1. Qualitative Comparison of Offset Surfaces Between the Central and Eastern Garlock Fault
Thomas M. Crane, Sally F. McGill
California State University, San Bernardino
ABSTRACT
The Garlock fault consists of three distinct segments, known as western, central and
eastern, together reaching approximately 260 km from the San Andreas fault to the
southern end of Death Valley. Although published slip rates are available along the Western
and Central Garlock fault, little is currently known of the Garlock fault earthquake history
or slip rate farther east. Using LiDAR and satellite imagery, the Central (CGF) and Eastern
Garlock fault (EGF) were analyzed for visibly offset features that may potentially be used
for estimating slip rate. Qualitative methods of assessing preserved alluvial surface
maturity were adapted and used to establish unit age categories. Qualitative comparisons
of surfaces considered to be similar ages reveal that the total offset at sites along the EGF
are less than half that of sites of comparable age along the CGF, suggesting a significant
reduction in slip rate across the intersection of the Brown Mountain, Owl Lake, and Garlock
faults. Digitally-measured offsets and their age groups were plotted in order to achieve
preliminary slip-rate estimates. The resulting plot confirms an eastward decrease in late
Pleistocene-Holocene slip rate at sites along the CGF and EGF. The CGF slip-rate estimate
taken from Slate Range West (SRW) and Slate Range East (SRE) sites in Pilot Knob Valley
was approximately 4.2 mm/yr, within error of previously published rates. The slip-rate
estimate from the Quail Mountains site (QMTNS), at the easternmost extent of the CGF,
was approximately 2.7 mm/yr. The slip-rate estimate from the Avawatz (AVA) section of the
EGF was approximately 1.0 mm/yr.
METHODS
Data Utilized: Large quantities of LiDAR (Light Detection And Ranging) data have been
made readily available by OpenTopography.org. The utilization of these high-resolution
DEM (Digital Elevation Model) datasets, in conjunction with satellite imagery and geologic
maps, allowed offset features along the Garlock fault to be analyzed, mapped, and
measured. Features with measurable lateral offsets were named according to their location
from west to east. Each offset measurement was given an error and quality rating based on
the confidence in constraining its boundaries, potential lack of preservation, or ambiguity
of its genesis
Relative Dating: All of these sites are located on military bases, which has made access
difficult. In the absence of quantitative dating methods such as radiocarbon or optically
stimulated luminescence (OSL) dating, qualitative techniques have been employed.
Physical characteristics examined in satellite and LiDAR imagery, including alluvial terrace
height, shape, dissection, darkness, and surface smoothness/roughness have been utilized
as a proxy for relative age of alluvial surfaces. All of these parameters were assessed for
each site and this permitted the distinction of set groups of relative ages for all surfaces
observed (see Table 1 for comparison of referenced unit ages). By assigning each measured
surface to a relative age group, variations in slip rate along strike can be examined.
Terrace Height: Relative ages of stream terraces are commonly assigned by height above
adjacent stream channels due to the process of younger channels down cutting and
infilling into older surfaces (Bull, 1991; Stoffer, 2004). Strike-slip faulting along the Garlock
has allowed the downstream portion of channels to be displaced and cut from their
source. This process tends to form ideal series of abandoned and raised surfaces. Sites
displaying a number of sequentially preserved terraces are highly valuable as they
generally record numerous total offsets for a variety of time scales.
Shape and Dissection: Low-lying, active and recently active channels show very strong bar
and swale topography. Intermediate-aged abandoned surfaces have sharp risers cut by the
adjacent modern channels, may show subdued bar and swale topography or have a very
flat surface indicative of desert pavement development, and minor incisions may be
present. Older surfaces have a terrace riser which is more subdued than those of
intermediate surfaces as the flat surface becomes more rounded in cross section, although
this is dependent on the recency of erosive events adjacent to the terrace. The oldest
surfaces appear very rounded to where the riser and surface may be indistinguishable.
Little preserved surface remains at this age and the material is dominated by incisions and
is recognized as ridge and ravine topography (Bull, 1991). Topographic profiles were
generated using DEM data in order to analyze the overall shape and degree of incision of
each surface in this study.
