SITING OF LARGE VOLCANIC CENTERS AT RELEASING FAULT STEPOVERS, WALKER LANE RIFT with emphasis on the Central Sierra Nevada Area, to complement field guide for daytrip to Sonora Pass Pdf version of powerpoint lecture prepared for the National Association of Geoscience Teachers Far Western Section Fall Conference High Sierra Institute, Baker Station, Sierra Nevada September 7 - 9, 2012 by Cathy Busby, Department of Earth Science, University of California at Santa Barbara 93106 Supported by NSF EAR 01252, 0711276, and 0711181 and USGS EDMAP 06HQA061 and 09HQPA0004 THIS PDF IS POSTED AT: http://www.geol. ucsb .edu/faculty/busby/NAGT2012 Pdfs of Busby’s papers cited herein are posted at: http://www.geol.ucsb.edu/faculty/busby/publications 1
Transcript
SITING OF LARGE VOLCANIC CENTERS ATRELEASING FAULT STEPOVERS,
WALKER LANE RIFTwith emphasis on the Central Sierra Nevada Area,
to complement field guide for daytrip to Sonora Pass
Pdf version of powerpoint lecture prepared for the NationalAssociation of Geoscience TeachersFar Western Section Fall Conference
High Sierra Institute, Baker Station, Sierra NevadaSeptember 7 - 9, 2012
by Cathy Busby, Department of Earth Science, University ofCalifornia at Santa Barbara 93106
Supported by NSF EAR 01252, 0711276, and 0711181 and USGS EDMAP 06HQA061 and 09HQPA0004
THIS PDF IS POSTED AT:http://www.geol.ucsb.edu/faculty/busby/NAGT2012
Pdfs of Busby’s papers cited herein are posted at:http://www.geol.ucsb.edu/faculty/busby/publications
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TWO MAJOR QUESTIONS:
1. When did the Sierra Nevada microplate begin to form?
Part I - Geologic signals of transtension in the WalkerLane belt.
2. What is the uplift history of the Sierra Nevada?
Part II - The record of Cenozoic paleo-channel fills.
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What is the Walker Lane belt?Dextral transtensional eastern boundary of the
Sierran microplate, currently accommodates20-25% of the plate motion between thePacific and the North American plates:
INCIPIENT PLATE BOUNDARY.
Unruh et al., 2003; McCrory et al., 2009; Faulds and Henry, 2008.3
Time- and space-transgressive processes can be observedfrom post breakup (Gulf of California in south)to new rift (Salton Trough and its geothermal fields)to RIFT INTIATION: Walker Lane Belt in north
Walker Lane Belt
short NE oblique slipnormal faults (BLACK)
are connected by longNNW dextral faults (RED)(note: yellow = GPS vectors)
…like Gulf ofCalifornia, but nosea floor spreadingyet.
(Jayko and Bursik, in press; Faulds andHenry, 2008)
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I nominate theCENTRAL part of theWalker Lane Belt“MOST LIKELY TOSUCCEED” first,because:
It has the structure that isthe most similar to Gulfof Californiatranstensional riftbasins:
1. NNW strike slip motionand E-W to ESE-WNWextension are partitioned .
2. Normal faults (black) areabundant, and strike slipfaults (red) are short.(Wesnousky. 2005; Surpless,2008; Jayko and Bursik, in press)
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Faulds and Henry (2008) did not includecentral Sierra Nevada range front (gray) inWalker Lane (peach)….
but our new structural work between Monoand Tahoe basins
and work by Jayko and Bursik from Monoto Long Valley (2012)
show it is transtensional = WALKER LANE
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II. It has abundant LateCenozoic to Holocenevolcanic rocks (shown ingray):
A signal of rifting.
Also good for dating structuresand clockwise rotations!
CENTRAL part of the Walker Lane Belt“MOST LIKELY TO SUCCEED” first(continued)…..
(Busby and Putirka, 2009)
The Tahoegraben, WalkerLane belt - homeof the newlydiscoveredPolaris strikeslip fault (Howleet al., 2009).
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Umhoefer (November 2011 GSAToday) points out that that Gulfof California rifting wasunusually FAST.
For one thing, there’s noevidence of a “Nevadaplano” inMexico, so the crust did not haveto be thinned as much……
The Nevadaplano Henry et al., in press, Geosphere special issue
(Busby and Putirka, 2009)
….but also….
