 1950 and 1960 by scientific
institutions were numerous for the time. Bathymetric data compiled in the early 1960s
[3, 4] revealed the varied morphological character of the adjacent ocean basin. Soon
afterwards, areas of the continental shelf were considered potentially petroliferous and
were surveyed by industry explorationists who informally shared with academic
colleagues that deformation on the slope indicated accretion. A concept of accretion
was published by Dickinson [5] suggesting that although ocean crust is carried down
with the descending lithosphere, lighter sediment is probably scraped off against a
more durable overriding plate. These off-scraped sediments and ophiolitic scraps were
presumably equivalent to the materials exposed in the Sambagawa formation of Japan
and in the Fransciscan formation in California, a vast tract of rock whose origin had
long puzzled geologists. Dickinson proposed steady-state accretion and that the terrain
between the arc and trench was proportional in width to the age of the arc-trench
system. Dickinson summarized the model during his seminal workshop in Asilomar in
1971, where the concept was debated between marine and land geoscientists. He
proposed a steady-state process of continental growth that was commonly referred to as
“the plate tectonic margin model”. The origin of the ophiolites of the Nicoya peninsula
thus became an interesting target of research.
Many proponents of plate tectonics were enthusiastic about the accretionary
hypothesis, and when Exxon released a multichannel seismic reflection record across
the MAT off Guatemala the model was convincingly backed by published data [6]. The
Exxon record showed many landward dipping reflections but at the time it was difficult
to differentiate between real reflections and diffractions without today’s more powerful
processing software systems. The Guatemalan example elevated the accretionary
hypothesis to broad consensus in the scientific community and the Guatemalan seismic
record was commonly cited as a type accretionary section. At this time, drilling in the
Nankai trough based mainly on industry records of JAPEX [7] provided further
evidence supporting accretion [8] and a tendency to equate all convergent margins with
accretion became common. With the Guatemalan record, Seely and his colleagues [6,
9] elevated the accretionary model from speculation to broad acceptance by
geoscientists and other proprietary data were later integrated in an interpretation of the
Oregon and Alaskan margins. Seely [9] interpreted reflection data off Oregon and
Alaska as he had interpreted the Guatemalan record inferring that slices were detached,
then rotated upward by younger underthrusting slices, to progressively build
continental crust. The actively accreting prism uplifted the shelf edge forming the
Figure 9.1. Shaded relief map of Cocos and Caribbean plates. Areas of Deep Sea Drilling Program (DSDP) legs 66, 67, 84 and Ocean Drilling Program (ODP) Leg
170 are indicated.
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seaward flank of forearc basins. Single-channel marine seismic reflection data of the
academic community were grossly inadequate to reveal the complex structure along
active convergent margins. In hindsight, even the early multichannel Exxon record
processed with digital methods available only in industry was also over interpreted.
Notwithstanding the focus on accretion in the published literature, evidence was
building that convergent margins were also subjected to tectonic erosion. As Miller
[10] and Rutland [11] pointed out, some older arc-trench systems have narrow forearcs
seaward of an exposed Mesozoic continental framework. This observation was not lost
on Creighton Burk. At the time, Burk was Director of the University of Texas Marine
Science Institute (UTMSI). In 1974, he formulated a program to investigate the
tectonics of the MAT from the Cocos ridge to the Riviera fracture zone. He and
colleagues on the Deep Sea Drilling Project (DSDP) Active Margins Panel reasoned
that investigating two adjacent areas one of which appeared accretionary and the other
erosional would yield insights greater than those derived from studies of either margin
type alone. In contrast to the accretionary Guatemalan margin, the southwestern
Mexico margin appeared to have no older accreted mass because Mesozoic rock crops
out along the coast as observed along the Chilean margin. Thus the volume of a
possible accretionary prism cannot accommodate all incoming sediment on the lower
plate and part of the prism must be missing. Site surveys by UTMSI produced the first
multichannel seismic reflection data off Acapulco Mexico and off Guatemala,
Nicaragua, and Costa Rica. These surveys formed a basis for selecting two transects
that were drilled during DSDP Legs 66, 67, and 84 (Fig. 9.1). Much to everyone’s
surprise, the Guatemalan margin proved non-accretionary [12, 13] and the Mexican
margin was interpreted as accretionary [14]. The Guatemalan margin yielded
Cretaceous limestone resting on igneous oceanic rocks of the upper plate within 4 km
of the trench axis. Clearly, steady accretion had not affected the margin during Tertiary
time [12, 15]. Although many of the landward dipping reflections interpreted as
accretionary thrusts were shown to be diffractions by later pre-stack depth processing,
rare landward dipping reflections do occur in the margin wedge. The shortcomings of
geophysics alone to assess basic convergent margin tectonic processes was also
experienced along the Japan, Marianas, Tonga, Peru, and most recently the Costa Rica
margins. Although the seismic reflection technique and the information content of the
seismic reflection method have improved greatly in the past 40 years, the accretionary
model that guided interpretations of geophysical data was appropriate only along some
margins. We return to this point after recounting the great advances made over the past
10 years with multibeam bathymetry (Fig. 9.2) and seismic reflection investigations
that include advanced pre-stack-depth processing.
9.2.2 Contribution of bathymetric mapping
Investigators at the Scripps Institution of Oceanography compiled bathymetric
information along the CAM most of which are summarized in Fisher [16].
Geoscientists compiling conventional bathymetric data in the late 1950s were mindful
of seafloor processes in constructing maps from widely spaced data. Fisher’s maps
showed numerous seamounts along the northern flank of the Cocos ridge. The many
transit legs of research vessels from ports in the area and through the Panama canal
became data of opportunity that Lonsdale and Klitgord [17] later compiled, data
included in the independent compilation of Case and Holcombe [18]. The latter
CENTRAL AMERICA: GEOLOGY, RESOURCES AND HAZARDS; BUNDSCHUH & ALVARADO (EDS)
Figure 9.2. Shaded relief map of the bathymetry and topography of Costa Rica and Nicaragua.
