Collaborative Research:
Crust-Mantle Interactions During Continental Growth and High-Pressure Rock
Exhumation at an Oblique Arc-Continent Collision Zone: The SE
Caribbean margin
Recent work in several island arc terranes including the Aleutians and
the Izu-Bonin arc has shown that their composition is mafic (Holbrook et al.,
1999; Suyehiro et al., 1996). This observation makes it difficult to reconcile
the concept that intermediate-composition crust has formed simply by
progressive welding of Aleutian-type island arc terranes to a central
continental core. The questions arise: 1) How can arc accretion to continents
produce intermediate composition crust? and 2) Are the accretion processes we
observe today the same as those that have operated throughout history to create
the continents (Lyell, 1830; Taylor, 1967; McCulloch and Bennett, 1994)? This
proposal addresses both of these fundamental questions on the origin of the
continental crust.
Island arc-continent collision is currently occurring in Taiwan (Kao et
al., 1998), northern Australia (Genrich et al., 1996), the Aleutians-Kamchatka
Peninsula (Geist et al., 1994), and along the South American-Caribbean plate
boundary (Ave Lallemant, 1997). The Aegean (Reilinger et al., 1997) is in a
pre/early collision stage. All of the Indonesian and related arc systems are
viewed as in an early pre-collision stage of accretion to southern Asia
analogous to Proterozoic growth of the southwestern U.S. (Karlstrom, personal
communication).
None of these are simple collisions. The first is a complex interaction
between the Eurasian plate and the Philippine plate, and occurs only over a
short distance with little study area. The second is in the early stages of
development, with the continental Australian plate still subducting northward
beneath the Java trench and the Timor arc, and hence does not lend itself to
understanding the true stages of accretion. The third is also an immature
collision of extremely limited geographic extent, with the mafic
Aleutian islands
colliding with Eurasia at a high angle. The latter two are
pre-collisional.
In contrast, the South Caribbean Plate Boundary Zone where the Leeward
Antilles Arc is colliding with South America (Figs. 1-3) covers a very large
area (~1,600 km east-west; ~106km2), and is an
eastwardly-diachronous (time transgressive) collision occurring over the last
50 million years, that allows us to study the processes and geometries of
arc-continent collision over a range of stages of development. Specifically,
this example allows us to analyze the common phenomena in plate tectonics of
arc-polarity reversal due to trench choking by the continental plate: Along the
eastern end of the plate boundary near Trinidad, choking and flipping has not
yet occurred, whereas further west it was achieved over 20 million years ago.
In the central segment, east of Margarita Island, the process is active and
ongoing today. This setting allows us to see what can happen within the
accretion zone after accretion has been completed, which is as important to the
understanding of continental growth as the accretion itself. The Leeward
Antilles arc appears to be felsic to intermediate, built on oceanic plateau
crust. Lastly, one of the products of accretion in this setting is the still
poorly understood process of exhumation of high-pressure/low-temperature
(HP/LT) metamorphic rocks. This arc-continent collision setting allows us to
trace the exhumation process for 50 million years and determine if large
lateral displacements provide an important path in the exhumation process. Such
attributes make the southern Caribbean boundary ideal for a modern study of arc
accretion, arc polarity reversal, and eclogite and blueschist facies
metamorphism tectonics.
Although arc-continent collision and arc accretion are universally accepted processes in the South Caribbean Plate Boundary Zone, a variety of plate tectonic models for the evolution of this zone have been proposed which imply a broad range of possibilities for (1) the number, age, geometry and polarity of subducted slabs, and (2) the chemical nature, origin, and degree of arc maturity of accreted fragments, along northern South America (Malfait and Dinkelman, 1972; Maresch, 1974; Ladd, 1976; Burke et al., 1978; Dickinson and Coney, 1980; Pindell and Dewey, 1982; Beets et al., 1984; Burke, 1988 ; Snoke et al., 1990; Pindell and Barrett, 1990). Since the development of a quantitative regional plate-kinematic framework (Pindell et al., 1988), the strong points of each of these models have been increasingly incorporated into a fairly standard and kinematically robust history of development recently summarized by Pindell et al. (1998), who refined it further using the regional history of sedimentation. In addition, this kinematic model has now been used to preliminarily test specific processes such as displacement partitioning, and associated arc-parallel extension and uplift of HP/LT rock assemblages (Ave Lallemant and Guth, 1990; Ave Lallemant, 1997). We now have an understanding of the paleogeographic history and associated processes involved in this complex zone of arc-continent accretion.