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… the capture of the much largerGuadalupe-Magdalena microplatesoff Baja resulted in a much biggerchunk of the margin beingtransferred (RIGHT) than thecapture of the little Monterreymicroplate did (LEFT) …
McCrory et al., 2009
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Both Baja and the Sierran microplate assumed a more northwardtrajectory by ~12 Ma, concurrent with the same change in themotion of the Pacific plate. (McQuarrie and Wernicke, 2005)
From 12 Ma to present, this has produced LARGE VOLCANICCENTERS IN TRANSTENSIONAL PULL-APARTS along theeastern margin of the Sierra Nevada: THE GEOLOGIC SIGNAL
Paleomagnetics of Pliocene lavaand sediments show clockwisevertical axis rotations, plus 13-25%extension.
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NOTE BLUE BLOB AT COSO: itʼs goingDOWN.Vertical GPS velocity (top).
Vertical velocity from InSAR (below).
Hammond et al., Contemporary uplift of theSierra Nevada, western U.S., from InSAR andGPS measurements: Geology, 2012.
Weʼll get back tothe RED over theSierra later.
Arrows showhorizontal velocityfrom GPS.
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ACTIVE very large volcanic centersin trantensional settings:
LASSEN“Beginning at 3.5 Ma, thenorthern Walker Laneincreasingly interacted withthe Cascade subduction zoneto produce transtensionalenvironments favorable tothe development of majorvolcanic centers”
(Muffler, Blakely and Clynne,2008 and in prep).
Pronounced linear alignments ofvolcanic vents, seismicity, and agravity low (Janik and MacLaren,2010).
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Weʼll talk about
Long Valley rift volcaniccenter (Bursik, 2008)
to compare to a
* Coso volcanic center (active)
*
MIOCENE CENTER we havediscovered:
“Little Walker volcaniccenter”: AncestralCascades arc, 12 - 9 Ma
INCLUDES SONORAPASS AREA
ACTIVE very large volcaniccenters in trantensional settings:
and also filled range-front, downto the east synvolcanic grabens
SHOWN IN LAVENDER.
Little Walker caldera =culminating silicic explosiveeruptions.
(Busby, 2012)28
Fault-controlled VENTS for the high-K volcanism include small,well -stratified trachybasalt or shoshonite cinder cones……
Sector of aproximal cone withsteeply-dippingstrata.
Volcanic bombs (map case for scale). Scoria fall deposits (pen for scale).
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High-K vents also include fault-controlled fissures marked bytrachyandesite scoria ramparts 8 - 12 km long and ~ 100-200 mthick…….
Strombolian blocksup to ~5 m across(left) in a red,nonstratified matrixof cinder blocks(field of view ~40 macross).
Close-upof scoriaramparts(map casefor scale).
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High-K vents also include trachydacite domes, lava flows, and tuffrings….
…while trachydacitewelded ignimbriteswere erupted from theLittle Walker caldera(“Eureka Valley Tuff”).
Flow-banding
Bomb sag
Fiamme
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Miocene “Little Walker volcanic center” (left) is STRUCTURALLY SIMILAR TOactive Long Valley rift center (right), SHOWN AT SAME SCALE:
NNW faults right transtensional, ENE faults left transtensional. Stars = vents.
From Busby, 2012 (Long Valley after Bursik, 2009).32
Prior to our work, it wasnot known that half theslip on central Sierranrange-front faultsoccurred during the 12 - 9Ma high-K volcanism,under transtension.
(Busby et al., accepted)
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….nor was it understood that the high-K volcanic rocks wereponded to thicknesses of >300-400 m in these grabens andhalf grabens…..
PRESERVED VOLUME of
Table Mountain Latite alone (in highlyglaciated terrane): 200 km200 km33
By far the most extreme intermediate-By far the most extreme intermediate-composition effusion we know of,composition effusion we know of,
and the only one fed by FISSURES.and the only one fed by FISSURES.
(1) Development of large volcanic centers at sites of maximum displacement onreleasing transtensional stepover faults.
(2) Extreme effusive eruptions along fault-controlled fissures, including intermediate-composition fissure eruptions of “flood lava”.
(3) Accumulation of unusually large-volume, widespread landslide deposits in grabensand rectilinear volcano-tectonic subsidence structures.
(4) Abrupt derangement of ancient E-W drainage systems of the“Nevadaplano”, by development of north-south grabens thatacted as funnels for lavas and fluvial channels.
When did the Sierran paleo-channels get beheaded?
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Henry, 2009
Oligocene Oligocene ““NevadaplanoNevadaplano””paleodividepaleodivide, with, with knownknownpaleochannels paleochannels in redin red: best: bestpreserved in preserved in unfaulted unfaulted SierraSierraNevada block of California.Nevada block of California.From From Cassel Cassel et al., 2009, based on Henry, 2008.et al., 2009, based on Henry, 2008.
