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MEASURING FAULT DISPLACEMENT RATES USING
IN-SITU COSMOGENIC 36Cl: THE DISPLACEMENT OF THE NAHEF EAST BEDROCK FAULT SCARP IN NORTHERN ISRAEL
A Thesis Proposal Presented
by
SaraE. Gran
to
The Faculty of the Geology Department
of
The University of Vermont
(March 15, 1998)
Accepted by the Faculty of the Geology Department, the University of Vermont, in partial
fulfillment of the requirements for the degree of Master of Science specializing in Geology.
The following members of the Thesis Committee
have read and approved this document before it was circulated to the faculty:
~ Advisor
./paul R. Bierman
Abstract
My master's thesis will estimate the age and rate of fault motion by measuring concentrations of in situ-produced cosmogenic 36Cl from the surface of a 10 m normal fault scarp. The limestone fault scarp chosen for this study is located in the Galilee of northern IsraeL Regional extensional activity exposed the scarp at the Earth's surface and cosmic rays began interacting with elements in the limestone, producing otherwise-rare isotopes. Since the cosmogenic reactions occur at a relatively steady rate, by measuring isotopes along an evenly-spaced down-dip profile, I will attempt to determine when successive increments of the scarp were exposed. I have mapped and sampled the scarp; these samples will be processed to determine isotope concentrations. Mathematical models, based on production rates modified for the surface geometry and rock chemistry, will then be developed in order to translate the isotope concentrations into model exposure ages. There are currently no fault-rate data for this particular fault system, located in the heavily-populated Beit-Hakerem Valley, with which to develop seismic hazard assessments. This scarp also represents the end of landform-creating extensional motion in the Northern Galilee. region, so displacement-rate and exposure age estimates will help constrain the timing of the local larger-scale tectonic activity.
Introduction
My master's thesis research addresses a significant and as yet intractable problem in
paleoseismology and seismic hazard assessment, dating earthquakes responsible for bedrock fault
scarps, and calculating faulting rates for such scarps. Most paleoseismicity studies have focused
on faults occurring in unconsolidated sediment where trenchable, fault-related colluvial wedges
develop on the down-dropped block and where 14C-dateable material is buried. This trenching
method is not applicable where fault scarps are formed entirely in bedrock and no colluvial wedge
forms. For this reason,most bedrock scarps remain undated (McCalpin, 1996). The continued
development of a new tool capable of providing numerical age estimates for motion on bedrock
scarps could significantly supplement the data available for seismic risk assessments. I will
address this problem by applying in situ-produced cosmogenic isotope dating to a particular
limestone fault scarp, the Nahef East Scarp (NBS), located in northern Israel (figures la and Ib).
The build-up of cosmogenic isotopes over time allows direct numerical dating of the
exposure of geomorphic surfaces (Bierman, 1994). As rock faults, and previously-subsurface
material is uncovered, cosmic ray bombardment will produce a distinctive set of otherwise-rare
isotopes inside the mineral lattices; the longer a fault surface is exposed, the higher the isotope
concentration will be. Fast cosmic ray nucleons striking the calcium-rich limestone cause the
reaction 40Ca(2n, 3p)36Cl (Lal, 1988). The 36CI concentrations on the fault scarp may indicate the
rate in this limestone for me to be able to distinguish between exposure ages which might be only,
several hundreds or thousands of years apart? The accepted production rate (scaled to sea-level,
high latitude) is about 50 atoms/(g Ca yr.) (Stone, 1996). This corresponds to about 4 x 10^5 atoms
of 36CI being produced in 20 grams of limestone per thousand years. I will need to be sure my
analytical method is precise enough to reliably distinguish between exposures occurring at least
every thousand years.
This study assumes that the NES was exposed due to tectonic activity instead of erosion,
and was not buried by sediment for any great length of time. If the scarp were exposed by
erosion, then my 36Cl ages will indicate a stripping rate instead of faulting rate. If the scarp were
buried during or after faulting, modeling an exposure age becomes very difficult. It is impossible
to establish with certainty that these situations did not occur, though preliminary field observations,
such as fault geometry and preservation of primary features, as well as lack of any current scarp
burial by colluvium, support a tectonic origin for the scarp.
