Rupturing Continental Lithosphere Understand spatial and temporal evolution of rifts. Focus on key processes, state parameters and physical properties

Rupturing Continental Lithosphere

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)

Original Goals: Modified from a summary by Rebecca Dorsey, University of Oregon

Science Highlights of the RCL Initiative

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 ?

Original Scientific Questions

Rupturing Continental Lithosphere


Modified from a summary by Rebecca Dorsey, University of Oregon

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 strike-slip 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.

GAME CHANGERS: New results that change the way we think about continental rifting, rupture, and underlying controls


Revised: New results also lead to new 5. Structural Evolution models for the Gulf of California


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 Initiative1. Styles of Extension

(a) North to south changes

NORTH: Widely distributed deformation, thick layer of new sediments, no central rift zone

SOUTH: localized deformation with central rift zone, thinner layer of new sediments, volcanism

(Gonzalez-Fernandez et al., 2005)

(Lizarralde et al., 2007)

(Dorsey and Umhoefer, 2012)

(b) Low angle detachment to high angle distributed faulting


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




Low angle detachment faulting in the Tiburon Basin





• 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



• ~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





1. Styles of ExtensionHigh-angle faulting and focused rifting in the Upper Delfín Basin


(Marin-Barajas et al., 2013)

(c)East to west migration of plate boundary deformation



Westward migration of extension in thenorthern Gulf of California

Rifting formed transtensional pull-apart basins in the EAST first



(Aragon-Arreola and Martin-Barajas, 2007)

Westward migration of extension in thenorthern Gulf of California

~2-3 Ma, the locus of rifting shifted west, forming new transtensional pull-apart basins




Location of active (dark gray) and inactive (light gray) pull-apart basins




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





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 Initiative2. Role of Sedimentation

Can create new type of hybrid crust at Ocean – Continent Transition.

What are the Thermal & Magmatic Effects of a thick sediment pile?

- 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.

Schmitt and Vazquez, 2006 EPSL

Syn-rift sediments, intrusions, vigorous hydrothermal circulation

Salton Trough

Quaternary rhyolites produced by episodic remelting of altered basalts, not fractional crystallization.

Science Highlights of the RCL Initiative2. Role of Sedimentation: Hydrothermal Activity & Magmatism

(Schmitt and Vasquez, 2006)

• 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)

Extending Continental Crust

Without Sedimentation:Boyancy force difference is large, resists deformation. Extension migrates, form new faults, strain is distributed.

Wide Rift (no sediment)

Narrow Rift (with sediment)

With Sedimentation:Boyancy force difference is small, promotes deformation. Extension continues on active faults, strain stays localized.

(Bialas and Buck, 2009) 2009

(Bialas & Buck, 2009)

Science Highlights of the RCL Initiative2. Role of Sedimentation: Rift Architecture

Salton Trough

Fuis et al., (1984)

… explains seismic refraction data, velocity structure

unmetamorphosed

basinal sediments

meta-sedimentary

rock and intrusions

“sub-basement”= basaltic

crustor

partially serpentin.

mantle

4-5

10-12

Dep

th

(km

)

0

20

Increasing seismic velocity (Vp) is typical of sedimentary basin fill.

Average Vp (5.65 km/s) is too slow for old crystalline rock. “Basement” is composed of metaseds & intrusions.

Faster velocities (7.5-8.0 km/s): could be basaltic crust (Fuis et al., 1984) or partially serp. mantle (Nicolas, 1985).

sediments

metaseds + intrusions

basaaltic crust

(gradual transition)

(abrupt increase in Vp)

Salton Trough: novel crust(Fuis and Mooney, 1991)

10

0

20

30

40

12 km

Science Highlights of the RCL Initiative2. Role of Sedimentation: Rift Architecture

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.


• 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 lithosphere at transtensional plate boundary

(Dorsey, 2010)

(Dorsey, 2010)

Colorado River → Salton Trough and Northern Gulf of California

Science Highlights of the RCL Initiative2. Role of Sedimentation: Rift Architecture & Crustal Recycling

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 …

1. Non-Volcanic Margins (Hyper-Extended): • Thin, magma-starved crust

• Mantle exhumed to near surface

Popular “END MEMBERS”O.C.T.

2. Volcanic Rifted Margins:• Thick mafic crust constructed by

robust syn-rift magmatism.

• But what about thick crust at non-volcanic margins?

• and other exceptions …


(Doré and Lundin, 2015)

O.C.T.

O.C.T.

3. Non-Oceanic “New” Crust: Geometry similar to that of volcanic margins, but crust isnot volcanic (at some margins).

Intermediate seismic velocities, crust is syn-rift sediments, with mafic magmatic intrusions.