The oceanic plate has four segments with different morphological character. Segments 1–3
were formed at the Cocos-Nazca spreading center and segment 4 at the East Pacific rise.
ODP Leg 170 sites are indicated by black circles filled white. DSDP Leg 84 site 565 is a white
circle filled black. Oil exploration drills sites onshore and offshore Nicaragua are indicated
by black filled circles. Black lines represent tracks of seismic profiles.
compilation included continental geology and both compilations depicted the seamount
covered ocean floor off central Costa Rica and the adjacent smooth ocean floor and less
complicated slope off Nicoya peninsula. This was termed the rough-smooth boundary
by Hey [19] who correlated it with the change in origin of ocean crust. The subducting
Cocos ridge was positioned opposite the uplifted Osa peninsula and off both Costa
Rican peninsulas the shelf is very narrow. At this time a first order morphology of the
continental margin was known as well as could be expected from conventional
bathymetry without GPS navigation.
A short multibeam bathymetric survey of the trench off Guatemala made with the
French R/V Jean Charcot demonstrated the more coherent information acquired with
multibeam [20]. Revealed were details of horst and graben as the plate bends into the
trench. These horst and graben underthrust the base of the continental slope, and are
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roughly mimicked by lower-most slope morphology where the plate interface forms a
scarp parallel to the trench axis. Disruption of the lower slope by seafloor relief of ~300 m
is minor but the slope morphology has a restless character that matches the poor
coherence of reflectivity in seismic reflection records. Tectonism of the middle and
lower slope is apparent from seismic data but without 100% coverage the tectonic
significance of multibeam bathymetry is not obvious. Nonetheless, Aubouin and coworkers correctly interpreted extensional tectonism and drilling established the nonaccretionary nature of all but the slope toe. With 100% coverage, extensive mass
wasting and extensional tectonics shaping slope morphology becomes apparent but this
was not achieved until 2003.
A multibeam system was installed on the Scripps R/V Thomas Washington during a
port stop in Puntarenas, Costa Rica in 1984. The instrument test data was preceded by a
survey off the Nicoya peninsula where the continental slope morphology is least
complicated [21]. It showed that even the simplest morphology was generously
endowed with small-scale gravity sliding as noted in DSDP drill cores [22]. Off
Guatemala the Charcot survey was expanded by R/V Thomas Washington and
combined with high-resolution seismic data the indications of sediment folding,
interpreted as accretion across the slope toe, were emphasized [23]. A small frontal
prism was in evidence despite upper plate basement sampled during Legs 67 and 84.
The GEOMAR Geodynamics group selected Costa Rica for investigation because
of a possible subducting seamount that had been partially mapped inadvertently by the
R/V Thomas Washington test survey. Scientific questions to investigate included the
fate of large seamounts in a subduction zone, whether they remain on the lower plate or
are sheared off, whether they form earthquake asperities, and whether they
mechanically erode the continental margin. R/V Sonne mapped about 400 km of the
CAM bathymetry with near 100% [24]. These data were expanded during several
subsequent cruises and current maps include detailed morphology of the continental
slope and incoming oceanic plate from northern Nicaragua to the Cocos ridge offshore
southern Costa Rica (Fig. 9.2).
9.2.3 Accretion versus non-accretion models
Seismic records acquired offshore Costa Rica by UTMSI in 1978 showed a thicker
slope sediment section off Costa Rica than offshore Nicaragua and Guatemala. Beneath
that sediment section is a rough yet strong reflective top of the rock comprising the
bulk of the continental margin, the so-called margin wedge. The base of the margin
wedge is defined by strong reflections paralleling the plate interface (Figs. 9.3 and 9.4).
Within the margin wedge are landward dipping reflections clearly differentiated from
diffractions [25]. A hole drilled offshore Costa Rica during DSDP Leg 84 failed to
reach basement for safety reasons and thus the accretionary model was not tested. The
seismic records off Nicaragua and Costa Rica were interpreted in accord with the
accretionary model [21, 25–27] although a non-accretionary Costa Rican model was
also proposed [13, 28]. The UTMSI seismic data offshore the Nicoya peninsula showed
some of the strongest coherent plate interface reflections of any collected at the time
and this area was chosen for the first academic 3-D reflection seismic experiment
across a convergent margin. The 3-D survey was collected across the lower slope and
extended with 2-D lines shot from the continental shelf to the ocean plate [25, 30, 31].
Within the 3-D volume of rock, landward dipping reflections were traced from the top
CENTRAL AMERICA: GEOLOGY, RESOURCES AND HAZARDS; BUNDSCHUH & ALVARADO (EDS)
Figure 9.3. Seismic reflection section from a 3-D survey of the lower slope offshore Nicoya
peninsula, Costa Rica [29].
of the margin wedge to the plate interface (Fig. 9.3). They were interpreted as recently
formed structures representing thrust faults, duplex structures, and “out-of-sequence”
faults. “Out-of-sequence” fault is a generic term derived from the constant accretionary
model to explain landward dipping reflections that exceed the length of initial
accretionary thrust faults. They are longer than the initially detached sections of
accreted oceanic sediment and are proposed to cut the accretionary prism once the first
thrust faults are rotated and become too steep to continue thickening the prism. Other
structures were interpreted as underplated duplexes seaward of a post-Eocene
accretionary prism forming the bulk of the continental margin (Figs. 9.3 and 9.4). A
major shortcoming of the interpretation was the lack of reliable velocity data and the
difficulty to balance structure in the context of an accretionary model [31].