Related Research
Previous cosmogenic isotope studies include discussions on the fundamental nuclear
physics involved with isotope production (Bard, 1997; Lal, 1988) and production rates (Zreda et
al., 1991; Nishiizumi et al., 1991). Other investigators have concentrated on determining the ages
of bedrock surfaces (Kurz et al., 1994; Zreda et al., 1991; Nishiizumi et al., 1991); glacial
landforms such as moraines and striated bedrock (Larsen, 1995; Brown, et al., 1991; Bierman et
al., in review); and wave-cut.ocean terraces (Stone et al., 1996). Herut et al. (1992) and Andrews
et al. (1986) discuss the abundance of 36Cl in the environment and the different ways it is
produced. In situ cosmogenic 36CI production rates and analytical methods for concentration
measurements are discussed in Stone, 1996; Bierman et al., 1995; Dep et al., 1994; Leavy et al.,
1987; and Stone et al., 1994. Several researchers have applied specifically 36CI dating to attain
exposure ages and erosion rates (Dep et al., 1996; Dockhom et al., 1991; Liu et al., 1994; Stone,
1994). There is an abstract (Zreda, 1995) that considers the possibility of measuring faulting rates
by measuring cosmogenic isotopes from a bedrock fault scarp. It is, however, a preliminary study
and has not been presented as a formal publication.
The tectonics oflsrael, especially the Dead Sea Transform (DST), has been extensively
studied in terms of plate motion and seismicity (Rotstein, 1987; Shapira and Feldman, 1987; Ben
Menabem, 1991; Enzel, 1994). The DST is an active left-lateral strike-slip fault extending
northward from the Red Sea (figure I a). The region containing the Nabef East scarp, about 30 km
to the west of the DST, is currently being mapped in detail and is part of a region-wide study of the
tectonic geomorphology of the Galilee region (Matmon, 1997). There are no specific references to
the seismic history of the NBS.
Research plan/methods
This project is composed of three distinct but interrelated components; field mapping and
sample collection (completed winter 1997-98), laboratory chemical analysis, and computer
modeling.
Mapping and Sample Collection. A l-kilometer section of the extensive series of en
echelon fault scarps was mapped and profiled using a Trimble 'Y-OO Real Time Kinematic Global
Positioning System, with horizontal and vertical precision errors of < 3 em (figure 2). The profiles
(cross-sections), oriented perpendicular to the fault surface, will be used to calculate total vertical
displacements (figure 3a).
I chose the location for 36CIsample collection during this mapping phase. The selected
scarp surface was planar and relatively fresh, indicating that little erosion had taken place. Erosion
of a surface makes exposure age determination complicated and more uncertain. The location was
also selected in order to detect as many faulting events as possible. At the sample location the
scarp is 10 meters high (measured down-dip), nearly the highest displacement (^) of the entire scarp
(figures 2 and 3).
The samples were taken with a paleo-magnetic rock-coring drill every 30 centimeters along
a single down-dip profile. I sampled 9.6 meters of the scarp, including three samples from below
the current scarp base (35 samples). At two heights, I took a second set of samples to serve as an
accuracy check. A large sample from the upper surface of the scarp was also taken, resulting in a
total of 38 samples (figure 3b).
Possible Outcomes and Interpretations
My master's thesis has several possible outcomes. By plotting sample position vs, isotope
concentration I should be able determine the character of fault motion, provided the 36Cl technique
is sensitive enough. One possibility is that I may be able to detect individual displacement events
by seeing distinct steps in the plotted data, indicating fault motion. Alternatively, the isotope·
abundances may suggest that the scarp was created with a single slip event. This would be quite
interesting, because empirical relationships suggest that a 7.5 magnitude earthquake is necessary in
order to produce a 10 meter displacement (McCaipin, 1996). A third possibility is that for a period
of time the fault slipped only a little bit at a time (less than my sampling interval of 30 em). In this
. situation, the height vs. isotope concentration curve should be smooth. There are an endless
number of combinations of displacement magnitudes and ages, but my preliminary models indicate
that I should be able to distinguish between the above scenarios (figure 4). Since there are many
possible "pathways" to reach the isotope concentrations measured on the scarp, it may be difficult
to determine a single, most-likely faulting scenario. In this situation, despite not being able to
determine when individual earthquakes occurred, I should still be able to estimate when faulting
started and.finished and calculate an average displacement rate.
