1. Becky Dorsey - University of Oregon
Crustal Recycling Along an Oblique-Divergent Plate Boundary: From the Colorado Plateau to the Salton Trough and Gulf of California
Becky Dorsey - University of Oregon
NSF MARGINS Program
Rupturing Continental Lith (RCL)
Source to Sink (S2S)
Today I’d like to talk about rapid input of sediment to structurally active basins along the oblique-rift plate boundary in the Salton Trough and northern Gulf of California. I’ve drawn a few lines on this satellite image to represent the system of linked transform and normal faults that are acting to accommodate oblique-divergent plate motion and create a network of transtensional stepover or pull-apart basins that are opening up along the plate boundary. The Colorado River enters the basin here at Yuma, and (click) has constructed a sizeable delta with distributariy channels that flow to the NW and the SE. Over time (click) the voluminous input of sediment from the Colorado River has filled and overfilled these basins, and along with the dextral motion on the SAF has forced progradatino of the Colorado R. delts to the SE into the northern Gul of Cal. The sediment is rapidly buried, heated and metamorphosed to create new transitional crust that exerts a strong control on crustal rheology, magmatism, rift architecture, and deformation style. So this represents a major, regional-scale crustal recylcing system. In this talk I’ll present a new estimate for the volume of sediment in the basins, and use that to estimate the rate of crustal growth and explore the implications for this and other rifted margins.
Gulf of California and Salton Trough:
Growth of an active oblique-rift boundary:
Basinal response to deformation
Erosion, transport, deposition
Role of Sedimentation in crustal evolution (not just a passive recorder of E. history)
Salton Trough
Gulf of Cal. 1

2. COLLABORATORS Gary Axen Amy Spears Susanne Janecke Bernie Housen
(N.M. Tech)
COLLABORATORS
Amy Spears
(WWU)
Susanne Janecke
(Utah State Univ.)
Bernie Housen
(WWU)
Mike Oskin
(U.C. Davis)
Molly Keogh
(U. Oregon)
Tom Peryam
(U. Oregon)
Kim Le
(U.C. Davs) 2

3. Nova Scotia margin (Wu et al., 2006)
Influence of Sediments on Crustal Architecture and Evolution
Build Transitional Crust (O.C.T.) at rifted margins (Fuis et al., 1984; Nicolas, 1985; Wu et al., 2006)
Thermal Effect of thick sediment:
- Warm the lithosphere due to insulation (Lizzaradle et al., 2007) ?
- Cool the lithosphere due to addition of cold seds ?
Nova Scotia margin (Wu et al., 2006)
Bialas and Buck (2009)
Sediments are one of the main recorders of tectonic events, but they may also affect the way compression or extension proceeds. It is now accepted that the unloading effect of erosion can change the pattern of compressional deformation in areas of continental convergence … In a similar way, we suggest that the load of sediments may promote localized deformation in areas of continental extension (Bialas and Buck, 2009).
Extension
with
Sedimentation
without
Wide Rift Mode
Narrow Rift Mode
Rift Architecture: sediment load promotes early transition to narrow rift mode (Bialas and Buck, 2009).
The weight of sediments reduces the difference in crustal buoyancy forces between adjacent blocks, allowing strain to localize.
Sediment 3

4. Gulf of California and Salton Trough
Today’s Talk:
* Brief Overview of Gulf-Trough Region
Exposed Late Cenozoic Section, Western Salton Trough: + Initiation (?) and growth of Pacific - North Am. plate boundary + First arrival of Colorado River sediment in tectonic lowland + Colorado River delta progradation and basin filling
Regional Sediment Budget, Mass Balance: Estimate volume of sediment in subsurface basins + Compare to volume eroded from Colorado Plateau + Explore implications for crustal growth and recycling 4

5. Inversion of Topography … prob. low by about 8-10 Ma. ?
~ 120 km E-W extension in B&R ~ 16 to 10 Ma (Wernicke and Snow, 1998; Fitzgerald et al., 2009; Colgan and Henry, 2009).
Inversion of Topography … prob. low by about 8-10 Ma.
topography
remnant ?
Colorado River drainage integrated at 6 Ma.
(Spencer et al., 2001; House et al., 2005), Entered Salton Trough ~5.3 Ma (Dorsey et al., 2007)
Colorado R.
36 Ma
20 Ma
6 Ma
0 Ma
10 Ma
McQuarrie and Wernicke (2005) 5

6. 2. Regional Integrated Strain Gans (1997), Fletcher et al. (2007)
12.3 Ma
12.3 to 6 Ma
6 to 0 Ma
TWO CONTRASTING MODELS for kinematic evolution of the Pac. - North America plate boundary: 12 to 6 Ma.
Baja Calif.
1. Regional Strain Partitioning Stock and Hodges (1989) Oskin and Stock (2003)
Late Miocene
12.3 Ma Ma
6 to 0 Ma
2. Regional Integrated Strain Gans (1997), Fletcher et al. (2007)
Baja Calif.
Models agree on past 6 m.y.
Fletcher et al. (2007) GSAB 6