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Although most of the high-K lavas were trapped inthe Sierra Crest grabenand the range-frontgrabens,
a handful of lavas escapedthe graben to westwarddown a “Nevada-plano”paleo-channelSHOWN IN TAN.
Carolyn Gorny, Senior Honors Thesis 2008, and Gornyet al., 2009; currently PhD student at Otago University.
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Our paleomagnetic work(with Chris Pluhar, CSUFresno - a Cal Tech alum)shows that one single lavaflow traveled 130 km to theSierra foothills and has aminimum volume of 25 km3
Note: andesites areconsidered “LARGEVOLUME” at 3 km3
Above from Gorny etal., 2009;
center photo by Busby;far right from theFresno Bee
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Prior to our work, it was notknown that the ancient E-WNevadaplano paleochannelswere disrupted by the 12 - 9Ma transtensional structures.
BEHEADEDPALEOCHANNEL FILL:
1 = Oligocene ignimbrite (seepink map unit, below).
2 = Andesites older than thehigh-K rocks (green mapunit, below).
3 (above) - a few high-K lavaflows rarely escaped, then -
4 (above) - all material < 9 Madiverted NORTH, parallel tomodern drainages east of thecrest.
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Derangement of E to W paleochannels/paleocanyons by N-Sgrabens:
Fluvial wedge dipping to north (right) in the Sierra Crest graben(sequence 4, above the high-K volcanic rocks).
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After 9 Ma, thelocus oftranstensionstepped north,dropping thehigh-K volcanicout of sight andcreating theEBBETTS PASSVOLCANICCENTER.
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A Lassen-sizedvolcanic centeronly 6.4 to 4 Maold, and onlyrecognized by ustwo years ago!Needs more workbut……
Megan Gambs, Senior HonorsThesis 2009; currently a PhDstudent at University ofWashington, Seattle
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TWO MAJOR QUESTIONS:1. When did the Sierra Nevada microplate begin to form?
Part I - Geologic signals of transtension in the Walker Lane belt.
2. What is the uplift history of the Sierra Nevada?
Part II - The record of Cenozoic paleo-channel fills.
Time permitting, a few words on this….
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OPTION 1:
OPTION 2:
I THINK THEREALITY LIESSOMEWHERE INBETWEEN.
(Busby and Putirka, 2009)
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Hammond et al., Contemporary uplift of the Sierra Nevada, western U.S.,from InSAR and GPS measurements: Geology, 2012.
CONTEMPORARYVERTICAL MOTION
Between 1 and 2mm/year
along entire lengthof range.
This rate could havegenerated the entiremodern range in
< 3 million years!
But thatextrapolation flies inthe face of manyother data.
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WHAT DO SIERRANPALEOCHANNELS TELL US?
Cenozoic volcanic rocks ofthe Sierra Nevada are largelypreserved in E-W“paleochannels” recognizedby Whitney (1880) andLindgren (1911).
THE SILICIC IGNIMBRITES ARE IMPORTANT FOR DEMONSTRATINGTHAT IT WAS DOWNHILL ALL THE WAY FROM EASTERN NEVADA TOCENTRAL CALIFORNIA (over 230 km, after Basin and Range extension isrestored):
CP
HIGH PLATEAU SLOPING WESTWARD IN EARLY CENOZOIC TIME(from Busby and Putirka, 2009)
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CENTRAL SIERRA NEVADA PALEOCHANNEL FILL,continued
Unconformity 2: Thermal uplift andextension, followed by…. …Sequence2: onset of arc volcanism at 16 Ma(purple).
2
2Early Miocenecolumnar-jointed lavaflow.
Alice Koernerexaminingpetrified woodin Miocenepaleochannelfill.
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Sierra Nevada microplate beginsto calve off the western shoulderof the Nevadaplano:
Unconformity 4,-regionalcorrelation andage least certain
Sequence 4Sequence 4 - Beheadedpaleocanyons locallyfilled with fault breccia -talus cone deposits, orwith local volcanicconstructs
CENTRAL SIERRA NEVADA PALEOCHANNEL FILL,continued
From Busby, 2012
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SOME ANSWERS???? From Cenozoic volcanic andsedimentary rocks….
1. When did the Sierra Nevada microplate begin to form?Using geologic signals of transtension in the Walker Lanebelt, Iʼd say by 12 Ma.
2. What is the uplift history of the Sierra Nevada? The recordof Cenozoic “paleo-channel” (paleo-canyon) fills suggestMiocene rejuvenation at 16 Ma, and especially at ~10-12 Ma.This is consistent with thermochronologic evidence for LateCenozoic rejuvenation.
