Exotic vs' Fringing Arc Models for the Growth Of Continents: Evidence from Mesozoic ArcRelated Basin

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Exotic vs. Fringing Arc Models for the Growth Of
Continents Evidence from Mesozoic Arc-Related
Basinsof Baja California, Mexico

• Cathy J. Busby
• Department of Geological Sciences,
• University of California,
• Santa Barbara CA 93101, USA
• busby_at_geol.ucsb.edu

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• The western margin of Mexico is ideally suited
  for testing two opposing models for the growth of
  continents along convergent margins
• the exotic arc model, where western Mexico grew
  through accretion of exotic island arcs by the
  consumption of entire ocean basins at multiple
  subduction zones with varying polarities
• (2) the fringing arc model, where extensional
  processes in the upper plate of a subduction zone
  dipping toward the continent produced arc-related
  basins, some rifted off the continental margin
  and others formed of new oceanic lithosphere that
  largely lay within reach of North American
  turbidite fans.
• In the fringing arc model, later phases of
  east-dipping subduction juxtaposed these terranes
  through transtensional, transpressional or
  compressional tectonics.

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Burro lobsters on the Pacific Coast of Baja
California, Mexico - held by Douglas Smith.
Thanks to him and other students and postdoctoral
researchers William Morris, Madeleine Fulford,
Ben Adams, Grant Yip, Lars Blikra, and Salvatore
Critelli. Collaborators include James Mattinson,
Paul Renne, and Elena Centeno-Garcia.
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Arc terranes compose at least a third of Mexico,
which was largely assembled in Late Paleozoic to
Mesozoic time. Excellent exposures in the Sonoran
Desert of Baja California provide spectacular
views of relatively unmetamorphosed, intact
Mesozoic arc terranes.
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• CONSTRUCTION OF CONTINENTS FROM ARCS
• What is the relative importance of
• accretion of exotic vs. semi-autocthonous
  elements
• volcaniclastic or epiclastic basin infilling
• 3. lateral translations
• 4. magma addition

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1. Exotic arc terranes accreted to nuclear
Mexico via closure of an entire ocean
basin (Gastil et al., 1978 Tardy et al., 1994
Lapierre et al., 1992 Dickinson and Lawton,
2001 Wetmore et al., 2002) 2. Marginal arc
terranes that fringed the continent (Rangin,
1978 Gastil and Miller, 1981 Phillips, 1993
Saleeby and Busby, 1992 Thomson and Girty,
1994 Busby et al., 1998 Busby,
2004) Tantalizing but sparse new detrital zircon
data support model number 2.
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Eastern Peninsular Ranges PZ-Mesoic Continental
margin assemblages intruded by Late Cretaceous
granitoids. Western Peninsular Ranges Early
Cretaceous oceanic arc. Vizcaino-Cedros Triassic
-Jurassic arc-ophiolite assemblages overlain by
forearc sedimentary rocks.
Busby-Spera and Boles, 1986
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Tectonostratigraphic Chart of Mesozoic Convergent
Margin Assemblages, Baja California, Mexico,
grouped by evolutionary phases 1, 2, and 3 (Busby
et al., 1998 Busby, 2004).
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Evolutionary Model for Arc Systems Facing
Large Ocean Basins Subduction initiated by
rapid sinking of old, cold lithosphere, but over
many tens of millions of years, progressively
younger lithosphere is subducted. Gradual
decrease in slab dip angle causes progressive
change from extensional to compressional tectonic
regime, and inboard migraton of arc axis. (Busby
et al., 1998)
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(Busby et al., 1998)
Intra-oceanic Arc-Ophiolite Systems (Late
Triassic to Late Jurassic) Supra-subduction zone
ophiolites with arc geochemical signatures
(Moore, 1985 Kimbrough, 1984), directly overlain
by arc volcanic-volcaniclastic rocks (Whalen and
Pessagno, 1984 Barnes, 1984 Moore, 1985
Kimbrough, 1984 Busby-Spers, 1987, 1988).
Detachment faults preserved on Vizcaino
(Sedlock, 2004) and reactivated on Cedros Island
(Smith and Busby, 1993).
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Speculative schematic reconstruction of active
and remnant arcs and backarc basins in Middle
Jurassic time (Busby, 2004).
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Mesozoic Arc-related basins, Vizcaino
Peninsula-Cedros Island
Triassic-Jurassic arc ophiolite complexes
(purple) underthrust by Cretaceous subduction
complex (brown) and overlain by Cretaceous
forearc basin sedimentary rocks (yellow and gold),
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Middle Jurassic Gran Canon backarc apron
deposited across rifted arc basement and steaming
new ocean floor. Overlain by Jurassic Coloradito
sedimentary melange with blocks derived from
southern Mexico (new DZ results from Kimbrough,
Moore and Centeno-Garcia).
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How to get continentally-derived clasts and
megablocks blocks dumped into intra-oceanic arc
basins. Also provides a mechanism for
amalgamating Triassic to Jurassic arc-ophiolite
terranes without significant shortening.
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Gran Canon Formation, Cedros Island Arguably the
least deformed, best exposed outcrop view of a
backarc apron studied to date. A more proximal
view than is afforded by most drill sites in the
western Pacific. Simple, uniform sedimentation
pattern typical of BABS isolated from terrigenous
sediment. (Busby-Spera, 1987, 1988 Critelli et
al., 2002 Busby, 2004).
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EVOLUTION Progradation of a deepwater
pyroclastic apron across a widening BAB during
growth of the SOURCE ARC arc from
deeply-submergerd to shallow-marine (tuff to tuff
breccia lithifacies) to an emergent arc
undergoing renewed rifitng (primary volcanic
lithofacies) to remnant BAB (epiclstic
lithifacies) All within 10-15 Ma. (Busby-Spera,
1987, 1988 Critelli et al., 2002 Busby, 2004)
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EXPLORATION OF MODERN ARC SEAFLOOR IN THE PAST
DECADE The question changed from Do deepwater
silicic calderas exist? To Why are there SO
MANY deepwater silicic calderas?
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• Lessons From Modern Extensional Arcs
• Voluminous silicic volcanism at calderas
• Regional-scale arc rifts
• e.g. Izu arc now, Tonga-Kermadec at 6 - 3 Ma
• 2. Localized arc rifts
• e.g. trailing edges of rotating blocks in western
  Aleutian arc
• e.g. over subducted obstacles (e.g. central New
  Hebrides arc)
• e.g. oblique impingement of backarc rift into
  volcanic front (e.g. Kermadec arc).
• (references in Busby, 2004)