7. Dorsey and Umhoefer (in press)
Basins of the Gulf of California & Salton Trough
Northern Gulf
Water depth: ≤200 m
Sed Thickness: 8-12 km accum. in past 6-8 Myr.
Total Crust: km
No true Ocean Crust
Central Gulf
Water depth: ~1,600 m
Sed Thickness ~3 km
Total Crust: 9-10 km
Ocean Crust since ~3 Ma
Southern Gulf
Water depth: ~2,500 m
Sed Thickness ~1 km
Total Crust: ~6 km
Lizarralde et al. (2007), and others
past 5-6 Myr
Today I’d like to talk about rapid input of sediment to structurally active basins along the oblique-rift plate boundary in the Salton Trough and northern Gulf of California. I’ve drawn a few lines on this satellite image to represent the system of linked transform and normal faults that are acting to accommodate oblique-divergent plate motion and create a network of transtensional stepover or pull-apart basins that are opening up along the plate boundary. The Colorado River enters the basin here at Yuma, and (click) has constructed a sizeable delta with distributariy channels that flow to the NW and the SE. Over time (click) the voluminous input of sediment from the Colorado River has filled and overfilled these basins, and along with the dextral motion on the SAF has forced progradatino of the Colorado R. delts to the SE into the northern Gul of Cal. The sediment is rapidly buried, heated and metamorphosed to create new transitional crust that exerts a strong control on crustal rheology, magmatism, rift architecture, and deformation style. So this represents a major, regional-scale crustal recylcing system. In this talk I’ll present a new estimate for the volume of sediment in the basins, and use that to estimate the rate of crustal growth and explore the implications for this and other rifted margins.
Dorsey and Umhoefer (in press) 7

8. 1. Late Cenozoic Basin Development, Western Salton Trough
Southern San Andreas Fault System
scec
Taking a closer look, in this map we can see the linked strike-slip and oblique normal faults that opening up the transtensional basins along the plate boundary. In the western Salton Trough, recent deformation has caused the older basins to be uplifted and eroded, for example here (click) in the Fish Creek Vallecito basin …
M 7.2 Earthquake
(April 4, 2010)

9. Fish Creek - Vallecito Basin
mod. from Winker (1987); Axen and Fletcher (1998) 9

10. Fish Creek - Vallecito Basin, western Salton Trough
Earliest Marine Turbidites (6.3 Ma)
Fish Creek - Vallecito Basin, western Salton Trough
Top of Sxn ~0.95 Ma
WSDF
2.65 and 2.60 Ma
Large Rock Avalanche
Alluvial-Fan Conglomerate
… we see a well exposed, thick section of late Miocene to early Pleistocene deposits that accumulated in the hangingwall of the west Salton detachment fault. The section provides a well dated record of the earliest arrival of Colorado River sand in the salton Trough at 5.3 m.y. ago. That was followed by a change from deposition of marine turbidites of the Latrania Formation (click) to fluvial sandstone and mudstone of the A. Diablo Fm, which records progradation of the progradation of the Colorado River delta.
Base of Marine ~ 6.3 Ma
Active Rift Basin, Steep Local Topography
Base of Sxn ~ 8.0 ± 0.4 Ma 10

11. Initiation of paleo-San Andreas fault at ~7-8 Ma?
Present Day
7-8 Ma 11

12. A. Diablo Fm., Palm Spring Gp.
Top of Sxn ~0.95 Ma
A. Diablo Fm., Palm Spring Gp.
2.65 and 2.60 Ma
marine turbidites, Latrania Fm.
… we see a well exposed, thick section of late Miocene to early Pleistocene deposits that accumulated in the hangingwall of the west Salton detachment fault. The section provides a well dated record of the earliest arrival of Colorado River sand in the salton Trough at 5.3 m.y. ago. That was followed by a change from deposition of marine turbidites of the Latrania Formation (click) to fluvial sandstone and mudstone of the A. Diablo Fm, which records progradation of the progradation of the Colorado River delta.
Oldest Col. R. sand
= 5.3 Ma
Locally - Derived

13. chert volc. qtz qtz metam. biotite plag. Sand Composition C-suite
Subsurface
Herzig et al. (1988)
volc.
C-suite
mixed-
source
L-suite
qtz
qtz
metam.
Colorado River - Derived
Outcrop
biotite
Colorado River sand is easily distinguished from locally-derived sand based on abundance of well rounded quartz sand with syntaxial qtz overgrowths, chert lithic fragments, and Cretacoeus forams reworked from the Colorado Plateau.
plag.
Locally - Derived