Where does all the sediment come from? Need large non-local input (e.g. Colorado River).

Nova Scotia margin (Funck et al., 2004)

O.C.T.

2. Volcanic Rifted Margins: Thick mafic crust constructed by robust syn-rift magmatism

New Type of Crust at Ocean-Continent Transition (OCT)


(Doré and Lundin, 2015)

O.C.T.

Iberia-Newfoundland:Magma-Poor,

hyper-extended

NW Europe-East Greenland, NW

Australia:Magma-

Dominated

N. Gulf of California:

Non-Oceanic “New” Crust

(Sawyer et al., 2007)



3. Role of Rift Obliquity Important factors:


Relationship between the orientation of the rift and relative motion direction (° of obliquity)


3. Role of Rift Obliquity Important factors:

Relationship between the orientation of the rift and relative motion direction (° of obliquity)


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.

• 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.

• Very small volumes of magma intruded during rifting can cause extension of other-wise strong, thick continental lithosphere.

(Behn and Ito, 2008; Qin and Buck, 2008)

Takei & Holtzman (2009, JGR)

(Behn and Ito, 2008)

Take-Home: Presence or absence of melt (crust or upper mantle) exerts a first-order control on rock strength, strain localization, and rift architecture.

Premise: Magma in the crust or upper mantle greatly weakens the lithosphere.

Science Highlights of the RCL Initiative4. Role of Magmatism

• 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)

(Ferraro et al., 2007) http://serc.carleton.edu

http://serc.carleton.edu


Wide-angle & multi-channel seismic data (Lizarralde et al. 2007, Nature). Crustal structure across 3 rift segments: Abrupt variability, unexpected.

Guaymas

AlarconCabo - PV

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

(wide rift)

(narrrow rift)

(narrow rift)

(Lizarralde et al. 2007)(Lizarralde et al. 2007)

Question: Is this a complete explanation?

Gonzalez-Fernandez et al., 2005 JGR

G

A

C

G

A

C

NORTH


SOUTH

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:– Extension rate– The thickness of the crust or lithosphere– Heat flow– Lower crustal flow– The potential temperature of the mantle

• 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:

– 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!


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


Variations in upper mantle seismic velocity (Vp and Vs) correspond to surface expression of volcanism

• 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.

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.

Source: DiLuccio et al. (2005); Clayton et al. (2006); Persaud et al. (2007).

Science Highlights of the RCL Initiative4. Role of Magmatism: Upper Mantle Structure

• 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, nature).

Shear velocity anomalies at 50-90 depth. Negative anomalies are slow. Contour interval is 0.5%. (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.

• Melting begins at ~160 km in the presence of a small amount of water, leading to low S velocities.

Wang et al. (2009)

Science Highlights of the RCL Initiative4. Role of Magmatism: Upper Mantle Structure


5. Structural Evolution: new ideas regarding the development of the Gulf of California

• 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

Gulf of CaliforniaT. Atwater movies: http://emvc.geol.ucsb.edu/1_DownloadPage/Download_Page.html

Science Highlights of the RCL Initiative5. Structural Evolution: Pre-rift Tectonics

• Previous interpretations:– Pre-Gulf Stage: Subduction, Arc Volcanism and Back-

arc 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 Initiative5. 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)


Pre-Gulf stage

Proto-Gulf stage


(Bennett et al., 2013)

Proto-Gulf stage

12.5-6 Ma

ModernGulf

6-0 Ma

Science Highlights of the RCL Initiative5. Structural Evolution: Syn-rift Tectonics

(Bennett et al., 2013)

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.

• 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.

Finite Element Model Set-up

Science Highlights of the RCL Initiative5. Structural Evolution: Microplate Coupling

(Plattner et al., 2009)

• 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)

(Dorsey & Umhoefer, 2012)

Gulf of Cal. dominantly a transform plate boundary

Forc

e (T

N/m

)

Time (Ma)

(Brune et al., 2012)

Thermomechanical models showing strain rate histories for orthogonal and oblique rifts (Brune et al., 2012). The results show that oblique rifts are significantly weaker.

Science Highlights of the RCL Initiative5. 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 GSA Bull.)▪ Older normal faults (black) accommodated large-

magnitude 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, right-stepping strike-slip faults.

▪ Initiation of strong dextral shear at ~7 Ma played an important role in localization of strain and onset of oblique rifting in the northern Gulf of California.

Science Highlights of the RCL Initiative5. Structural Evolution: Oblique Extension & Strike-Slip Faults

(Darrin et al., 2010)

Documents

Rupturing Continental Lithosphere Understand spatial and temporal evolution of rifts. Focus on key processes, state parameters and physical properties