Transcript Science Highlights of the RCL Initiative

Science Highlights of the RCL Initiative
Rupturing
Continental
Lithosphere
Modified from a summary by
Rebecca Dorsey, University of Oregon
Original Goals:
Understand spatial and temporal evolution of rifts.
Focus on key processes, state parameters and physical
properties that control them
- Link properties, processes, observations, and
modeling
- Observations: one orogenic rift (Gulf of California)
and one cratonic rift (Red Sea)
- Use experiments and data to address 4 thematic
questions (below)
Science Highlights of the RCL Initiative
Rupturing
Continental
Lithosphere
Original Scientific Questions
Modified from a summary by
Rebecca Dorsey, University of Oregon
1. What forces drive rift initiation, localization,
propagation and evolution ?
2. How does deformation vary in time and space, and
why ?
3. How does crust evolve, physically and chemically, as
rifting proceeds to spreading ?
4. What is the role of fluids and magmatism in
continental extension ?
Science Highlights of the RCL Initiative
GAME CHANGERS: New results that change the way we think
about continental rifting, rupture, and underlying controls
1. Styles of Extension Important factors: (a) Different styles of extending the
lithosphere; (b) Structural evolution of normal faults in rifts; (c) Pre-rift tectonic
histories (subduction, collision); (d) Faulting style controls shape of
sedimentary basins.
2. Role of Sedimentation Sediments not just a passive record of earth
history. Exert a direct control on rift process, magmatism, crustal composition,
formation of ocean basins. Includes critical link to interior fluvial system
(Colorado River) – “source to sink”. Can create new type of hybrid crust at
Ocean – Continent Transition.
3. Role of Rift Obliquity Important factors: (a) Oblique extension and strikeslip faults; (b)Relationship between the orientation of the rift and relative motion
direction (° of obliquity); (c) Rift obliquity affects resulting morphology of the rift
zone (Gulf of CA vs. Red Sea).
4. Role of Magmatism
Pre-rift volcanism depletes the upper mantle → less syn-rift magmatism. Less
magma makes lithosphere effectively stronger, so deformation migrates (not
localized). This produces a WIDE RIFT zone and longer time to rupture.
Revised: New results also lead to new 5. Structural Evolution
models for the Gulf of California
Science Highlights of the RCL Initiative
GAME CHANGERS: New results that change
the way we think about continental rifting,
rupture, and underlying controls
1. Styles of Extension Observations:
(a) North to south changes
(b) Low angle detachment to high angle
distributed faulting
(c)East to west migration
of plate boundary
deformation
(Gonzalez-Escobar et al., 2013)
Science Highlights of the RCL Initiative
1. Styles of Extension
(a) North to south changes
(Gonzalez-Fernandez et al., 2005)
NORTH: Widely distributed deformation, thick layer of
new sediments, no central rift zone
(Lizarralde et al.,
2007)
(Dorsey and Umhoefer, 2012)
SOUTH: localized deformation with central rift zone,
thinner layer of new sediments, volcanism
Science Highlights of the RCL Initiative
1. Styles of Extension
(b) Low angle detachment to high angle
distributed faulting
Science Highlights of the RCL Initiative
1. Styles of Extension
Northern Gulf: Upper
Tiburon and Delfin Basins
Location of next
seismic line shown:
extends across the
Upper Delfin Basin
to the Upper
Tiburon Basin
(Gonzalez-Fernandez et al., 2005)
Science Highlights of the RCL Initiative
1. Styles of Extension
Low angle detachment faulting in the Tiburon Basin
(Gonzalez-Fernandez et al., 2005)
Science Highlights of the RCL Initiative
1. Styles of Extension
Northern Gulf: Upper Tiburon and Delfin Basins
• Localized crustal thinning begins ~6 Ma via a
major low-angle detachment fault in the
Upper Tiburon Basin