14. Delta Prograded during abrupt increase in subsidence rate, fluvial conditions persisted during rapid subsidence.
Requires large increase in sediment flux rate. Seen in other supply-driven delta systems (Goodbred and Kuehl, 2000; Carvajal and Steel, 2006)
~ 4 Ma
Top of Sxn ~0.95 Ma
2.65 and 2.60 Ma
Colorado Delta Progradation
Continuous Fluvial
Base of Sxn ~ 8.0 ± 0.4 Ma 14

15. Colorado R.
Axen (2008)
Linked slip on southern SAF and west Salton detachment fault. Space created by lithospheric rupture is filled with basaltic intrusions from below and voluminous sediment input from above (mainly Colorado River). 15

16. 2. Regional Sediment Budget, Mass Balance
Rapid Sediment Input to active oblique-rift basins during past 5-6 Myr.
Sediment builds new (recycled) crust as it is buried and metamorphosed ...
Regional-scale crustal recycling system.
NASA
satellite view looking SE along the Pac-NAM plate boundary, GoC and S.T.
Calif.
North
America
Gulf of
Pacific
Plate
Next:
Calculate Volume of sediment in basins
Rate of crustal growth
Implications for rift-margin evolution
Trough
Colorado R.
Today I’d like to talk about rapid input of sediment to structurally active basins along the oblique-rift plate boundary in the Salton Trough and northern Gulf of California. I’ve drawn a few lines on this satellite image to represent the system of linked transform and normal faults that are acting to accommodate oblique-divergent plate motion and create a network of transtensional stepover or pull-apart basins that are opening up along the plate boundary. The Colorado River enters the basin here at Yuma, and (click) has constructed a sizeable delta with distributary channels that flow to the NW and the SE. Over time (click) the voluminous input of sediment from the Colorado River has filled and overfilled these basins, and along with the dextral motion on the SAF has forced progradatino of the Colorado R. delta to the SE into the northern Gul of Cal. The sediment is rapidly buried, heated and metamorphosed to create new transitional crust that exerts a strong control on crustal rheology, magmatism, rift architecture, and deformation style. So this represents a major, regional-scale crustal recylcing system. In this talk I’ll present a new estimate for the volume of sediment in the basins, and use that to estimate the rate of crustal growth and explore the implications for this and other rifted margins.
Salton

17. SOURCE: Colorado River
Catchment Area: 630,000 km2 4th largest in conterminous U.S.; ~10-15 times the area of the sink.
Dissolved Load (TDS): ~ 400 ppm (early 1900’s) ~ 800 ppm (modern)
Sediment Discharge: x 108 t/yr (pre-dam) ~ 1.0 x 105 t/yr (modern) (Meade and Parker, 1985)
SINK: Basins in Salton Trough and northern Gulf of California
Opened by oblique divergence along plate boundary since ~6-8 Ma.
Colorado River sediment arrived in Salton Trough at ~5.3 Ma ...
has dominated basin fill since then.
Rapid subsidence and sediment accumulation (~2-3 mm/yr)
High heat flow: greenschist facies metam. (~300°) at 2-4 km depth.
Looking at it now with north up, the Colorado River has a large catchment area, 4th largest in the conterminous U.S., that is about times larger than the area of basins receiving the sediment. The river had a very large pre-historic sediment discharge, x 10^8 t/yr, that was reduced by about 3 orders of magnitude as a resulty of dam construction. Sedimentary basins in the S.T. and northern Gulf have formed by oblique divergence since ~6-8 m.y. ago … read the rest.

18. Estimate VOLUME of Colorado River - derived sediment in subsurface basins …
So we want to use this information to estimate the volume of …

19. Estimate VOLUME of Colorado River - derived sediment in subsurface basins …
DATA: recent seismic studies, information about basin depth and crustal structure.

20. Estimate VOLUME of Colorado River - derived sediment in subsurface basins …
DATA: recent seismic studies, information about basin depth and crustal structure.
AREA of 6 main depocenters; multiply by depth …
UNCERTAINTIES:
total basin depth (requires crustal model)
sed. composition and age
volume of intrusions at depth

21. Shallower, unmetamorphosed basins: well imaged in sesimic-reflection studies
Altar Basin
Pacheco et al. (2006)
Tiburon Basin
Aragon & Martin (2007)

