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Dr. C'cnn T. sc:.borg April 25,196h CML: ;., L 4.sc f=cg7 Co. =i;; ion
,nc::..; cn 25, D. C.
Scar Sir:
n en effort to reconcile differences in interpretation of the geolo y l
of iode. n liced, Julius Schlockcr and Ecnuci Scnilla, C. A. WahrMftig of th i
"Jnivcroay of California at Dcrkc1cy, D. H. I.cmor; of tha U. S. G. S., cnd rt
- 2. C. P.crlicvc in the rolo of goclogic conculte.nt to P. G. ari E.,
toguthor vith cursciv;c virited the propocod recetor cito on Farch 18,.196h.
All por-tinent., col. gic data, both previously known and found on thic trip, vero dis-j cussed.n t'.
field.
Wo believe th;.cc arc all in eccentic1 apocnont as to the ch..rym.:.:,,ns tht can bo ncdc at tha sito but vc discpce as to their rol-
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ativo significance and interpretation.
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O pr ;ous discgocnont with interpretations cf Schlocker end Eonilla
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t t.nount of displaco.:cnt of the Shaft Fault. Uc stcted in our l
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.:. cnd C. that the Sheft Fsult, which is c fractured zonc in the g reni.
f
.iu and cuts the loucr beds of tM overlying sedinants, hcc.t r.a..
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u of disp 1cccacnt, cnd thct thic cicpiccoment is princip117
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,.r, that is, the bed above the dippin;; fault-phne have r.c -
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.terpcot to the beds bc1cu tl.c fcult-phne, indicating a ten-raul-c.tcral displacement cieng the SMft Fcult of probt.bly scverci
.m e., tas.nau.
c..,d by Schlocker cnd -""', the cvidence fer cuch disphec-
~nt being oi_. 40 of a thin diko of ticrite and en irrocular]y abyd,qnc-sito bc0 or ai.s of bodies.
Offcst.. of the diorite dika indicct:.; right laterci. otic.., while displacement cf ti.e pcy.ctito irdicated lef t 1::sr:1 no-tion.
,.. ccr :. tod that the diorito diha to ;other with enclosing shc r cono, l
vhich ne es11cd a vein in our letter to ?. G. and E., hcd indeed been offcct by c rn t for a distance of a foot or so, but wo n.intained that this was c ptrcn and older fault then the Shcft Fcult.
Cn this last visit to the cito; Uchrb 23 fcund evic:ence that:the diorito dike ncy have been offect by c fcult for r ance of28 to"30' feet in a right 1ctoral acnco:
i c diorito dihc uc
~. L t cf the Shaft Fruit which Ms similar texture and nincralogy to tMt c: L..
c.ci iko.
The evidence icoks reasonably good tut this much disp 1cco-ncnt ec...av; taken place but it is by no necns cortcin 3
cr.d it is clocr thct if the c J.:o t:ro displaced this much it uas by a fault tMt i;:
3 nuch oldcr thr.
- c.c Shaf t Fc _
- .
A long history of events is recorded in the gcnite ttich j
bacrs on cny n;crpretation given to thc novenent of the Shft Fault.
j These.
I cvants events cre as follows:
in its hte cooling stages, shcaring of pcnite ::.: emplacement of pt.nito uith pop.ctito fernc !
i % cnphconont of diorite in local sencs within c;:. roughly east-wec; direct-
)
i cr nero of thcc shaars,
- c. shocring parallel to diorite c'ike or dihc%
.rfcet of dikes in a rigt h..
senso by north-south trending e.. cars, furuncr : cst-west'bhocring pce-c._.. to diorite dike or dikes, hydrotc.:: mal develo r.crie of secondary nincrci..
j i
(chy and chlorite) in cast-wcst shecrs.
j Of critical :dportance to tho tinc j
af formation of the Shaft Fcult and to.hc cnount of its displacement is tho i
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rc ence of 4.n cast-west trending she:.r conc to the cast of the SMft Fcult l
j cco nap) and almost on lino with the en:.ar cono contcining the diorite dihe act of t.':e Shaft Fault. This shoc tion cc,he shear zono to the vest but it doc;cno contains the scne chloritic c1tcr-
.c; contain diorito. The Shcft cult dc.
not contain chloritic alteration. Fa:. thcro been 2c to 30 fcct cf tD.'.