Schedule for Completion Winter 1997-8 Completed mapping and sampling. Spring and summer 1998 Defend proposal, process samples, collect accelerator, ICP, ICP-MS and spectrophotometry data. Fa1l Develop models for age determination, present Progress Report, present preliminary findings at GSA national conference in Toronto. Winter 1998-9 Write. Spring and Summer 1999 Finish writing. Fall 1999 Defend Thesis
Funding I have several funding sources for this project. My living and travel expenses during field
work in Israel were paid for by Israeli collaborators at Hebrew University. Airfare to Tel Aviv
was provided by the DVM provost's office through an International Educational Incentive Grant. I
have support from Israeli and Lawerence Livermore collaborators, as well as a UVM
SUGRIFAME research grant, for travel, sample processing, and AMS analyses.
"
MEDITERRANEAN SEA
GALILEE
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Tal Rllkkn CnlUc!lOUI IIdlnntl TJboda.
o:::::J CY CD c:::::LJ Jurudc ndhlnu (^) o He lIordo. II I 2' ~,.. OJ .. '!I,t11:l1'Pi' FluJ.t. Q^ Zellnkh^ Ii'^ :^. ............. o 1 ft-'\I 2 3'7>D
Figure la. Structural map of northern Israel (after Michelson, et aI., Figure I b. Topographic map of field area.^ Nahef East Scarp is shown as a 1987). Field area consists of Cretaceous sedimentary rocks. Many heavy dashed line in the center of the figure.^ (after Society for the Protection of more, smaller normal faults exist in the area than are shown on this Nature in Israel, I:50,000 map series) map.
profile of sample
~ 320.
c o 0.g
300 ..
280 +,-----+----+-----'---'----:------+-----'----- o 20 40 60 80 100 120 140 horiz. dist. (m)
Figure 3a. Cross-sectional profile of sample locality. Total vertical displacement is calculated by extrapolating the lower block surface and measuring the vertical distance perpendicular to the lower block. In this location, the measured scarp height is 10.85 m and the vertical displacement is 7.7 m. Note the increased roughness of the upper block, this is due to karst' weathering of the limestone.
sample locations on profile
312 T'-----'---------------------------,^5
I
310+
/ 'rOo
J 308 r ~ i
j 306 J
1<i
0;"
304·
302
,
52.00 54.00 (^) 58.00 60.00 62.00 64.00' 66.00 68. horizontal (m)
Figure 3b. Close up of sample profile showing locations of all samples. Samples were taken at 30 ern downdip intervals. 3 samples were taken from below the scarp base, and one from the upper surface. At 3 m and 6 m above the base, I took replicate samples. No samples were taken from the upper I m of scarp because it was highly weathered.
, ,
q
..•..
Model CI36 concentrations'
700000 t
600000 1 -c.., 0 ~ E 9 -e
I
500000 t
i
,I
400000 t
300000 t
i 200000 + , , !
,I
100000 t i
,I
o : -300 - Q (^100 300) 500 (^700 ) position on scarp (em)
1000000
900000
800000
Figure 4_ Hypothetical "Cl concentrations from different faulting scenarios. See Appendix 2 for model mechanics. These models are for the development of a vertical scarp (a vertical scarp is simpler to model). For each scenario, exposure starts at 100 Ka. This model ignores isotopic decay.. Series #1 (diamonds) represent isotope accumulation if the scarp moved all 10.meters the day before I sampled (all dosing received from upper, unfaulted surface). #
. (squares) is ifthere was constant creep from 50 Ka-25 Ka. #3 (triangles) is if there. was a -- single fault motion at 25 Ka. #4 (open circles) is if there were a series of 5 two-meter. displacements occurring between 25 Ka and the present. The "Cl in the uppermost sample. (on.the far right of each line) is proportional to when the scarp first developed. In #1, the top sample was essentially never exposed, and therefore has the smallest amount of "Cl. #3 and #4 both represent faulting that started 25 Ka. #2 has the highest "Cl, .because in that situation faulting started the earliest, at 50 Ka.. Note that the slope of #2 and #4 are. parallel, except that #4 represents discrete fault motions: The slope of that line is proportional to long-term displacement r~tes. - .''' .. _.--0--'
Leavy, B.D., F. M. Phillips, D. Elmore, P.W. Kubik, and E. Gladney (1987). "Measurement of cosmogenic 36Cl/CI in young volcanic rocks: an application of accelerator mass spectrometry in geochronology." Nuclear Instruments and Methods in Physics Research B29: 246-250.