• Crust thinned from ~30 km to ~19 km (this
includes ~8-10 km of added sediments)
• Extension in Upper Tiburon basin ended ~2-3
Ma
Science Highlights of the RCL Initiative
1. Styles of Extension
Northern Gulf: Upper Tiburon and Delfin Basins
• ~2-3 Ma the focus of extension shifted from the
Upper Tiburon basin to the Upper Delfin basin
• This included
– the change from low-angle detachment faulting in the
Upper Tiburon basin to high-angle normal faults and
the development of a narrow rift zone within the
Upper Delfin basin
– Some basaltic(?) magmatism, mostly intruded as sills
into lower basin sediments
Science Highlights of the RCL Initiative
1. Styles of Extension
High-angle faulting and focused rifting in the Upper Delfín Basin
(Gonzalez-Fernandez et al., 2005)
(Marin-Barajas et al., 2013)
Science Highlights of the RCL Initiative
1. Styles of Extension
(c)East to west migration of plate boundary
deformation
Science Highlights of the RCL Initiative
1. Styles of Extension
Westward migration
of extension in the
northern Gulf of
California
Rifting formed
transtensional pullapart basins in the
EAST first
(Aragon-Arreola and Martin-Barajas, 2007)
Science Highlights of the RCL Initiative
1. Styles of Extension
Westward migration
of extension in the
northern Gulf of
California
~2-3 Ma, the locus
of rifting shifted
west, forming new
transtensional pullapart basins
(Aragon-Arreola and Martin-Barajas, 2007)
Science Highlights of the RCL Initiative
1. Styles of Extension
Location of
active (dark
gray) and
inactive (light
gray) pull-apart
basins
(Aragon-Arreola and Martin-Barajas, 2007)
Science Highlights of the RCL Initiative
1. Styles of Extension
Why the westward shift?Not fully known
but the current hypothesis is…
• Thick sediments deposited from the Colorado
River may play a role in insulating the
lithosphere and creating lateral heat flow,
which in turn affects lithospheric strength and
resulting strain
• Prior volcanism in Baja and mainland Mexico
may have also affected lateral heat flow
Science Highlights of the RCL Initiative
GAME CHANGERS: New results that change
the way we think about continental rifting,
rupture, and underlying controls
2. Role of Sedimentation
Sediments not just a passive
record of earth history.
Can create new type of hybrid crust at Ocean –
Continent Transition.
Science Highlights of the RCL Initiative
2. Role of Sedimentation
Can create new type of hybrid crust at Ocean –
Continent Transition.
Science Highlights of the RCL Initiative
2. Role of Sedimentation: Hydrothermal Activity & Magmatism
Salton Trough
Quaternary rhyolites
produced by episodic
remelting of altered
basalts, not fractional
crystallization.
Schmitt and Vazquez, 2006 EPSL
Syn-rift sediments, intrusions, vigorous hydrothermal circulation
What are the Thermal & Magmatic Effects of a thick sediment pile?2006)
(Schmitt and Vasquez,
- Warm the lithosphere due to insulation
(Lizzaralde et al., 2007)
?
- Cool the lithosphere due to rapid addition of cold material ?
- Distribute magmatic products (sills) and favor melting ?
- Enhance (S & V 2006) or Inhibit (Liz. 2007) hydrothermal circulation ?
More questions than answers (in this pres.) - great topic, needs work.
Science Highlights of the RCL Initiative
2. Role of Sedimentation: Rift Architecture
Extending Continental Crust
Without Sedimentation:
Boyancy force difference is
large, resists deformation.
Extension migrates, form
new faults, strain is
distributed.
Wide Rift (no
sediment)
(Bialas & Buck,
2009)
With Sedimentation:
Boyancy force difference is
small, promotes
deformation. Extension
continues on active faults,
strain stays localized.
Narrow Rift (with
sediment)
(Bialas and Buck, 2009)
2009
• Buoyancy forces related to sedimentation favor formation of a narrow rift (above).
• Or, depression of isotherms due to thick sediment … might strengthen lithosphere?