During the 1991 and 1992 R/V Sonne cruises 76 and 81, not only multibeam
echosounding but also magnetic data [32], seismic refraction data [33], and
multichannel seismic reflection sections [34] were acquired. The refraction data
indicated higher acoustic velocity in margin wedge rock (~5–6 km/s) than that derived
from time processing of the 3-D seismic reflection data. These velocities were similar
to those in upper plate crustal rock of the Guatemalan margin [35] where the margin
Figure 9.4. Seismic reflection profile FM-CR20. Stack produced at the Institute of Geophysics
University of Texas at Austin [30]. Post-stack deconvolution and time migration applied at
GEOMAR. ODP Leg 170 drill holes and DSDP Leg 84 site 565 are shown. The margin wedge
extends to ~ 7 km of the deformation front. The frontal sediment prism is composed of
reworked slope sediment. All oceanic sediment is underthrusted past site 1040 [36]. Location
on Fig. 9.2.
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wedge had been shown to be igneous rock, and not indicative of young accreted
sediment [31, 33]. In addition to the high velocities, the bathymetry offshore Central
Costa Rica indicated only a small frontal prism. The subducting plate with seamounts
and ridges appeared to erode the margin because opposite the seamounts the slope was
indented [24] (Fig. 9.5). A non-accretionary origin explained most readily the structure
imaged in reflection sections [34]. Modeling of magnetic anomalies was consistent
with a margin wedge composed of igneous rather than sedimentary rock [32]. Drilling
results from ODP Leg 170 confirmed that all sediment in the incoming plate is
underthrust and no classical accretionary prism occurs, although the margin wedge rock
was not unequivocally sampled [36]. In fact, the small “accretionary prism” at the
lower slope was shown to consist only of tectonized slope sediment without transfer of
oceanic sediment from the lower to the upper plate (Fig. 9.4). Thus the former
accretionary prism is now termed a frontal sediment prism. Consequently, the steady
state accretionary model for Costa Rica was no longer the consensus interpretation.
9.3
CHANGES IN THE PARADIGM
9.3.1 Long-term subsidence and subduction erosion
Tectonic processes controlling the past 5 m.y. evolution of the CAM became much
clearer once multibeam bathymetry was available and structure was imaged in true
depth with multichannel seismic reflection sections (Figs. 9.5 and 9.6). DSDP and ODP
drill holes provided a lithostratigraphy of sedimentary units and benthic microfossil
fauna documented paleo bathymetry to reveal a history of massive subsidence. These
data indicated that a large mass of the CAM was missing and was presumed to be
tectonically eroded during Neogene time. Evidence for Neogene margin subsidence
offshore Costa Rica, Nicaragua and Guatemala supports the structural evidence for
subsidence observed in the seismic data and this evidence is discussed below.
In Costa Rica, large-scale Neogene subsidence and upper plate extension has been
interpreted from structure in seismic images and it is recorded in DSDP Leg 84 and
ODP Leg 170 cores. The top of the margin wedge is a rough low-relief surface overlain
by sedimentary strata that locally show onlap (Fig. 9.6). The upper plate landward of
the frontal prism appears extended [30, 37]. Numerous normal faults offset the
sedimentary strata and some appear to continue as discrete reflections deep into the
margin wedge. Most likely, the surface at the top of the margin wedge is an erosional
unconformity that subsided from the surf zone to its current depths beneath the outer
shelf and the middle and lower slope (Fig. 9.6).
The margin wedge lower boundary, the plate interface, is imaged as a reflective
interface that retains a high amplitude signature for about 50 km from the trench and to
about 12 km depth. Reflection and onshore-offshore refraction velocities constrain
determinations of upper plate thickness from the lower slope (± 200 m) to the coast
(± 500 m). If the margin wedge unconformity was formed by surf zone erosion near a
former coast, the crust there may have been 14–16 km thick, similar to upper plate
thickness beneath the current coast [33, 38, 39]. The margin wedge in central Costa
Rica is currently only 10 km thick beneath the outer shelf and about 3.5 km thick
beneath the middle lower slope (e.g., km 35 on Fig. 9.6). If the unconformity was
formed near sea-level, the upper plate must be thinned. Although faulting of the upper
plate may account for some of the thinning, the small offsets along these faults is not
sufficient to explain the much larger thinning observed and thus basal erosion and
CENTRAL AMERICA: GEOLOGY, RESOURCES AND HAZARDS; BUNDSCHUH & ALVARADO (EDS)
Figure 9.5. Perspective view of the shaded relief of bathymetry and topography of Costa Rica
and Nicaragua. The Talamanca cordillera reaches ~3800 m high and the trench axis ~5700 m
deep. Main morphological features are labeled.
removal of material is a plausible explanation for subsidence. Upper plate extension is
an apparent response to basal erosion and thinning rather than the main cause of margin
subsidence. The margin wedge unconformity and normal faulting has been mapped
across the entire slope of Costa Rica [30, 37, 40] and a similar unconformity separating
igneous basement from overlying strata has been observed beneath the continental
slopes of Nicaragua and Guatemala. Since onlap produces a time-transgressive
unconformity, the age of the unconformity probably varies along the CAM.
Although the distinctive unconformity at the top of the margin wedge is seen
regionally across the middle–upper slope, it becomes more irregular farther down slope
and the upper plate is increasingly dismembered. Locally the unconformity is more
disrupted where lower plate relief (seamounts and ridges) has subducted and
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stratigraphy is poorly imaged in seismic records because of the deformation [37]. The
margin wedge dismemberment beneath the lower slope is consistent with subduction
erosion there.