However, this model implies certain crust- lithosphere
inter-relationships within the South Caribbean Plate Boundary Zone. We are
still far from having a clear picture of the geometry of the various slabs and
deep crustal/upper mantle elements which have played primary roles in this
4-dimensional history of arc-continent accretion and HP/LT metamorphic rock
exhumation, especially in central and eastern Venezuela and Trinidad where
accretion and reversal are ongoing. With carefully designed experiments to
define these geometries within the plate boundary zone, we can learn the manner
in which lithospheric slabs and crustal fragments interact during the plate
boundary evolution, and therefore gain a dynamic, time-transgressive
understanding of the detailed processes of ongoing arc-continent accretion, and
attendant HP/LT rock exhumation.
The Caribbean-South America plate boundary has been accreting the
Antilles Arc (the Venezuelan-Dutch archipelago) to the South American craton
along an ever-lengthening plate boundary consisting of a trench at either end
of a right-lateral transpressional fault system (Figs 1-7; Table 1). The
right-lateral compressional plate boundary zone connects the NW-dipping Lesser
Antilles subduction zone where the American plate is subducting beneath the
Caribbean plate in the east, to the SE-dipping subduction zone off western
Venezuela and Colombia in which the Caribbean plate subducts beneath South
America. Teleseismic tomography and seismicity patterns show the Caribbean
plate overriding the Atlantic seafloor southwest of Trinidad beneath the Gulf
of Paria (Figs 8-9), whereas to the west the Caribbean plate is seen subducting
beneath continental South America (Figs 9-10). Thus, subduction polarity
reverses across the length of the right-lateral transpressional zone. The Aves
Ridge/Lesser Antilles Arc, built on plateau-like, 12 km thick Caribbean oceanic
crust, is accreting to South America along this boundary, with a 200-500
km-wide fold and thrust belt/foreland basin system developing between the two
subduction zones. The core of the folded belt lies within the transpressional
boundary, exposing HP/LT accretionary metamorphic rocks in a series of boundary
parallel belts. Cessation of arc magmatism, exhumation of the metamorphic
belts, and development of the fold and thrust belt/foreland basin system all
appear to have developed diachronously from west to east. In a larger context,
it has been suggested that the Caribbean Plate and the Scotia Plate formed and
migrated relatively eastward from deep mantle corner flow around the Nazca
Plate as a result of Nazca-South American convergence (Russo and Silver, 1996).
Plate tectonic/terrane reconstructions of the Caribbean-South American plate
boundary qualitatively support this hypothesis, showing lithospheric fragments
being swept around the northern margin of South America and accreting to the
continental landmass (Norton, Exxon Production Research).
As we describe in subsequent sections, aspects of this plate boundary
make it an excellent study area for the processes associated with
island-arc-continent accretion, subduction polarity reversal, and HP/LT rock
exhumation
In the west the plateau-like Caribbean plate is subducting beneath the
felsic-intermediate Leeward Antilles arc and the metamorphic belts of the
Caribbean Mountain system, both of which were formerly part of the Caribbean
plate and are now accreting to South America. Passive and active seismology
will image the subduction polarity reversal, and zones of crustal strain
associated with arc accretion
The Antilles arc is being dismembered laterally by arc-parallel extension
along the margin and its fragments are being accreted to the continent. This
extension is the result of displacement partitioning within the curved arc (see
Fig. 11) and displacement along en echelon strike-slip faults. Available
geochronology suggests that arc magmatism ceased from west to east as
subduction polarity reversed and the arc began accreting to the continent. U-Pb
and Ar-Ar dating will provide precise timing of magmatic history and cessation
along the length of the Leeward Antilles arc, either confirming or refuting the
diachronous cessation of magmatism as a function of arc-continent interaction
age.
High-pressure metamorphic assemblages are being exposed in the
metamorphic belts forming the transpressional zone in the core of the Caribbean
Mountain system. These rocks appear to show a west-to-east decrease
in metamorphic
ages, we conjecture that exhumation is proceeding west-to-east with accretion
of the island arc. Field studies and metamorphic thermobarometry will confirm
this pattern of exhumation as a function of arc location.