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..:encnt along the Shaft Fcult, tr~ic chet.: cone would appccr en t b =.
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cf tho Shcft Fcult 23 to 30 fcet to the ncrth. h docc not cccur th=;.
1 q c.:. cs cro exco11cnt in thct crec cnd tht, offcct chocr cono could not "o s
tr.:.0 L. :: to:..
h*o bolicyc, thereforc, tct thic ic the cc:ic checr conc tMt ecn-tc:.i the.;.cr.ito di. :., : cot of the Shaft Fcult, cnd tht its fcv inches cf rijdt, latcr:1 dispic0cnont arc the boct indicction of the total uncunt of h.,c; ; disp 1ccc:c.cnt of the Shf t.ccult.
l 0.. tb:.r coologic plcn, Schicckcr cn:. Beni11c chou the Sbft Fat.lt ac 1
1 c cono of becck: bounded by tuo noro or it.,c pcrc11c1 fcult: which cc;;pletcly crocc the hctten of the sito cnd cre ccpt.rt ted frca eccl. other en averc;c d:. -
1 tance of about two feet. On their crccc-ccction the bounding fcults cro ccp-cratcc cbcut fivo foot at the t op s of tha chf t on the couth sid'.:, and only cns of thc:. is shcrn cutting the overlying ccdinantary bcdc. L'o fduhc cre chcun i
on thcir phn or scetion diverging in ctrike cnd dip frc. th: bc;:iin; frcc-turec of the Shcft 7ault. ^uo offcut: of the diorite dike cro chun on thcir Cccio;ic phn of the chc.ft botten tj ch occur en f:.ults that do not cppct.r to l
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c c==., uit? the Shf t Fcult. Thcir n.pping of theco offcots ic cectrcte'y
- /.ictcd: so ti;ht cro thsco sr:11 f:xitc that thir continuance fcr =cre few inchec beyond the offcot cocnente of dike in cither direction ecn-
- r.n nct h ccte'. iched, although it cecn
- cortcin tnt they muct continuo throu-h l
r.
renite. Both of these sncil fcults differ in ctriko frc a tha Shcft 7:.dt t f: c. 20 to ho degrees, and both cf thc= dip ccetucrd approvinctely 35 c':-
i grces, 5thile the Shaft Fault dipa rectucrc cpproxinctoly 70 dec ecs. Fcult:.n; j
with t-is ccno direction and conso of dispiccenont is not confincd to tho v_-
cirit,< of the Shcft 7:dt cs succected by Schlockert: and Eonilh 's pub 21sMc ncp:
=-11 fcult '.ith cbout 5 rc:t er c.'. ht k.tcral crf:ct dicniccc.: th; cbc.
- no in which the diorito diks cecurs cen
- 35 fcct rest of the Shcft l
?:.u a,.:nn c sinihr fiult dispice.. the d:crito diko sever:1 inchec co..erc::.in-i l
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- ':,cly _G foot cast of tho Shaft Fcuh. A2 noct c11 of theco cncil, vry tight i
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= uC-hocled faults with richt htcrcl displacencnt strgko appro: cit.mtcly l
north.:.uth, while nenbors of tho Shaft Fcult ctriko N 1: 0 3.
Ac ccn to cocn on the :uip, the diorite dike or dikes can be tr:.ced to the Shaft Fault fren j
tho ucc;, but can bo trcced only to a point cicht fcot frca the Shcft Fcult on the cast whoro it cppecrs to stop c;cinct encther very tight fault.
I Uc believ; that this fault is part of the old= north-scuth cycten of faults rcth-er th n of the Shaft Fault systen.
'.!httht:: cr not there cro two cyctens of faults, as to stated in our 1ct-tc; to P. G. cn:13. and reaffirm hero, onc systc= coologicc11y old cnd shm:in-peccibly 3C fcc., of dispicconent, the other cyctc= yor. ; cnd shou-lng only en:[
foot er so x icplacenant, the evidence of the enount of displacencnt uhcre the S'; t Fcuh cuts the surface of the grcnito - the cncient cranite curfcce on which the Pleistocene sedinantcry bcds ucro deposited - is unccuiveccl.