Consistent with geophysical evidence for subsidence of the CAM in Costa Rica are
results from studies of DSDP Leg 84 site 565 cores, and site 1041 of ODP Leg 170
cores. The lithologies of rock from the unconformity at the top of the margin wedge
currently at ~4 km (site 1042, Leg 170), have shallow-water affinities [41, 42]. The
latter sites recovered igneous basement. The site 1042 cores just above the margin
wedge unconformity recovered ~25 m of carbonate cemented limestone breccia
underlain by ~10 m of a breccia composed of clasts of red chert, doleritic basalt and
mafic rock [36]. The contact between material from the high velocity body and
sediment was not recovered because the site 1042 was located at the frontal tip of the
margin wedge and a thrust truncated the stratigraphic section. The chert and igneous
rock from the breccia are similar to rocks found in the Nicoya complex cropping out
onshore [36]. Analysis of the carbonate cemented limestone breccia indicates a beach
to near shore depositional environment [41]. Drilling at Sites 565 and 1041 penetrated
much of the sediment overlying the margin wedge and good recovery provided a
relatively continuous benthic foraminiferal stratigraphy. The depth at which the
foraminiferal assemblages lived changes from shallow water at the base of the section,
to abyssal fauna in the upper section consistent with the present depth of the sites [43].
The beach to near shore carbonate cemented limestone breccia is ~16 Ma old and
neritic fauna (water depth < 300 m) appears in sediment older than 5–6.5 Ma indicating
slow average subsidence. However, the more detailed depth information from benthic
foraminifera shows a sudden acceleration in subsidence offshore Nicoya peninsula
starting at ~5–6.5 Ma when that area of the margin subsided rapidly to upper-middle
bathyal depth (< 800 m water depth). A renewed increase in subsidence rate occurred at
~1.8 Ma when the slope deepened to abyssal depths (> 2000 m) or the current core
depth of ~3200 m. [43]. These observations are explained by long-term margin
subsidence caused by subduction erosion and thinning of the upper plate.
In Nicaragua, drill hole information along a seismic reflection/refraction transect
across the Sandino basin shows that the basin depocenter was located beneath the
current continental shelf during Cretaceous to ~Middle Eocene time (Fig. 9.7). The
Cretaceous–Paleogene sediment units rapidly pinch out along the basin seaward flank.
Beneath the current upper continental slope, a thin Cretaceous–Early Paleogene
sediment sequence becomes indistinct down slope [44] but probably extends to the
middle slope where dredging recovered fragments of igneous basement rock and
Cretaceous limestone [45]. From about Late Eocene to latest Oligocene or Early
Miocene time (~26–23 Ma) the outer shelf seafloor was sufficiently elevated to form a
barrier to sediment transport and most deposition was restricted to the inner shelf area
where a stable depocenter accumulated sediment about 5 km thick. A major change in
basin configuration and the beginning of long-term regional subsidence of the seaward
part of the margin occurred in latest Oligocene or Early Miocene time. When the outer
shelf subsided, Neogene sediment unconformably covered the Cretaceous–Early
Paleogene units. Since Early Miocene, a 70-km-wide swath of the Nicaraguan margin
forming the current upper continental slope subsided about 2 km at the position of the
current shelf break, and probably 3–4 km along the middle to-upper slope transition
(Fig. 9.7).
In Guatemala, paleo-depths from benthic foraminifera of sites 568–570 on the
middle-upper slope yield similar evidence for long-term subsidence as off Costa Rica
Figure 9.6. Pre-stack depth migration of Sonne-81 line 4 projected on bathymetry. Line 4 is 58 km long from the slope toe to the outer continental shelf. The top
of the margin wedge is an unconformity cut by normal faults with small offset. Thrust faulting occurs only at the lower continental slope in the small frontal
sediment prism where the slope drainage system is disrupted. Location on Figure 9.2. Modified from Ranero and von Huene [37].
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[42]. Benthic foraminiferal assemblages indicate a progressive subsidence of the slope
that might have started in latest Oligocene or Early Miocene (~26–23 Ma) time.
Subsidence began earlier (~26 Ma) in the middle slope (site 569 currently at 2799 m
depth). Subsidence at site 568 (currently at 2030 m deep) started about 19 Ma and at
site 570 (currently at 1720 m deep) about 11 Ma. The record of vertical tectonism
shows a migration of margin subsidence toward the continent.
The Neogene subsidence record in Guatemala and Nicaragua (~26–23 Ma) begins
at approximately the same time but is somewhat later in Costa Rica offshore Nicoya
peninsula (~17 Ma). The rapid subsidence pulse at ~5–6.5 Ma in the latter area is not
observed in Nicaragua or Guatemala. Initiation of widespread subsidence in the CAM
might be related to a major kinematic reorganization of the plates in the eastern Pacific.
Soon after the Farallón-Pacific spreading center collided with the North American plate
in the Late Oligocene [46] a latest Oligocene or Early Miocene (26–23 Ma) change in
plate kinematics led to the opening of the Cocos-Nazca spreading center [46, 47]. The
plate kinematic reorganization produced a change from oblique to normal convergence
along the MAT and was accompanied by an increased rate of spreading along the East
Pacific rise [48] adjacent to the newly formed Cocos plate. Fast convergence rates and
arrival of a younger and shallower slab at the trench may have induced subduction
erosion along the plate boundary [42, 44]. The rapid pulse of subsidence at 5–6.5 Ma
recorded locally offshore Nicoya peninsula is coeval with the arrival of the topographic
swell associated with the Cocos ridge [43]. Studies of the Talamanca cordillera
opposite the subducting Cocos ridge indicate that widespread calcalkaline volcanism
ceased about 3.5–5 Ma [49, 50]. Adakitic rocks probably produced by partial melting
of ocean crust were emplaced ~3.5 Ma [51] and denudation ages from fission track
analysis indicate uplift of the cordillera to about this same time [52]. These
observations indicate a 5–7 Ma arrival time of Cocos ridge at the trench. The arrival of
Cocos ridge further decreased the subduction angle of the incoming plate as far north as
Nicoya peninsula and after an initial uplift a former shelf area subsided to its current
abyssal depth [43].