22. For deeper basins, use crustal model of Fuis et al
For deeper basins, use crustal model of Fuis et al. (1984) – Salton Trough: Lithosphere is fully ruptured: Unmetamorphosed seds are 4-5 km deep; Basement = [metaseds + mafic intrusions]
Salton Trough
sediments
metaseds + intrusions 10
12 km 20
basaaltic crust 30
Fuis and Mooney (1991) 40
… explains seismic refraction data, velocity structure
Increasing seismic velocity (Vp) is typical of sedimentary basin fill.
unmetamorphosed
basinal sediments
10-12 km in 5.3 m.y. requires accum. rate of mm/yr … consistent with measured rates (Van Andel, 1964; Herzig et al., 1988; Dorsey et al., in press).
4-5
(gradual transition)
For the deeper parts of the basins that cannot be imaged with seismic reflection, I use the crustal model of Fuis et al. (1984). According to their model, unmetamorphosed sediments fill the basins down to about 4-5 km depth, and the deeper basement consists of metasedim. rocks and mafic intrusions that are filling the space created by lithospheric rupture and separation of the Pacific and North American plates across the plate boundary. This model was developed to explain observed seismic refraction data that show a gradual increase in seismic velocites with no abrupt increase at the sediment-basement contact. The average velocites in the basement layer are too slow for pre-Cenozoic crystalline rock but fit well with velocities predicted for a mix of metasedim. rocks and mafic intrusions.
Average Vp (5.65 km/s) is too slow for old crystalline rock ( km/s).
Consistent w/ metaseds & intrusions.
meta-sedimentary
rock and intrusions
10-12
(abrupt increase in Vp)
“sub-basement”
= basaltic crust or
partially serpentin.
mantle
Faster velocities ( km/s) could be basaltic crust (Fuis et al., 1984) or partially serp. mantle (Nicolas, 1985).
Depth (km)
Fuis et al., (1984) 20

23. For deeper basins, use crustal model of Fuis et al
For deeper basins, use crustal model of Fuis et al. (1984) – Salton Trough: Lithosphere is fully ruptured: Unmetamorphosed seds are 4-5 km deep; Basement = [metaseds + mafic intrusions]
Salton Trough
sediments
metaseds + intrusions 10
12 km 20
basaaltic crust 30
Fuis and Mooney (1991) 40
Northern Gulf of California
Delfin Basin
Tiburon Basin
sediments
sediments
5 km
metasedim. rx + intrusions
10 km
lower crust
Gonzalez et al. (2005)

24. RESULT: Total volume of Colorado R. sediment in subsurface basins
BRACKET VARIABLE PARAMETERS:
Salton Trough Northern Gulf
Depth to base of unmetamorphosed sediments km km
Depth to base of metased. rocks and intrusions km km
Volume % of intrusions in metasedim. rocks % %
Thickness of non-C.R. seds at base of section m m
Depth x Area: min. and max. volumes for the 6 basinal domains …
RESULT: Total volume of Colorado R. sediment in subsurface basins
is ~ x 105 km3

25. Compare to volume of rock eroded from Colorado River catchment (two estimates):
(1) Spatially averaged erosion on Plateau (Pederson et al., 2002), corrected for ratio of pre- to post-6 Ma erosion (Flowers et al., 2008), plus modest inputs from the Virgin and Gila rivers: ~ 2.0 x 105 km3.
(2) Multiply pre-dam sediment discharge ( x 108 t/yr; Meade and Parker, 1985) by time since first arrival of C.R. sand in the Salton Trough (5.3 Ma), and simple density conversion: x 105 km3.
* Preliminary, needs more work *
Volume of sediment stored in plate-boundary basins (~ x 105 km3)
is roughly equal to volume of rock eroded from Colorado R. in past 5-6 m.y.

26. Rate of Crustal Growth:
1. Input of sediment to plate-boundary basins
= volume / time / distance along strike
= x 105 km3 / 5.3 m.y. / 500 km along strike
= ~ km3 / m.y. / km
2. Magmatic accretion at seafloor spreading centers:
= km3 / m.y. / km (slow and v. slow spreading rates)
= km3 / m.y. / km (medium to fast spreading rates)
3. Magmatic accretion at island arcs:
= km3 / m.y. / km (Philippines)
= km3 / m.y. / km (other west Pacific arcs)
= km3 / m.y. / km (Izu-Bonin arc)

27. Lithospheric Rupture, Sedimentation, and Crustal Recycling

28. Salton Trough and Northern Gulf of California
sediments
metaseds + intrusions
basaaltic crust
Fuis and Mooney (1991)
Salton Trough and Northern Gulf of California
Insights into crustal evolution and structure at ancient rifted margins.
Nova Scotia margin (Wu et al., 2006)
“Novel type of crust”: rifting and basin filling (Moore, 1973; Fuis et al., 1984; Nicolas, 1985).
Surface Processes: Important mechanism of crustal growth and recycling … similar in scale and rate to magmatic accretion at subduction zones and slow spreading centers.
May be important at other rift and oblique-rift margins where large continental drainage is captured following tectonic collapse and subsidence of a pre-existing orogenic highland.