No of.' at of the contours of this surfcco is chcun by ScN ckcr and Sorilh Q1ct 2 of their report and reproduced h=o).
Thic surfcco was forned by wrocion, peccibly rava action, beforo the Pleistocene cedinents vero depcc'ited,
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mnd it clips stcoply south:restward in thc direction of the tr ce of the Shcft t
F.-
- .t,; thuc in the idcol direction for detceting ictcrcl or vertical novencnte f
cf the ihaft Fcult. Any lateral or verticc1 novc=cnt of the Shaft Fcult uculd.
- Jfcct these contours and produce a encil ridge clong the fault tracc.
Schicchcr cnd Bonille chou no such offsets c1though they night not havo noticed a dicplccc-
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j ncnt of a feat cr so.
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1 72.us, wo belicyc that the tn:.1 ancunt of di::phcenent of :.ny fculta paccir th-ough the recetcr shcft that has cccured sinco the P1cietcccnc codihe n,ary bcds wora deposited is of the crc.cr cf eno foot. Vcrticci c11chen:1de: on the fault surfcce indicate,,r:t -ho Ic.::t ncycnont, ct 1ccr.;t, was ver;ical. The cgo of the codi.cntry xds is certt. inly cre.:.tcr thcn h2, COO years cnd more likoly betroer, c:0,CCO cnd 600,0C0 yccra, ba::cd upcn corrections with Pleistocene cvori.,3 ciscwhcre. The disphecnont in the j
s:.dir.sr.ts van probcbly the recu1T, c. :nro uMn one displacenant, as evid-enced by the brcadth and ancstono ing natur,. of the SMft Fcult and by tM 21ccr evider.cc of puro vertical notion nest recon,tly contra::ted with cvid-cnce of strC.c-slip novononts within the sedinantary bcds cc noted by Ecnil'r and Schlock:r.
Ur.,:.ng the abovo figurce and the e.:tec cf knmin novenants of the San j
Androct, Fault in historic times, a crudo estinato of the chances of disphec-Tr.t o.: curring withinthe sito during the ncrc, 200 yccrs ccn bo sdo. Two
.arge m.rth.uckes with accenpanying certh novcncnts havo occurred clong the 1:.n,nraat Scult in this region sinco about 1000, one in 1638 and cnothcr in
'ZL If r:. cxtrcpolato from thic inadequate ccnple cnd scy that the avc :.
r x,rio. ':ctu re.cf or quakes is 65 years, then thro hcvc been botucen 500 cn IC,Z' aartn:w ko: conparable to that of 1906 which h:ve shcken the cito cin::c tr.c sce. nontary beds overlying the sito ucro deposited. As the h0,0C0 yo:.r nininr. agt c. the sediments is unroa2fictic cnd is cinply the li.it cf detec-tien cf the ccrbon-1b nothod, wo can ::.y nero recconcbly thct th re have bccn botue:. 3,CT.~. cnd 10,000 major carth.u:..:cs :.n this arca. If, c:: Schlockcr cnd Bon 11'. believo and as we beliove, the Shcfi, Fault hcc noved ncro thcn orco durin; 11: tino interval with diaph:'araanto of, coy. h in@ s cach t'no, then the ch:...:cs cro about,1/500 for disruption of the sitw during the next 200 years. If, es Wahrhaftig. believes, there has been cnly ona novenant of this fault, z.mn the chances decrease to 1/1500.
It secna worthwhile noting that einec r.11 controversy cbout tho seisnic haecrcs of the site localicos on tho Shcft Fault cnd possibic dispiccenento clong it, a simplo solution would be to novo the hole 'so. thct the Shr.ft Fcult docs n:t inter cet it~.
It was fortuitenc th::t the Shaft Fcult crosscd the p
particub r spot sc1cetod for the hoic. It would cec verthuhile to r:plerc; b" wens of tunn::ic, the necessary 30 -ho fact westwcrd to asecrtain thathc
- . i.hur fault occurs.
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