9.4
THE FOREARC SANDINO BASIN
Sandino forearc basin is located beneath the broad continental shelf from the Gulf of
Tehuantepec to Costa Rica (Figs. 9.7 and 9.8). This basin has been subsiding active
since Late Cretaceous and contains a 9–15 km thick sediment accumulation at its
depocenter [44, 53]. The structure of the basin is similar from Guatemala to Nicaragua.
Its depocenter is located near the current coastline and the infill progressively shallows
towards a basement outer high located beneath the current shelf edge. Evolution of the
basin from Nicaragua to Guatemala was similar during the Tertiary. Although there
may be a difference in the pre-Eocene tectonics at each segment of the margin, it is also
possible that previous interpretations are different because of incomplete data [44]. The
pre-Middle Eocene basin imaged near the present coastline off Nicaragua (Fig. 9.7)
was not clearly imaged off Guatemala. Offshore Nicaragua, seismic stratigraphy and
backstripping indicate that rapid basin subsidence occurred during early development
of the Sandino basin (Cretaceous–Paleocene). In Costa Rica, Late Cretaceous deep
water sediment rests over Nicoya complex igneous rock, and deformation of the basin
has uplifted similar strata that crop out along the coastal region of Nicaragua [28, 54]
indicating a single development of the entire forearc basin along Middle America.
Figure 9.7. Pre-stack depth migration of a cross section composed of three multichannel seismic reflection profiles across the Nicaragua margin. Seismic
stratigraphy has been calibrated with Corvina-2 and Argonaut-1 wells. The continental shelf is underlain by the more than 9 km deep Sandino basin. Location
on Fig. 9.2. Modified from Ranero et al. [44].
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The Late Eocene and Oligocene section in Nicaragua and Guatemala have similar
structure. The strata show landward thickening and are absent from the outer high and
upper slope probably indicating subsidence restricted to the shelf. A transition from
deep to shallow water sediment in Guatemala [55] Costa Rica [28, 56] and Nicaragua
Pacific margin in the Late Eocene indicates similar conditions regionally. From
Nicaragua to Guatemala a phase of renewed subsidence from the latest Oligocene to
the Quaternary is documented by sediment deposition across the outer high and upper
slope. In Early–Middle Miocene time tectonic shortening deformed strata in the
Nicaraguan segment (Fig. 9.7) creating an angular unconformity beneath the shelf. A
similar unconformity locally separates Oligocene from Miocene sections in Guatemala
(Fig. 9.8). Therefore, the entire Sandino basin developed as a unit and the similar Late
Cretaceous to Oligocene tectonic histories indicate a major regional tectonic event.
Along the CAM, the basement of the forearc basin is probably composed of igneous
rocks of oceanic origin. The Nicoya complex onshore Costa Rica belongs to the
Caribbean basaltic province [57–59] and it is possible that the basement underlying the
forearc basin of the CAM is part of a single volcanic province. Landward of the MAT,
the magnetic anomalies offshore Nicaragua and Costa Rica include a broad positive
anomaly (Fig. 9.9). Magnetic model ling has shown that this can be explained with an
offshore extension of the igneous rocks that crop out on the Nicoya peninsula [32].
Wide-angle seismic studies also show that the igneous complex extends seaward to
near the continental slope toe [33, 39]. The continuity of the magnetic anomalies across
the forearc of Costa Rica and Nicaragua indicates that the same igneous basement
extends across the entire region (Fig. 9.9). The origin of a sharp magnetic boundary
across the coastline of northern Costa Rica in the Nicoya peninsula area is not yet clear
but may be related to the Chortis-Chorotega block transition and is older than the
development of the forearc basin. The Caribbean basaltic province is interpreted as
formed largely by initial melting of the head of the Galápagos plume (~90 Ma) and
subsequent obduction (~80 Ma) [57, 58, 60, 61]. This large igneous body was obducted
along at the Antilles easterly dipping subduction zone. It is proposed that this obduction
terminated subduction and eventually caused inversion of subduction polarity [60].
Subduction along the Middle America trench probably began on the western edge
of the Caribbean basaltic province at ~75 Ma, as suggested by the oldest volcanoclastic
Cretaceous sediment overlying the igneous complexes [54]. Subduction initiation is
associated with rapid subsidence of the upper plate [62, 63] that might have caused the
initial development of the Sandino basin along the western rim of the Caribbean plate.
Subduction initiation is associated with rapid subsidence to several-km depth at 150–
200 km from the trench, driven by sinking of the slab into the mantle [62, 64, 65]. As
subsidence rates decreased after initial subduction, the basin was filled with sediment
(Middle–Late Eocene) from a developing volcanic arc. Subsidence seems to be
currently active across the continental shelf and slope.
9.5
QUATERNARY TECTONICS
A compilation of multibeam bathymetry ~600-km-long and 100–150 km-wide offshore
Costa Rica and Nicaragua provides an unprecedented map of the active tectonics
exhibited as seafloor relief (Figs. 9.2 and 9.5). Data were acquired with the
Hydrosweep system during R/V Sonne cruises 76, 81, 107, 144 and 150, R/V M. Ewing
cruises 0005 [66] and 2001 [67], and with a Simrad system during R/V Sonne cruise
Figure 9.8. Post-stack time migration of line GUA2 across the Guatemala margin. The line was collected in 1977 by R/V Ida Green. Re-processing includes poststack deconvolution, frequency-wavenumber noise attenuation, dip-dependent trace interpolation and migration. The Sandino basin may be more than 10 km
deep underneath the continental shelf and thins rapidly towards the outer shelf. Basement extends underneath the continental slope to a point very close to the
trench and is covered by a thin and disrupted sediment section. The top of the basement is indicated by the black-filled circles where observed. The pelagic
sediment section of the oceanic plate is strongly tilted at the trench axis indicating that faults are active across the entire oceanward trench slope. Turbidites
partially fill a half graben being underthrust at the deformation front. The top of the subducting plate can be followed ~35 km underneath the overriding plate
and it shows that the half graben structure is preserved.