The available images of mantle structure
and seismicity patterns suggest that the lithospheric plate geometry along this
boundary controls development of surface structures, although available
tomographic images are of too low resolution to provide details. The passive
source seismology will provide considerably higher resolution tomographic
images of the lithosphere, as well as better constraint on seismicity
patterns.
The corner flow hypothesis predicts that seismic anisotropy patterns will
follow the trend of the plate boundary in a broad region up to 400km wide. The
passive source seismology will map mantle flow as an anisotropy signature
across the length of the plate boundary.
Portions of slabs may have broken off during choking of the original
Benioff zone by South America. We will image these as well as the edge of the
South American craton against which they are subducting.
Plate geometries beneath the strike-slip boundary and particularly near
the zones of transition from subduction to strike-slip are poorly known (Figs
9-10), and thus the details of polarity reversal and its consequences for
crustal evolution are unknown. Imaging this region will assist with
interpretations of regional seismicity.
In the eastern zone we will be able to understand the origin of the
subduction related Matur’n Basin-Gulf of Paria gravity low, the largest
on-land negative Bouguer gravity anomaly in the world. This low appears over
the region where a tear between nonsubducting South America and the subducting
Atlantic plate has been hypothesized.
The zone of crustal-scale orogenic float between the two plates is
currently cut by E-W transcurrent faults that participate in the strain
partitioning. In addition to establishing the strain partitioning, these faults
and their relations with flanking sedimentary basins can be assessed to
understand large-scale isostatic rebound resulting from plate truncation and
overthrusting, and creating regional unconformities.
Logistically, this boundary also makes an excellent study area. Marine
seismic acquisition is possible over most of the Antilles arc and into the
center of the metamorphic belts comprising the core of the fold and thrust
belt. The islands and peninsulas enable field study of the nature and chemistry
of basement. Onshore-offshore seismic profiling, seismic velocity
determination, and correlation with surface geology are also facilitated by the
islands in the arc, and by islands and peninsulas of exhumed high-pressure
rocks forming the interior of the folded belt system. The coastal geology of
much of the plate boundary is well exposed and well studied in reconnaissance.
(Venezuela and the archipelago, despite the tropical latitude, is an arid,
semi-desert region with excellent exposures for geologic studies and sampling).
Reconnaissance geophysics in the form of seismicity and regional tomography
studies have been completed, providing considerable guidance in locating of
active source seismic transects and passive seismic arrays. Petroleum
exploration provides a wealth of sedimentary and upper crustal information on
the foreland fold and thrust belt and the offshore regions, further allowing us
to pick our seismic transects carefully. We have thousands of kilometers of
shallow petroleum exploration data to examine to aid in choosing the exact
locations of our transects, and for interpretation of shallow structure. We
have developed important working relationships with PDVSA-INTEVEP, FUNVISIS,
and a number of Venezuelan universities. The former is a Venezuelan oil company
conglomerate. FUNVISIS is the Venezuelan seismological institute. PDVSA has
agreed to undertake and pay for permitting the experiment with the Venezuelan
government (A number of Rice, UH, and UT graduates work for PDVSA). FUNVISIS is
proposing a number of seismic investigations to complement our continental
margin passive and active source seismic investigations, described below and in
an appendix.
We are proposing detailed land and marine passive seismic imaging from
the Guyana shield to the central Caribbean to investigate upper
mantle structure
in northern South America and the Leeward Antilles. The imaging will be based
on travel time and waveform tomography, receiver function analysis and
shear-wave splitting. Detailed seismic reflection and onshore-offshore
profiling along the length of the boundary and along four boundary normal
corridors representing the plate boundary at different times of development
will provide high resolution velocity-reflectivity images of the crust and
upper mantle. Field geologic studies, U-Pb and Ar-Ar age dating, geobarometry
and geothermometry will date and characterize the timing and cessation of arc
volcanism and the exhumation histories of the metamorphic belts in the study
corridors. The formation and history of the E-W through-going strike-slip fault
systems and their relation to the collisional orogen will be assessed to
understand the role of strain partitioning and isostatic response resulting in
new basins and uncomformities. Geodynamic modeling of coupled continental
tectonics and mantle dynamics in the arc-continent collision setting will
constrain the physical conditions that can cause subduction polarity reversal
(e.g., the buoyancy and volume of arc crust required, the influence of
rheologic factors such as a weak lower crust). Modeling will also be used to
see how structural elements in the mantle lithosphere, e.g., cratonic roots,
can effect patterns of crustal deformation in an arc-continent setting. This
aspect of the modeling will be able to map the conditions under which mantle
structures are related directly to deformation patterns in the upper crust and
the conditions under which some form of decoupling can occur. By comparing
model predictions to the proposed geologic and seismic datasets, physical
factors that are not accessible by direct observations alone can be
constrained; specifically, the nature of deformation within the lower crust
and, by association, its rheology.