Surface Darkness: Arid environments such as the Mojave Desert exhibit rock varnish
accumulation on the surface of pebbles and cobbles over time. As desert pavements
develop, the varnish accumulates and darkens the surfaces of the clasts forming the
pavement. Varnish is easily destroyed by abrasion during stream transport (Bull, 1991).
Therefore a stable surface is required for varnish accretion. On Google Earth and National
Agriculture Imagery Program (NAIP) imagery, the varnished pavements contrast with the
light-colored fresh alluvium in active and recently active channels. There is a general
progression of darkening of preserved alluvial surfaces over time. Surfaces will tend to
become darker until new drainage incisions develop and destroy the stable desert
pavements.
Smoothness/Roughness: The smoothness of surfaces (or conversely, roughness) can be
quite apparent in LiDAR hillshade images. Modern channels and young surfaces have
significant bar and swale topography, along with creosote bushes strewn throughout. As
surfaces age and are offset and uplifted by a fault, bar and swale topography becomes
progressively more subdued. Vegetation becomes sparser as well. The net effect is an
increase in smoothness, or decrease in roughness, of alluvial surfaces over time.
Correlation with Dated Surfaces: Comparison of the relative age groups with published
studies of dated surfaces also allows tentative absolute ages to be inferred and rough
estimates of slip rate to be calculated, which must be verified in future field studies with
direct dating techniques. In Table 1 we show the relationship between our assigned units
(column 1) and those of published studies for which absolute ages have been measured or
suggested (columns 3-7). Our unit labels and inferred absolute ages are taken from Helms
et al. (2003) for the Holocene and latest Pleistocene surfaces (Qal0 – Qal6), and are also
consistent with absolute ages determined by Rittase (2014). For older surfaces (Qal7-Qal9)
our age inferences are guided by Miller (2007) and by Rittase (2014).
Site Comparison: By thoroughly mapping, measuring, and assessing offset features based
on multiple characteristics analyzed remotely, the sites with well-constrained lateral offsets
were carefully compared. Once the relative ages of surfaces were established qualitatively,
and offset measurements of said surfaces were measured digitally and justified, the data
were plotted on a scatter diagram showing the inferred age versus the amount of left-
lateral offset (Figure 7).
Absolute Dating: In June 2014 we obtained access to the SRE site on the China Lake Naval
Air Weapons Station and were able to collect samples for OSL dating from Qal3 and Qal4
(Figure 3). OSL dating results should be available later this year, which will allow calculation
of a firmer slip rate from these units at SRE.
INTRODUCTION
The focus of this study is offset features along the central (CGF) and eastern (EGF)
segments of the Garlock fault. Particular focus is in Pilot Knob Valley, the easternmost
extent of the CGF, as well as near the Avawatz Mountains, the easternmost extent of the
EGF. The CGF and EGF are separated by the intersection of the Brown Mountain and Owl
Lake faults with the Garlock fault, just east of the Quail Mountains (Figure 1). This
intersection is characterized by a widening of the fault zone with many splays showing
primarily vertical displacement. Beyond this intersection the EGF trends due east for
another 50 km as a 5 km wide zone of multiple splays, although recent left-lateral slip
appears to be concentrated within a narrower zone, known as the Leach Lake strand of the
Garlock fault (Clark, 1973; Brady, 1986).
Little is known of the earthquake history or slip rate of the EGF and studies of the CGF have
been mostly limited to areas west of Pilot Knob Valley (McGill and Sieh, 1993; Dawson et
al., 2003; Madden and Dolan, 2008; McGill et al., 2009; Ganev et al. 2012). Piecing together
the spatial variations in slip rate and slip-rate history of these segments may help constrain
the tectonic significance of the Garlock fault (Hill and Dibblee, 1953; Davis and Burchfiel,
1973; Humphreys and Weldon, 1994; McGill et al., 2009).
Figure 2. Slate Range West (SRW) site
Figure 7. Scatter plot of Pilot Knob Valley (PKV, sites SRW and SRE combined), Quail Mountains
(QMTNS), and Avawatz (AVA, Avawatz 2 and 3 combined). Linear regression lines were fitted to
each series of points.