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254 TECTONICS AND GEODYNAMICS
163. Editing with MB system software [68] eliminated spurious soundings and
accepted soundings were converted to depth with water velocities measured during
different cruises. There data allowed gridding at ~100 m node spacing. This bathymetry
resolves tectonic structure less than 0.5 km wide from which sedimentary and tectonic
processes can be interpreted. A compilation of magnetic data shows the structure of the
incoming oceanic plate formed at the East Pacific rise and Cocos-Nazca spreading
center (Fig. 9.9). The comparison of magnetic and bathymetric data shows the control
of structure formed at the spreading centers on the configuration of the oceanic plate
and on the deformation during bending at the trench.
The emphasis of previous Costa Rica margin studies was on segmentation of the
ocean plate, which correlates, with large-scale tectonic segmentation of the upper plate
[37, 40, 47]. The current bathymetric and magnetic compilations sharpen the
correlation between lower ocean plate segmentation and upper plate tectonics.
Expanded coverage offshore Costa Rica and Nicaragua shows this upper/lower plate
similarity that we interpret as a cause-and-effect relation between lower plate character
and upper plate tectonism.
9.5.1 Segmentation of the incoming oceanic plate
The along strike variability in ocean plate relief and water depth result from a
combination of magmatic and tectonic processes. The oceanic Cocos plate subducting
beneath much of CAM is divided into two provinces that were generated at different
mid-ocean spreading centers (Fig. 9.1). Most of the oceanic lithosphere entering the
MAT today originates from the East Pacific rise (EPR) and has the low relief
morphology and low-amplitude magnetic anomalies common to fast-spreading ridges
[19, 48]. Along the southern boundary of the Cocos plate, crust was generated along
the Cocos-Nazca spreading center (CNS) by intermediate spreading with a rough
topography and high-amplitude magnetic anomalies [69]. The resulting “rough-smooth
boundary” that separates these two provinces enters the MAT offshore the Nicoya
peninsula in northern Costa Rica [47]. Detailed magnetic mapping (Fig. 9.9) clearly
defines the location of this feature, other tectonic boundaries and seafloor spreading
magnetic lineations in the Cocos plate which resulted from the break-up of the Farallón
plate at 23 Ma and the early history of the subsequent Cocos-Nazca spreading (Fig.
9.10).
The identification of seafloor spreading anomalies provides the age of the Cocos
plate along the MAT. The oldest plate of 24 m.y. is found offshore Nicaragua. To the
north, the EPR-generated crust becomes progressively younger until it is almost zero at
18°N where the EPR meets the MAT offshore Mexico. To the south of Nicaragua, the
age of the Cocos plate decreases rapidly to ~13 Ma. offshore the Costa Rica-Panama
border where the Cocos plates terminates at the Panama fracture zone against the
Nazca plate. No clear Wadati-Benioff seismicity is observed underneath Panama and
lithospheric underthrusting may not be active in that region.
Magmatic processes at the Cocos-Nazca spreading center and the Galápagos hotspot
govern crustal formation from the 20 km beneath the crest of Cocos ridge to ~6
km offshore central Costa Rica [33, 70, 71]. The high heatflow on and adjacent to
Cocos ridge and the much lower than normal heatflow offshore Nicoya peninsula [67]
are probably also related to magmatic history and fluid circulation in ocean crust but
causes for strong lateral changes have not yet been determined. Magmatism also
CENTRAL AMERICA: GEOLOGY, RESOURCES AND HAZARDS; BUNDSCHUH & ALVARADO (EDS)
Figure 9.9. Magnetic anomalies in Nicaragua and Costa Rica compiled from R/V Sonne cruises
76, 107 and 144, R/V M. Ewing cruises 0005 [66] and 0104 [67], and R/V R. Revelle
delivery cruise [90]. Data processing includes corrections for daily variations of the magnetic
field. In addition, detailed maps of an aeromagnetic survey across Costa Rica were digitized
and merged with the marine data. The contour interval is 100 nT. Different patterns of
magnetic anomalies result from the different types of oceanic crust.
constructed large volcanic edifices adjacent to Cocos ridge. The tectonic process of
bending-related normal faulting seems greatly influenced by crustal thickness and the
orientation of tectonic fabric created at the spreading center with respect to the axis of
bending [72]. Seafloor relief in the study area defines 4 distinct oceanic plate segments
(Figs. 9.2 and 9.5). The lithosphere of segments 1–3 was formed at the Cocos-Nazca
spreading center whereas the crust of segment 4 was formed at the East Pacific rise.
Segment 1 consists of the broad Cocos ridge whose crest is the shallowest seafloor area
(water depth 2.5–1.5 km) of the MAT. The shallow seafloor corresponds to the
anomalously thick ocean crust of Cocos ridge produced ~14 Ma ago by the interaction
of the Galápagos hotspot with the Cocos-Nazca spreading center [73]. Paralleling
Cocos ridge are large tilted normal fault blocks indicating extension perpendicular to
the ridge axis along segment 1. Steep normal fault scarps dip toward the Cocos ridge
axis and bound a large graben over the thickest crust (Figs. 9.2 and 9.5). The sediment
strata in the graben are not tilted, indicating that the large faults are not recent and
related to the bending of the plate, but probably old structures formed during the
creation of the Cocos ridge. Smaller ridges, conical seamounts, and the Quepos plateau
characterize segment 2. The conical seamounts and Quepos plateau are 13–15 Ma old
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256 TECTONICS AND GEODYNAMICS
Figure 9.10. Isochron map of the Cocos plate offshore Nicaragua and Costa Rica with ages
derived from identification of seafloor spreading anomalies [47]. Numbers indicate ages in
m.y. of identified magnetic lineations. Dotted lines show tectonic boundaries, double arrows
indicate crust formed at the East Pacific rise and at the Cocos-Nazca spreading center.