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I digress..Busby (in press) Geology POSSIBLE
DISTINGUISHING FEATURES OF DEEPWATER EXPLOSIVE
AND EFFUSIVE VOLCANISM
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Back to the Gran Canon Formation.
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Pillow lava (me for scale)
Pillow breccia (boot for scale)
Perfect pillow with keel (hammer for scale)
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Record of arc rifting on the backarc
apron Felsic subaqueous pyroclastic sediment
gravity flow deposits cut by basaltic fissures.
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Last phase of BAB formation before or during.
Transtensional amalgamation Why???? To bring in
continental detritus, To accrete SOFTLY, and to
fit in with THE BIGGER PICTURE..
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THE BIGGER PICTURE
(Busby et al., 2005)
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Western Peninsular Ranges (red) Early
Cretaceous oceanic arc Vizcaino-Cedros region
(orange) Forearc sedimentary rocks uncomformably
on Triassic-Jurassic arc-ophiolite assemblages.
Busby-Spera and Boles, 1986
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Early Cretaceous extensional arc-forearc
system Arc separated from continental margin
by narrow BAB with interfingering clastic input
from both sides, and no backarc ophiolite
remnants. (Busby, 2004)
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Reconstruction of the Early Cretaceous Arc to
Forearc Region Syndepositional normal faults
drop intra-arc and forearc basins to bathyal
water depths. No accretionary wedge (Sedlock,
1988, 1993, 1996) instead basement rocks
downstep to the trench, where arc-derived
detritus was carried into each sub-basin by
turbidity currents (Busby and Boles, 1986).
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Early Cretaceous Extensional Forearc Coarse-graine
d slope apron (Left) passes upward into turbidite
wedge (Right)
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Cretaceous Alisitos arc View of an Intact
Oceanic Arc, From Surficial to Mesozonal Levels
(Busby et al., JVGR, in press)
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Alisitos arc terrane (red) 300 X 30 km oceanic
arc terrane Sutured against PZ-Early Mesozoic
continental margin assemblages (blue) and then
intruded by a major continental margin arc
batholith (pink) Rosario segment 120,000 scale
mapping by Busby et al., in press.
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TIME-INTEGRATED VIEW OF AN INTACT OCEANIC
ARC Why study ancient arc terranes? The time
progression - cannot be inferred from
along-strike changes on modern margins because of
lateral variation in parameters. The opportunity
to look deep.
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I. EXTENSIONAL OCEANIC ARCIntermediate to
silicic explosive volcanism, culminating in
caldera-forming ignimbrite eruptions(Fackler
Adams and Busby, 1998).
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• RIFTED OCEANIC ARC Mafic effusive and
  hydroclastic rocks and widespread dike
  swarms(Fackler Adams and Busby, 1998).