CONCLUSIONS
This study has shown through remote, qualitative comparisons that the late Pleistocene-
Holocene slip rate of the Garlock fault appears to decrease between the central and
eastern segments. The slip rate estimates were found to be 4.2 mm/yr for Pilot Knob Valley
and 2.7 mm/yr for Quail Mountains in the Central Garlock fault, and 1.0 mm/yr for Avawatz
in the Eastern Garlock fault. This is significant in helping constrain the role of the Garlock
fault in partitioning slip between the San Andreas fault and the Eastern Calfiornia Shear
Zone (ECSZ). Past studies attempted to understand the Garlock fault’s history and tectonic
role (Hill and Dibblee, 1953; Davis and Burchfiel, 1973; Guest et al., 2003; McGill and Sieh,
1991). Davis and Burchfiel (1973) suggested that total displacement of the Garlock fault
must increase westward from its eastern end, based on recognized features offset by tens
of kilometers. The findings of this study are in agreement with Davis and Burchfiel’s (1973)
observations.
Optically stimulated luminescence ages will soon be available (from Ed Rhodes and his
students) for Qal3 and Qal4 at SRE and will allow firmer estimates of slip rate to be
calculated for this location and time period. OSL sampling of units of other ages and at
other locations is needed to confirm the preliminary results presented here.
REFERENCES
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Table 1. Unit labels and inferred ages used in this study (columns 1 and 2) in comparison to
correlative units in other studies.
DISCUSSION
As relative unit ages were assigned based on qualitative analyses of preserved surfaces, the
measured offsets corresponding with those unit ages were compared across sites. SRW
and SRE in Pilot Knob Valley, and QMTNS at the eastern extent of the CGF, all display units
of multiple ages with measurable offsets. These successively preserved units are ideal for
measuring the average offset over various timescales. In the Avawatz section of the EGF
these ideal sequences were absent. Rather, various ages of preserved and offset surfaces
were found throughout the Avawatz section. Individual sites pertinent to this study often
only exhibited one or two offset units. This may be due in part to greater topographic relief
near the Avawatz mountains, causing less consistent drainage paths and more irregular
deposition than what is found in Pilot Knob Valley.
The majority of offsets found along the EGF are terrace risers or bent drainages incised into
older units. Measurable left-lateral offsets of younger units were rare, or unrecognizable
due to resolution limits of the LiDAR data. Offsets smaller than 3 m are virtually
indistinguishable from bar and swale or other topographic variability, although vertical
scarps are visible in units as young as Qal2 throughout the EGF. This suggests the most
recent earthquakes had a component of vertical slip and may have left-lateral deformation
of up to 3 m, as recorded in previous studies (McGill and Sieh, 1991). Similar scarps are
found in young units throughout the CGF as well, implying the most recent earthquake
event may have occurred across the CGF and EGF together, but either a greater total left-
lateral slip has accumulated per event or a greater quantity of events have occurred in the
CGF than the EGF.
Directly comparing units determined to be approximately the same age shows significant
differences in total accumulated slip dependent on the location of the site along the
Garlock fault. Figure 7 is a plot of qualitatively estimated age vs measured offsets. Although
many more offsets were found and measured, these were the offsets most confidently
attributed to qualitatively established unit ages, a prerequisite for comparison. X-axis error
bars span the ranges of time within each assigned unit age (which can be referenced in
Table 1), and Y-axis error bars were determined by digital measurement in ArcGIS. Linear
regressions were formed for data series PKV (sites SRW and SRE), QMTNS, and AVA (AVA2
and AVA3 site areas combined). The geographic distribution of these sites along the CGF
and EGF can be seen in Figure 1. The distinct regression lines for each of the three regions
clearly show a decrease in average estimated slip rate from west to east.
The oldest offset unit observed in this study is the Qal8 unit of AVA2 (Figure 5). By
reconstructing the western and eastern margins boundaries of Qal8, offsets AVA2-G-6 and
AVA2-G-7 were measured to be 77 – 134 m and 50 – 120 m respectively. Although offsets
on the order of ~ 100 m are observed in PKV and QMTNS, they are found in much younger
units.
Offsets of unit Qal7 were found in both QMTNS and AVA, but not PKV. QMTNS-4 and
QMTNS-5 (Figure 4) offsets were measured to be 95 ± 15 m and 100 ± 15 m, respectively.