• Sediment thermal blanket inhibits hydrothermal circulation → more melt extraction
→ thicker basaltic crust at young ocean spreading centers (Lizarralde et al. 2007)
Science Highlights of the RCL Initiative
2. Role of Sedimentation: Rift Architecture
Salton
Trough
sediment
s
metaseds +
intrusions
0
12
km
1
0
2
0
basaaltic
crust
Salton Trough: novel
crust
(Fuis and Mooney, 1991)
3
0
4
0
… explains seismic refraction data, velocity
structure
0
Increasing seismic velocity (Vp) is
unmetamorphose
typical of sedimentary basin fill.
d
basinal
sediments
45
meta-sedimentary
rock and
intrusions
Fuis et al.,
(1984)
“sub-basement”
= basaltic crust
or
partially
serpentin.
mantle
Depth
(km)
1012
2
(gradual
transition)
Average Vp (5.65 km/s) is too slow for old
crystalline rock. “Basement” is
composed of metaseds & intrusions.
(abrupt increase in
Vp)
Faster velocities (7.5-8.0 km/s): could be
basaltic crust (Fuis et al., 1984) or
partially serp. mantle (Nicolas, 1985).
Science Highlights of the RCL Initiative
2. Role of Sedimentation
SSIP: Salton Seismic Imaging Project
(Virginia Tech, Caltech, USGS)
Coordinated Data Collection and
Analysis:
- onshore seismic refraction &
reflection
- offshore seismic refraction &
reflection
- onshore broadband teleseismic
Map of the Salton Trough region showing
topography, faults, locations of active-source shots,
receivers, etc. (Images courtesy of Liang Han and
John Hole, VA Tech)
Results support hypothesis
of Fuis et al. (1984):
• Metasedimentary rock to
depths of 10-12 km
• Sediment mostly derived
from Colorado River
• New (recycled) crust
formed in past 5-6 m.y.
Science Highlights of the RCL Initiative
2. Role of Sedimentation: Rift Architecture & Crustal Recycling
Colorado River → Salton Trough and Northern Gulf of California
(Dorsey, 2010)
• Erosion of large area on Colorado Plateau
• Transfer sediment via large river into deep
basins at active oblique-rift margin
• Sediment rapidly converted to new crust by
burial and heating in deep basins
• Processes linked by rifting and rupture of
(Dorsey, 2010)
lithosphere at transtensional plate boundary
Science Highlights of the RCL Initiative
2. Role of Sedimentation
New Type of Crust at Ocean-Continent Transition (OCT)
Text-Book Image of a Rifted
Continental Margin. Generic.
Newer studies show that there are
different types of rifted margins, each
with unique O.-C. Transition …
O.C.T
.
Popular “END MEMBERS”
1. Non-Volcanic Margins
(Hyper-Extended):
• Thin, magma-starved crust
• Mantle exhumed to near surface
O.C.T
.
2. Volcanic Rifted Margins:
• Thick mafic crust constructed by
robust syn-rift magmatism.
(Doré
and
Lundin,
2015)
O.C.T
.
• But what about thick crust at nonvolcanic margins?
• and other exceptions …
Science Highlights of the RCL Initiative
2. Role of Sedimentation
New Type of Crust at Ocean-Continent Transition (OCT)
3. Non-Oceanic “New” Crust:
Geometry similar to that of
volcanic margins, but crust is
not volcanic (at some margins).
O.C.T.
Nova Scotia margin (Funck et al., 2004)
Intermediate seismic velocities,
crust is syn-rift sediments, with
mafic magmatic intrusions.
Where does all the sediment
come from? Need large nonlocal input (e.g. Colorado River).
2. Volcanic Rifted Margins:
Thick mafic crust constructed
by robust syn-rift magmatism
(Doré
and
Lundin,
2015)
O.C.T
.