These geodynamic, geophysical, and geologic studies will concentrate
along 5 of key transects (Figs. 12-13) which are characteristic for 0 Ma
(Present) to 50 Ma (Eocene) accretion and collision. To spatially sample the
region well, we have added additional MCS seismic lines between the main
transects, as suggested by previous reviewers of this proposal. Geologic
studies will of course cover the entire study region. Geodynamic modeling will
be done for 2-D and 3-D structures. Broad spatial sampling is necessary for
understanding the time-transgressive nature of arc accretion, HP/LT rock
exhumation, and crust-mantle interplay.
1) Trinidad/0Ma (Present): A NW-SE
trending transect through Trinidad
will establish the pre- and syn-collisional crust and upper mantle geometry
(Atlantic lithosphere dips to the west and northwest) and will ascertain the
cause of the negative Bouguer gravity anomaly over the Matur’n Basin/Gulf
of Paria (Fig. 3). This is the initial condition of the collision/accretion
process.
2) 65W/15Ma
(Miocene): This NS trending line
through
Eastern Venezuela will examine the
Miocene-Pliocene collisional crust-upper mantle geometry. West- to
northwest-dipping Atlantic crust is hypothesized to be cut by incipient
south-dipping Caribbean lithosphere subduction.
3)
67.5W/30Ma (Oligocene): The Central Venezuela line will establish Oligocene
collisional crust-upper mantle geometry: How long and how far the South
American crust has been subducted northward and the Caribbean southward. The
two are recognized by the expected opposite vergence of structures in the upper
crust.
4)
70W/50Ma (Eocene): The Western Venezuela
line will show the Eocene collisional slab geometry similar to the 30
Ma line but influenced by the northward
extrusion of the Maracaibo block. This and the 30Ma line are the final condition.
5) Arc/12N/0-50
Ma: This EW trending line along the Dutch and Venezuelan Leeward Antilles
islands examines possible crustal thickness changes resulting
from heterogeneous
arc-parallel stretching (Fig. 11). Seismic velocities and
fieldwork will constrain
the proportions of mafic-versus-sialic rocks; deformation structures will
help elucidate the processes responsible for the exhumation of
high-pressure
metamorphic rocks.
In addition there are a number of collaborating projects by Venezuelan
scientists: FUNVISIS is proposing three seismic investigations to complement
the studies in this proposal. Some funding is already in hand, other funds are
being requested from the Venezuelan government and oil industry. 1) FUNVISIS is
now upgrading the permanent Venezuelan seismic network with 20 BB seismographs
(Guralp CMG-40T sensors) and 30 short-period stations (1 Hz, vertical component
S13s) installed in the seismogenic zones in northern Venezuela. Seven
additional portable BB seismographs will be used to augment the passive
experiments proposed here. The U.S. effort is designed to make use of the
FUNVISIS permanent and temporary stations. 2) FUNVISIS is requesting funds from
the Venezuelan National Council on Scientific and Technological Investigations
(CONICIT) that will add 3 land shotpoints along each of four onshore-offshore
transects we are proposing to record them as land refraction profiles, thus
extending their landward the coverage. 3) FUNVISIS is requesting funds from
PDVSA-INTEVEP to conduct a seismic refraction/reflection profile across the
Serran’a del Interior fold and thrust belt, tying one of our
onshore-offshore transects from the metamorphic hinterland near the coast to
the foreland basin on the edge of the craton in the continental interior.
FUNVISIS will also undertake detailed gravity measurements along all seismic
profiles. 4) PDVSA-INTEVEP has agreed to undertake and pay for all permitting
for the experiment. Additionally they will provide financial support for two
Venezuelan students to pursue Ph.D. studies at the U.S. institutions. (See
attached letter).