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Time slice 1 Incipient, leaky ring fracture on
central subaerial edifice. Southern
volcano-bounded marine basin fed from both sides.
Northern fault-controlled marine basin
down-drpped from nonmarine ennvironment into deep
water, with caldera sited along the fault.
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Time slice 2 Cataclysmic caldera-forming
eruption on subaerial edifice. Ignimbrite crosses
relatively low-gradient shoreline into southern
volcano-bounded basin with some integrity, but
slides into northern fault-bounded basin as hot
mega-blocks in debris avalanche deposit.
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Time slice 3 Continued rapid tectonic subsidence
indicated by stacking of photo-zone rudist reefs
on south margin of central subaerial edifice
(north margin too steep). NOTE OVERALL VERY HIGH
TECTONIC SUBSIDENCE RATES OF 2 KM/MY.
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Time slice 4 widespread mafic volcanism, as well
as diking (except where occluded by resurgent
granitic magma). Continued rapid subsidence
indicated by rudist reef buildup on volcanoes in
southern basin. Emplacement of mafic plutonic
rocks. ALL FOUR TIME SLICES (5 KM) REPRESENT LESS
THAN 1.5 MA!!!
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Facies on Central Subaerial Edifice Extensional
arc Fluvial-debris flow volcaniclastic rocks,
andesite flows, ring-fracture silicic lava domes,
caldera fill, and strongly-welded outflow
ignimbrites. Rifted arc Mafic flows and mafic
volcaniclastic fluvial- debris flow deposits.
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Facies in Southern Volcano-Bounded Marine Basin
Extensional arc Pyroclastic flows enter marine
basin on relatively gentle slopes and maintain
their integrity. Rudist bioherms and minor beach
conglomerates developed on the relatively gentkle
slopes. Rifted arc Small-volume, central-vent
mafic eruptions.
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Facies in Northern Fault-Bounded Marine
Basin Extensional arc Pyroclastic flows
disintegrated into turbulent suspensions that
mixed with water as they traversed the rugged,
fault-controlled basin margin. Fault margin
controlled siting of caldera. Slumping and mass
wasting events common due to steeper topographer
and greater seismicity. Rifted arc Multiple,
unhindered conduits for ascent of mafic magmas,
which erupted largely in deep water.
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Excellent preservation sea star resting trace
(Busby, 2004).
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Subaerial edifice Lava dome breccias Beach
conglomerates
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Liquifaction of substrqate where a pyroclastic
flow entered the sea (Busby et al., 2004)
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Ultra-welded lineated ignimbrite in slide blocks
within deepwater debris avalanche deposit.
Peperitic margin on ultrawelded ignimbrite slab
at contact with enclosing wet volcaniclastic
sediment host. (Busby, 2004).
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Correlations between 60-km long Rosario Segment
and 40 km-long San Quintin segment.
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• Two types of units are widespread enough to
  permit stratigraphic correlation
• Tuff of Aguajito ignimbrite
• Deepwater debris avalanche with plastic clasts of
  tuff of Aguajito
• Note column 2 shows submergence of subaerial arc
  edifice rifting?

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Phase 3 Compressional Arc
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Decreasing slab dip and increasing convergence
resulted in BAB closure, and development of a
compressional continental margin arc.
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High-standing, deeply dissected continental arc
shedding coarse debris into forearc basin.
Underplating of blueschist metamorphic rocks and
gravitational collapse of the wedge top results
in extensional basins on the accretionary wedge.
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Late Cretaceous flood of conglomeratic turbidites
into the forearc basin (Vizcaino Peninsula).
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Mid-Cretaceous flood of conglomeratic turbidites
into forearc basin Provenance switch from
undissected island arc (volcaniclastic) to deeply
dissected continental arc (with plutonic and
metamorphic rocks). (Busby, 2004)
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Late Cretaceous trench-slope basin of extensional
origin, Cedros Island Fault-controlled submarine
canyon.
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Late Cretaceous trench-slope basin of extensional
origin, Cedros Island Propogation of a second
normal fault results in transfer-zone ramp
followed by basin broadening as it filled.
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Late late Cretaceous to Paleocene Oblique
subduction produces forearc strike-slip basins
(intra-arc equivalents recognized in California
Busby-Spera and Saleeby, 1990).
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Rosario embayment of the Peninsular Ranges
forearc basin complex - far better exposed than
northern equivalents in California. Evidence for
stike slip RAPID (i.e. 2 -5 my) alternation
of nonmarine and bathyal marine strata,
and normal and reverse faults (with folds)
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Coastal nonmarine deposits indicate subsidence
rates of 1 km/Ma. Thick overbank deposits stuffed
with dinasaur bones!
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A Campanian log jam.
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Fabulous exposures of coarse-grained turbidite
complexes (Morris and Busby-Spera, 1988)
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Fabulous seacliff exposures of submarine canyon
fill (Morris and Busby-Spera, 1988)
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Fabulous exposures of coarse-grained turbidite
complexes (Morris and Busby-Spera, 1990)
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Turbidite channel-interchannel deposits within
valley-levee complex.
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Slumps on levees on deepwater valley-levee
complex.
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Busby et al Geology 2002 and in prep Pacific
margin example of catastrophic sedimentation at
the Cretaceous/Tertiary boundary triggered by
bolide impact.
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ONE THIRD OF MEXICO IS MESOZOIC ARC
TERRANES!!! Both my 1998 2D model.and my 2004
3D modelare relatively fixist (i.e. accretion
largely an upper plate process) Detrital zircon
studies in progress will constrain 3D models
(with collaborators Elena Centeno Garcia, George
Gehrels, Marty Grove, Dave Kimbrough, and
others).
Long may you run!