Science Highlights of the RCL Initiative
2. Role of Sedimentation
IberiaNewfoundland:
Magma-Poor,
hyper-extended
N. Gulf of
California:
Non-Oceanic
“New” Crust
NW Europe-East
Greenland, NW
Australia:
MagmaDominated
(Sawyer et al., 2007)
Science Highlights of the RCL Initiative
GAME CHANGERS: New results that change
the way we think about continental rifting,
rupture, and underlying controls
3. Role of Rift Obliquity Important factors:
Relationship between the orientation of the rift
and relative motion direction (° of obliquity)
Science Highlights of the RCL Initiative
3. Role of Rift Obliquity Important factors:
Relationship between the orientation of the rift
and relative motion direction (° of obliquity)
Science Highlights of the RCL Initiative
GAME CHANGERS: New results that change
the way we think about continental rifting,
rupture, and underlying controls
4. Role of Magmatism
Pre-rift volcanism depletes the upper mantle
- leads to less syn-rift magmatism
Less magma makes lithosphere effectively
stronger, so deformation migrates (not localized).
This produces a WIDE RIFT zone and longer
time to rupture.
Science Highlights of the RCL Initiative
4. Role of Magmatism
Premise: Magma in the crust or upper
mantle greatly weakens the lithosphere.
• Viscous properties of the mantle are
sensitive to very small melt fractions.
• Even 1% melt can cause dramatic reduction
in effective viscosity (strength).
• Small melt fractions in mantle lithosphere
may lead to weakening & strain localization.
Takei & Holtzman (2009,
JGR)
• Very small volumes of magma intruded
during rifting can cause extension of otherwise strong, thick continental lithosphere.
(Behn and Ito, 2008; Qin and Buck, 2008)
Take-Home: Presence or absence of melt
(crust or upper mantle) exerts a firstorder control on rock strength, strain
localization, and rift architecture.
(Behn and Ito, 2008)
Science Highlights of the RCL Initiative
4. Role of Magmatism
http://serc.carleton.edu
(Ferraro et al., 2007)
http://serc.carleton.edu
• Ignimbrite pulses (Oligocene - Miocene) related to removal of the Farallon plate from the
base of the North American plate after the end of the Laramide orogeny.
• Rapid increase in subduction angle due to slab roll-back drove extension and magmatism,
eventually leading to direct interaction between the Pacific and North American plates.
(Ferrari et al., 2007, GSA Special Paper 422)
Science Highlights of the RCL Initiative
4. Role of Magmatism
Pre-Rift Magmatism controls magma supply and rift width, abrupt variations between adjacent rift segments:
• Narrow Rift Segments magmatically robust, thicker mafic crust: inferred to overlie fertile undepleted mantle
• Wide Rift Segment minor syn-rift magmatism: mantle dehydration & chem. depletion due to pre-rift volcanism
NORTH
Question: Is this a complete explanation?
Wide-angle & multi-channel seismic
data (Lizarralde et al. 2007, Nature).
Crustal structure across 3 rift segments:
Abrupt variability, unexpected.
Gonzalez-Fernandez et al., 2005 JGR
G
G
(narrrow rift)
A
A
(wide rift)
C
C
(narrow rift)
(Lizarralde et al. 2007)
(Lizarralde et al. 2007)
SOUTH
Science Highlights of the RCL Initiative
4. Role of Magmatism
Previous observations and assumptions:
•
The width of rifts, narrow vs. wide, and the amount of magmatism (from almost none to 2–3 x the
predicted amount) are thought to be controlled by:
–
–
–
–
–
•
Some models suggest extension rates are the most important factor determining rift geometry:
–
–
•
•
•
For example, narrow rifts may result when “extension rates outpace thermal diffusion” and stretching
and necking occurs (Lizarralde et al., 2007, after England, 1983)
Wide rifts may form when extension rates are slow, allowing cooling of lithosphere which thus maintains
strength and deformation is spread out, thus preventing necking (Lizarralde et al., 2007; Hopper and
Buck, 1996)
Other models suggest that crustal thickness and heat flow are more important in controlling rift
geometry:
–
•
Extension rate
The thickness of the crust or lithosphere
Heat flow
Lower crustal flow
The potential temperature of the mantle
Wide rifts may result from warm thin lithosphere, i.e., are a function of crustal thickness and heat flow
(Hopper and Buck, 1996 and references therein)
The temperature of the mantle is thought to control the amount of magmatism present during
rifting
In all of these models, the predicted controlling factors would all operate over large areas and thus
all rift segments in a region would behave similarly
Not the case in the southern Gulf of California!