Triangles show the locations of volcanoes. The 100 km isodepth contour of the WadatiBenioff zone is shown [47].
[73] and are emplaced on 21–18 Ma old lithosphere [47]. The age and geochemistry of
the seamounts is essentially the same as Cocos ridge indicating emplacement during the
hotspot activity in the adjacent segment 1 lithosphere [73]. The ocean crust of segment
2 is 7–8 km thick [33, 71] and it is flexed into the trench more than the contiguous
segment 1. Bending at segment 2 is partially relieved by normal faulting near the trench
axis (Figs. 9.2 and 9.5), that strikes obliquely to the trench axis and to the seafloor
spreading magnetic anomalies (Figs. 9.9 and 9.10). Segment 3 with a 6 km thick crust
[33] has the smoothest morphology displaying a few bending-related faults with
relatively small vertical displacement. Bending-related faults strike roughly parallel to
the trench and are perpendicular to the magnetic anomalies, implying that bending,
rather than reactivating the inherited tectonic fabric, forms new faults. The boundary
between segment 3 and segment 4 is a low ridge that strikes perpendicular to the trench
and marks the juncture between the lithosphere formed at the Cocos-Nazca spreading
center and at the East Pacific rise. The crust in segment 4 is ~5–6 km thick [33, 74] and
is pervasively faulted as it bends into the trench. Faulting is roughly parallel to the
trench axis and to magnetic lineations (Figs. 9.9 and 9.10) and thus parallel to the fabric
formed at the East Pacific rise (Figs. 9.2 and 9.5). In this segment, trench depth and the
width of the faulted area increase to the NW to ~5500 m depth.
Bending-related faulting produces a profound geochemical and mechanical change
of the incoming oceanic lithosphere. The pervasive system of faults cuts across the
entire crust and into the upper mantle to at least 20 km below the seafloor [72].
Figure 9.11. Perspective shaded relief view of the bathymetry from central Costa Rica showing seamount subduction and the Nicoya slide.
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258 TECTONICS AND GEODYNAMICS
Faulting remains active across the entire trench oceanic slope and as the plate continues
bending new faults form up to the trench axis. This extensional system provides open
paths for fluid percolation deep into the oceanic plate, leading to serpentinization of the
peridotites of the mantle and perhaps also important hydration of the crust [72]. As the
oceanic plate subducts, the progressive pressure-temperature increase leads to
metamorphic dehydration reactions liberating fluids from the slab that are released into
the mantle wedge of the overriding plate, triggering arc volcanism [75]. The
segmentation of the oceanic plate is also displayed in the variability of the amount of
bending-related faulting at the trench which might also imply differences in the amount
of hydration of the plate along the subduction zone.
In summary, the morphology of the subducting oceanic plate, the amount of
bending and the associated increase in trench depth, change greatly in segments of the
CAM studied. Controlling parameters appear to be the changes in crustal thickness, its
age and thermal structure, and the angle between seafloor spreading fabric and the axis
of bending. This segmentation of the incoming plate is parallel by a similar spatial
segmentation in the character of the overriding plate.
9.5.2 Parallel segmentation of subducting and overriding plates
Remarkable is the parallelism between morphotectonics of the continental slope, the
shelf, the land, and the morphological segmentation of the ocean plate. Active forearc
tectonism is influenced by local uplift over subducting seafloor relief and the
subsidence from subduction erosion at the front and along the base of the upper plate.
Thus, changes in character of the subducting oceanic plate influence differences in the
Quaternary tectonic evolution of the continental margin. The influence of the
subducting oceanic plate character is additionally correlated to distinct differences in
seismic activity occurring along the four segments of the margin. The volcanic arc also
displays along-strike variability that corresponds to changes in the character of the
incoming plate.
Segment 1 of the incoming plate subducts offshore the Osa peninsula of
southeastern Costa Rica. It contains the thick and buoyant Cocos ridge and correlates
with singular morphotectonic features landward of the continental slope to the arc.
Retreat of the margin is greatest opposite the colliding Cocos ridge which implies
accelerated Quaternary erosion (Figs. 9.2 and 9.5). Opposite the subducting ridge,
uplift of a broad area of the continental shelf culminates in the Osa peninsula. The
peninsula displays margin perpendicular topography parallel to the large tilted fault
blocks on the ridge [40]. Landward of Osa peninsula, the Fila Costeña thrust and fold
belt is the only area along the margin where active contraction is significant. Further
landward, arc volcanism is extinguished and the Talamanca cordillera has been uplifted
[76, 77] exposing mid crustal granodioritic plutons [49]. Rough relief from river
incision characterizes the region where arc volcanism ceased and a drainage system
developed (Fig. 9.5). Large earthquakes of MW 7–7.5 have nucleated in this area but
during the past 20 years small earthquakes in this segment have been relatively few
compared to those in the adjacent middle segment [78].