Liu, B., F.M. Phillips, D. Elmore, P. Sharma (1994). "Depth dependence of soil carbonate accumulation based on cosmogenic 36CI dating." Geology 22: 1071-1074.
Matmon, A. (1997-8) Personal communication.
McCalpin, James P. (1996). Paleoseismology. Academic Press: San Diego. 588 p.
Michelson, ,H. A. Flexer and Z. Erez (1987). "A comparison of the eastern and western sides of the Sea of Galilee and its implication on the tectonics of the northern Jordan Rift Valley." Tectonophysics 141: 125-134.
Rotstein, Yair (1987). "Gaussian probability estimates for large earthquake occurrence in the Jordan Valley, Dead Sea rift." Tectonophysics 141: 95-105.
Shapira, Avi and L. Feldman (1987) "Microseismicity of three locations along the Jordan Rift." Tectonophysics 141: 89-94.
Stone, J.D. (1997). Personal communication.
Stone, J.D., G. L. Allan, et al. (1996). "Cosmogenic chlorine-36 from calcium spallation." Geochimica et Cosmochimica Acta 60: 679-692.
Stone, 1., K. Lambeck, L.K. Fifield, J.M. Evans, and R.G. Cresswell (1996). "Late Glacial age for the Main Rock Platform, western Scotland." Geology 24 (8): 673-676.
Stone, J., G.L. Allan, L.K. Fifield, J.M. Evans, A.R. Chivas (1994). "Limestone erosion measurements with cosmogenic chlorine-36 in calcite - preliminary results from Australia." Nuclear Instruments and Methods in Physics Research v. B 92 p. 311-316.
Zreda, M. G. (1995). "Dating of a late Quaternary bedrock fault scarp using cosmogenic chlorine 36." EOS, Transactions, American Geophysical Union (Supplement 46) 76: 685.
Zreda, M. G., F. M. Phillips, et al. (1991). "Cosmogenic chlorine-36 production rates in terrestrial rocks." Earth and Planetary Science Letters 105: 94-109.
[,
Appendix 1 '6Cl Sample Processing (Modified from Stone, 1997)
(1) Sample preparation Trim cores of weathered surfaces (remove top 0.5 em) Measure length, weight of core Break into 1 em pieces Grind in jaw crusher, plate grinder Sieve, save 125-250 and 250-500 micron fractions Rinse 250-500 micron fraction in DI water until all fines are washed away Dry in oven overnight
(2) Leaching (to remove meteoric J6CI) Tare and label clean 600 mL beaker Add 20-25 g dry sample
Wet with about 100 mL MQ water. In separate beaker, mix about 15 mL 2N RNa, and
50-100 mL MQ. Pour into rock sample beaker slowly (to avoid foaming). Try to get even dissolution of grain boundaries. Once reaction has died down, cover and let sit for 12-24 hrs. Swirl occasionally. Rinse 3-4 times with MQ and repeat leaching procedure Rinse thoroughly with MQ (at least 5 times) to remove ALL Ca(NO,), Dry overnight. Sample should decrease in weight by about 3 g.
(3) Sample loading Tare and label a clean 600 mL pyrex beaker Add -20 g of sample to beaker, weigh. Use more sample if Cl content is really low.
(4) Dissolution In fume hood, wet sample to a slurry with MQ and gradually add 10 mL 2N RNa, per gram of sample. Avoid foaming (add only 10 mL at a time). It will take about 200 mL acid to dissolve 20 g sample. Once dissolution is complete, swirl to homogenize and then allow insoluble fraction to settle overnight. Weigh beaker, trying to avoid stirring up sediment.