Science Highlights of the RCL Initiative
4. Role of Magmatism
Sutherland et al., 2012…
• Suggest that a tear in the subducting slab between the north and
south GOC, just north of the Alarcón segment, may be responsible
for the differences in extensional styles
• Note that low-angle detachment faulting/ductile deformation is
found in the north (Upper Tiburón) whereas the southern rift
segments are symmetric and display brittle deformation
• Suggest the tear in the slab created different thermal regimes
north and south of the tear which changed the strength of the
lithosphere
Science Highlights of the RCL Initiative
4. Role of Magmatism: Upper Mantle Structure
Variations in upper mantle seismic velocity (Vp and Vs) correspond to surface expression of volcanism
Large-scale model for the Gulf of California region based on receiver functions and surface wave inversions.
Triangles are the NARS-Baja stations and the green circles are locations of receiver functions.
• Panels reveal pronounced low-velocity anomalies associated with major centers of seafloor spreading.
• In South: Asthenosphere anomaly is sharp and well defined, associated with the spreading center
• In North: Asthenosphere upwelling is diffuse, two possible explanations: (a) the spreading process has
been altered by large sediment load from the Colorado River, or (b) extension is caused by stretching.
Source: DiLuccio et al. (2005); Clayton et al. (2006); Persaud et al. (2007).
Science Highlights of the RCL Initiative
4. Role of Magmatism: Upper Mantle Structure
Wang et al. (2009)
Interpretation of anomalous mantle velocities along profile AB.
• The most prominent anomalies are the low-velocity
anomalies centered at depths of 60–70 km.
• Low-velocity anomalies are interpreted as centers of
enhanced melt concentration and upwelling.
Shear velocity anomalies at 50-90 depth. Negative anomalies
are slow. Contour interval is 0.5%. (Wang et al., 2009).
• Melting begins at ~160 km in the presence of a small
amount of water, leading to low S velocities.
• In the northern Gulf, low-velocity anomalies are centered slightly to the west of the plate boundary.
• This suggests a dynamic component of upwelling that keeps melt production centered beneath the
original location of rifting even as the plate boundary migrates to the east …
• or possibly that a remnant slab is missing from the upper mantle in the north (Wang et al., 2009,
Science Highlights of the RCL Initiative
5. Structural Evolution: new ideas regarding
the development of the Gulf of California
Science Highlights of the RCL Initiative
5. Structural Evolution: Pre-rift Tectonics
•
•
•
•
•
Subduction and magmatism followed by oblique transform deformation
Prior Work: plate reconstructions for past 40 m.y. (Atwater and Stock, 1998)
Paleo-East Pacific Rise entered subduction zone, initiated transform margin
Baja micro-plate “captured” by Pacific plate, now moving NW relative to North Am.