Segment 2 of the incoming plate subducts offshore central Costa Rica. The
subducting ridges and chains of seamounts that are 2 to 3 km high breach the margin
front and leave seafloor grooves up the slope indicating material removal. Subducting
seamounts breach the lower slope during initial collision with the margin (Fig. 9.11)
CENTRAL AMERICA: GEOLOGY, RESOURCES AND HAZARDS; BUNDSCHUH & ALVARADO (EDS)
Figure 9.12. Prestack depth migration of Sonne-81 line 6 indicating active tectonic erosion
above a subducting seamount as it tunnels underneath the continental plate. Dots mark the
plate boundary, with black dots delineating the seamount flanks. This segment of line 6 is
parallel to the continental margin and shows the lateral variations in margin wedge thickness.
The margin wedge is 0.5–0.7 km thinner above the subducting seamount than on either side.
Tectonic extension of the overriding plate caused by uplift above the seamount is too small to
explain the thinning. The thinning is probably due to ongoing tectonic erosion by the
seamount. Location on Fig. 9.2. Modified from Ranero and von Huene [37].
and as they tunnel beneath it they apparently erode the underside of the upper plate
(Fig. 9.12). The continental slope is uplifted above the subducting seamount causing
mass wasting at the seafloor. Mass wasting produces slumps and slides locally several
tens of km long. The largest slide is the Nicoya slide with a headwall detachment
running more than 50 km across the middle slope (Fig. 9.11). The slide toe runs about
5 km up the ocean trench slope and it could have created a tsunami wave about 30 m
high [79]. Seamounts and ridges remain attached to the subducting plate beneath the
continental shelf where they produce uplift (Fig. 9.13). Subducted basement highs
beneath the shelf induce local increase of the stress on the plate interface and are areas
of earthquake nucleation [37, 80–82]. The largest events associated with chains of
subducting seamounts are Mw < 7 [37]. Some seamounts are associated with local uplift
on land that may separate the Panama block from northern Costa Rica along margin
perpendicular faults [83, 84]. In this segment the volcanic output is greater than along
other segments of the arc, and the arc volcanoes have recently migrated landward [85]
perhaps related to the accelerated tectonic erosion associated with seamount subduction
here [37, 40]. Where large seamounts on the ocean plate are sparse and small,
roughness of seafloor forms from bending-related normal faulting. Segment 3 and the
contiguous portion of segment 4 exhibit a relatively smooth subducting oceanic plate
morphology (Figs. 9.2 and 9.5) mainly opposite the Nicoya peninsula. There the
continental slope displays the most stable morphology of the margin with gentle dips
and well-developed canyon systems in the upper-middle slope (Fig. 9.5). Heatflow at
the trench and lower slope is exceptionally low [67, 86]. Some of the largest historic
earthquakes have nucleated in this segment [78] adjacent to the smoothest subducting
topography in the area. The oceanic plate of segment 4 displays more bend-faulting and
morphological roughness that increases towards the NW (Figs. 9.2 and 9.5). The
vertical displacement of individual faults increases progressively NW from a few tens
of m to as much as 500 m. A parallel change in continental slope morphology occurs
along the margin. The slope of the margin changes gradually from the simplest
structure offshore Nicoya peninsula to a margin characterized by an upper, middle and
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260 TECTONICS AND GEODYNAMICS
Figure 9.13. Prestack depth migration of Sonne-81 line 5 showing the subducted extension of
the Quepos plateau underneath the continental shelf. Note the uplift of shelf strata above the
subducting ridge. Location on Fig. 9.2.
lower slopes with different morphologies and dips. The upper slope dips gently and its
many canyons develop at the slope break and coalesce downslope. A change to steeper
dips defines the transition to the middle slope, characterized by many normal faults and
the mud diapirs associated with fluid flow [87] and chemosynthetic fauna [88]. Also,
the middle slope dip increases progressively towards the NW, where slump scars
become abundant (Figs. 9.2 and 9.5). The lower slope is an area characterized by
terraces that display an en echelon pattern that mimic and strike parallel to the
morphology of the half grabens in the subducting oceanic plate. The terraces are
formed by the riffling of the thin apex of the overriding plate over subducting
topography indicating a largely dismembered upper plate. Segment 4 ocean floor
subducts in the area where the damaging 1992 large tsunami earthquake occurred [89].
9.6
SUMMARY
Early in the exploration of the Pacific basin the CAM was recognized as a Pacific type
margin with a trench and Wadati-Benioff zone. It was recognized as a subduction zone
when the plate tectonic paradigm first developed. Later it was significant in developing
the knowledge to correctly differentiate between accretionary and erosional convergent
margin end-members. When only seismic data are available the constant accretionary
end-member model must be applied with greater care than it was invoked in the past.
Landward dipping reflections in seismic records are not only derived from accretionary
processes but in erosional margins they may represent extensional structures or
inherited fabric and their origin requires further investigation. The CAM displays the
effects of subducting lower plate character on tectonics of the upper plate. The effects
of relief, crustal thickness, thermal structure, and the orientation of original oceanic
crustal fabric also shape continental slope structure, induce a pattern of earthquake
seismicity, influence the potential for tsunamigenic submarine sliding, and correlate
with changes in the volcanic arc. Oceanic crustal thickness and temperature correlate
with trench depth and bending of the incoming plate. Orientation of oceanic crustal
fabrics subdue or accentuate bend faulting which imparts subducting plate roughness
and correlates with rates of erosion. These interactive geological processes are not yet
fully understood and are the topics of ongoing research. The differences in the
character of the subducting plate are sharply segmented along the CAM and segment
CENTRAL AMERICA: GEOLOGY, RESOURCES AND HAZARDS; BUNDSCHUH & ALVARADO (EDS)
boundaries in the lower plate correspond with features in the upper plate. This
segmentation also separates earthquake magnitude and epicentral patterns. Subducting
seamounts are commonly associated with large submarine slides. Thus, the forefront
research along the CAM continues to contribute significantly to a basic understanding
of natural hazards and promotes progress towards the mitigating of their damaging
effects.
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