(5) Splitting for chloride analysis and carrier spiking Tare and label a clean 50 mL centrifuge tube. Tare and label two 15 mL centrifuge tubes to receive spec. aliquots Tare and label 15 mL centrifuge tube for ICP fraction. Transfer about 13 mL of solution to the 50 mL tube, weigh. Divide this fraction between small centrifuge tubes: 4 mL in spec viall, 8 mL in spec vial
- Weigh those tubes and calculate how much is left in 50 mL vial. Dilute remainder to 40 mL, weigh. Transfer I mL to rcp tube, weigh ICP tube. Dilute rcp fraction to 10 mL. Discard remainder of clear solution from 50 mL centrifuge tube.
Weigh carrier bottle Pipette 2-3 mL carrier into sample beaker (make sure no carrier sticks in pipette). Re-weigh carrier bottle, calculate amount removed.
IStella Model for CI-36 concentrations
Controls, 'or factors that remain
o 0'^ o
constant
position (^) displacement top of scarp
--- 00 Iconstants for^ limestone^ I constants for^ chalk scaiing factor Po
0 0 0 0 attenuation (^) lime Rho lime (^) attenuation chalk' Rho chalk 'Graph 1
module for 1/2 production received Module for total amount of CI
through upper limestone block produced for a given sample
(always constant regardless of position, adding all pathways'
position and displacement)
amount of CI
Po
vertical
ck prod at depth
top of scarp
o
downblock prod at depth upblock prod at depth scaling factor
module for production at depth
through lower block (1/2 p'o, turns
off as soon as position is exposed)
module for production on exposed
scarp face (1/2 Po, only turned on
after this position is exposed
Po
switch for Ib depth prod,
scaling factor downblock prod at surf
scaling', factor'
I~'^ -
App~>'\d~X 2- V 1'1+, fy-p lio'\tl -t;,,,- 16 V'tO cLd o amounCoCCI36(t) = amount_oCCI36(t - dt) +^ (atoms_in_per_uniCtime)^ *^ dt INIT amounCoCCI36 = 0
. INFLOWS: 11 atoms_in_per_unit_time = time_factor*(downblock_prod_aCdepth+downblock~prod_aCsurf+upblock_prod_aCdepth) o attenuatlonchalk^ =. 160 DOCUMENT:. attenuation factor for cosmic rays penetrating to depth in chalk
o attenuation^ lime^ =^160 DOCUMENT: attenuation factor for cosmic rays causing reactions at depth
o downblockjprod^ jatdepth^ =
o
switch_fo r_Ib _ depth_p rod* Po EXP (-rela tive_positon * R ho_ chalk/a tten uation_chal k) * scaling_facto r downblock_p rod_acsurf = Poscaling_factor*switch_for_exposed
-0 Po = 18 DOCUMENT: Production 'rate at surface, corrected for latitude and elevation. This is atoms of CI produced per gram limestone per year
o position = 600 DOCUMENT: position of sample (0 is base of scarp, 1020 is top of scarp) in cm
o relatlvaposlton^ =^ vertical-displacement DOCUMENT: depth of sample relative to downdropped block (depth through which rays must penetrate)
o Rho_chalk = 2. DOCUMENT: density of chalk in gm/cmA
o Rho_lime = 2. DOCUMENT: density of limestone in gm/cmA
o scaling_factor =^ 0.
DOCUMENT: scaling factor accounting for only half production coming from each hemisphere (1/2 from upper block side, 1/2 from down-dropped block side (whether it's surface or at depth)
o sWitch_for_exposed = IF(vertical-displacement<=0)THEN(1 )ELSE(O) DOCUMENT: switch to turn on this exposure when this position becomes uncovered
o switch_for_lb_depth~prod = IF(vertical-displacement>0)THEN(1 )ELSE(O) DOCUMENT: switch to turn off at-depth production through down-dropped side when sample position becomes exposed at the surface (then the downblock prod at surface gets turned on). Only oneot these will be on at a given time
o tlrne jactor = 100
o top_oCscarp = 1020 DOCUMENT: Height of scarp in meters
o upblock_prod_at_oepfh =. scaling_factor(PoEXP(-((vertical*Rho_lime)/attenuation_lime)))·
o vertical = top_oCscarp-position DOCUMENT: vertical distance from top of scarp to this sample position
/(p