… Gulf of California opened by oblique extension along former (Miocene) volcanic arc
38 Ma
30 Ma
20 Ma
10 Ma
6 Ma
0 Ma
T. Atwater movies: http://emvc.geol.ucsb.edu/1_DownloadPage/Download_Page.html
Gulf of California
Science Highlights of the RCL Initiative
5. Structural Evolution
• Previous interpretations:
– Pre-Gulf Stage: Subduction, Arc Volcanism and Backarc east-west (ENE-WSW) extension present up to 12
Ma
– Proto-Gulf Stage: 12-6 Ma change to Pacific-North
American dextral transform boundary
• Transform faults west of Baja
• Proto-Gulf east of Baja with continued ENE-WSW extension
– Gulf of CA stage: 6-present
• P-NM boundary shifts to Gulf around 6, Baja mostly coupled
to Pacific plate
• Gulf opens via a series of oblique transtensional faults,
transform faults and normal faults
Science Highlights of the RCL Initiative
5. Structural Evolution
New Interpretations from MARGINS:
revising the Proto-Gulf Stage:
• Some dextral slip west of Baja microplate and
some east during this stage, 12-6 Ma
• Southern GOC : transtension began around
12 Ma (Sutherland et al., 2012)
• Northern GOC: dextral shear is recorded
onshore in mainland Mexico, east of and prior
to the opening of the northern GOC and may
have started as early as 11.5 Ma but certainly
by 8 Ma (Bennett et al., 2013)
Science Highlights of the RCL Initiative
5. Structural Evolution
Pre-Gulf
stage
(Bennett et al.,
2013)
Proto-Gulf
stage
Science Highlights of the RCL Initiative
5. Structural Evolution: Syn-rift Tectonics
Proto-Gulf
stage
12.5-6 Ma
(Bennett et al.,
2013)
Modern
Gulf
6-0 Ma
Science Highlights of the RCL Initiative
5. Structural Evolution: Microplate Coupling
A key question for rifting studies is: How and where (and why) does strain localize
as rifting progresses to plate rupture? Geodetic studies suggest that microplate
coupling is a primary driving force for rifting in the Gulf of California. (Plattner et al., 2009)
Finite Element Model Set-up
• Stable Baja California microplate moves
primarily (~96%) with PAC motion, but it
nevertheless moves independently.
• Thus, Baja California is partially coupled to
the Pacific plate (Plattner et al., 2007, 2009).
• Pacific plate “drags” the Baja microplate
to the NW, opening Gulf of California rift.
High interplate coupling (frictional tectonic stresses) can reproduce
observed kinematics of the Baja California microplate as seen from
geodetic rigid-plate motions. Plattner et al. (2009 Geology).
AND … Relative Plate Motion is highly oblique.
This is an important yet often overlooked point.
Science Highlights of the RCL Initiative
5. Structural Evolution: Oblique Extension & Strike-Slip Faults
Gulf of Cal. dominantly a transform plate boundary
Force (TN/m)
(Brune et al., 2012)
(Dorsey & Umhoefer, 2012)
Time (Ma)
Thermomechanical models
showing strain rate histories for
orthogonal and oblique rifts
(Brune et al., 2012).
The results show that oblique
rifts are significantly weaker.
• Using a simple analytic mechanical model and numerical, thermomechanical modeling techniques,
Brune et al (2012) found that oblique extension significantly facilitates the rift process … because oblique deformation requires
less force to reach the plastic yield limit than rift-perpendicular extension.
• “The model shows that in the case of two competing non-magmatic rifts, with one perpendicular and one oblique to the direction
of extension but otherwise having identical properties, the oblique rift zone is mechanically preferred and thus attracts more
strain”. (Brune et al., 2012 JGR)
Science Highlights of the RCL Initiative
5. Structural Evolution: Oblique Extension & Strike-Slip Faults
Model predictions are supported by recent field
studies in northern Gulf of Calif. & coastal Sonora
Tectonic model for coastal Sonora, late Miocene
time (Darin et al., 2010; Bennett et al., in press
Bull.)
▪ GSA
Older normal faults (black) accommodated largemagnitude NE-SW extension from ~10-6 Ma.
▪ At ca. 7 Ma newly initiated and/or reactivated faults (red)
localized dextral strain into a series of en-echelon, rightstepping strike-slip faults.
▪ Initiation of strong dextral shear at ~7 Ma played an
(Darrin et al., 2010)
important role in localization of strain and onset of
oblique rifting in the northern Gulf of California.