Hans Ave Lallemant met with representatives of FUNVISIS, PDVSA-INTEVEP
and Simon Bolivar University on May 4, 2000, in Caracas, and worked out a
preliminary protocol for cooperation with the U.S. investigators and the
Venezuelan government, universities, and oil industry. Alan Levander, Colin
Zelt, and Terry Wallace will visit Venezuela in the fall of 2000 to plan the
experiments, and develop student-faculty exchanges with our Venezuelan
colleagues. We anticipate that a number of Venezuelan students will enroll in
the Universities of Texas, Arizona and Rice University as part of this
collaboration.
This proposed project and the cooperating investigations study island
arc-continental accretion, the mechanics and P-T-d-t paths of burial and
exhumation of HP/LT rocks, and the development of folded belts. To do this we
are proposing extensive geological investigations in the arc and metamorphic
belts to provide dates on volcanic activity and uplift histories along the
length of the margin, as well as the sequencing of sedimentary basin
development. We will image the crust and mantle structure beneath the
Caribbean-South American continent, the transpressional regime, and the
Caribbean Sea. The deep seismic reflection/refraction investigations and the
mantle tomography studies will jointly image the crust-mantle interactions
which have produced the surface geology. The 1000 km by 500 km area for the
active source studies encompasses a large part of the SE Caribbean plate
margin. The land and marine teleseismic imaging we are proposing encompasses an
even greater area than the active source investigations, and will image the
lithospheric and sub-lithosphereic mantle beneath the southern Caribbean plate
and as far south as the Guyana shield of the South American craton.
This project is an outgrowth of a two day workshop on the Caribbean-South
American plate boundary organized by Professor H.G. Ave Lallemant, and held at
Rice University in October 1997, which was attended by approximately 40
academic scientists (including most of the U.S. PI’s) and about 60 oil
industry representatives, including employees of PDVSA-INTEVEP. This proposal
involves 10 PIâs at 6 U.S. universities, and 1 Caribbean expert who will
be paid through Rice (James Pindell). Additionally, this project is coordinated
with investigators at 3 Venezuelan institutions: The Universidad Central de
Venezuela, Simon Bol’var University, and FUNVISIS. The geologic
investigations are designed to begin in the first year and continue through the
third year. Active source seismic experiments are planned for the second year
(winter 2002), with data analysis in the third year. Passive source seismic
experiments are planned for the second year and first half of the third year,
with data analysis extending through the third year.
We note that criticisms of last year’s submission of this project
included inadequate PI support, and inadequate attention to program
coordination and management. We have addressed these criticisms: PI support has
been increased for the project managers. Project coordination will be enhanced
by annual workshops, following the highly successful example set by the CDROM
project. All PI’s have budgeted for annual workshops. Management will be
enhanced by additional secretarial support, and addition of a post-doc for
coordinating seismic studies, and helping organize workshops. We have also
included a passive OBS experiment in this request that was lacking from last
yearâs submission.
Direct
participants in this proposal are :
Rice University - Alan Levander: PI: Project Coordinator/ MCS
profiles
Ave Lallemant: PI: Geology Coordinator/Venezuela field
studies
Colin Zelt: PI: Active Source Seismology Coordinator/
Onshore-Offshore
profiling, wide-angle data analysis
Adrian Lenardic: PI: Geodynamic modeling
James Pindell: Research scientist: Cenozoic basins and
kinematic studies
Inci Ertan: Post-doc: Thermobarometry
Univ. of Houston - Peter Copeland: PI: Antilles field studies
Ar-Ar thermochronology on Antilles and Venezuela
Univ. Georgia - James E.
Wright: PI: Antilles field studies,
U-Pb thermochronology
Univ. of Arizona - Terry Wallace: PI: Passive Seismology
Coordinator
Land passive seismology
Univ. Ca. San Diego - Frank Vernon: PI: Marine passive seismology
Univ. of Texas - Paul Mann: PI:
Seismic interpretation and tectonics
Gail Christeson: PI: OBS seismology and crustal structure
Participating Scientists (see accompanying letters in Supplementary
Information Section)
Universidad Central de Venezuela :
Professor Marino Ostos Stratigraphy in Serran’a
Universidad Simon
Bol’var: Professor J.
Rigueiro Onshore seismology
Professor Professor J. Castillo
Onshore Seismology
Dr. Jesus
Castill Gravity
FUNVISIS -
Dr. Michael Schmitz, Dr. Gustavo Malave Land active Seismology
Dr. Herbert
Rendon Land passive seismology
Dr. Frank
Audemard Onshore Geology
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