The Qal7 unit offsets of AVA were much smaller at 40.2 ± 10 m for AVA2-G (Figure 5), 36.0
± 5 m for AVA3-E (Figure 6), 37.4 ± 5 m AVA3-E (Figure 6), and 38 – 45 m for AVA3-F
(Figure 6). An additional Qal7 offset of 40.2 ± 5 m was measured at AVA3-M, but is not
shown in these figures. Thus, features deemed to be of Qal7 age in the Quail Mountains
are offset more than twice as much as features of similar apparent age in the Avawatz
Mountains.
Offsets of unit Qal6 were found in all three regions. In PKV, offset PKVW-A-1 (Figure 2)
measured 95.5 ± 5 m. The QMTNS Qal6 offset, QMTNS-3, is nearly half the size of PKV,
measuring between 41 – 68 m (Figure 4). Multiple Qal6 offsets are found within the AVA
series, and all are far smaller than PKV and QMTNS. AVA2-E-3 (Figure 5) is 8.3 – 19.9 m and
AVA2-F-1 (Figure 5) is 11.6 ± 2 m. The east-facing riser of AVA3-K-1 is 9.6 ± 3, but is not
shown in these figures.
Similar relationships between the regions were found in offsets of unit Qal5. PKVW-A-1
(Figure 2) measured 43.6 ± 5 m and QMTNS-2 (Figure 4) measured 23.3 ± 3 m, whereas in
AVA there are two Qal5 offsets: AVA2-B-1, a west-facing riser offset 9.9 ± 3.0 m and AVA3-
L-5, an east-facing riser offset 6.2 ± 2.4 m (not shown in figures).
Offsets of surfaces younger than Qal5 are limited in PKV and QMTNS, while absent in AVA.
The Qal3 offset PKVE-2 (Figure 3) measured 18.3 ± 2 m, four times greater than the Qal3b
offset of QMTNS-1 (Figure 4) measuring 4.1 ± 1.5 m, showing a decrease in offset per unit
age toward the east in the youngest units.
Figure 3. Slate Range East (SRE) site. Orange dots show approximate locations of OSL pits
shown
Figure 4. Quail Mountains (QMTNS) site
Figure 5. Avawatz 2 (AVA2) E, F, and G sites. Sites AVA2-A through AVA2-D are west of this areaFigure 1: Regional view of the study area spanning the Central (CGF) and Eastern (EGF) Garlock
fault. The intersection of the Brown Mountain fault (BMF) and Owl Lake fault (OLF) with the
Garlock fault represents the boundary between the CGF and EGF. The diamonds represent the
approximate locations of sites shown in Figures 2 – 5
Figure 6. Avawatz 3 (AVA3) E, and F sites. Sites AVA3-A through AVA3-D are west of this area,
and AVA3-G through AVA3-P are east of this area
Measurement Name Offset (m) Unit Description
PKVW-A-1 43.6 ± 5 Qal5 West-facing riser of Qal5 offset from primary drainage
PKVW-B-3 95.5 ± 5 Qal6a West-facing riser of Qal6a offset from primary drainage and recaptured by young drainage
Measurement Name Offset (m) Unit Description
PKVE-1 6.7 ± 1 Qal2 West-facing riser of Qal2 to east edge of primary drainage
PKVE-2 18.3 ± 2 Qal3 West-facing riser of Qal3 to east edge of primary drainage
PKVE-3 30.6 ± 3 Qal4 West-facing riser of Qal4 offset across the fault
PKVE-4 41.5 – 45.9 Qal4 Eastern edge of remaining Qal4 offset across the fault, potential maximum
Measurement Name Offset (m) Unit Description
QMTNS-1 4.1 ± 1.5 Qal3b West-facing riser of Qal3b
QMTNS-2 23.3 ± 3 Qal5 West-facing riser of Qal5
QMTNS-3 41 – 68 Qal6 West-facing riser of Qal6 upstream to minimum and maximum potential downstream
riser position
QMTNS-4 95 ± 15 Qal7 West-facing riser of Qal7 upstream to projected probable location of downstream riser
QMTNS-5 100 ± 15 Qal7 East-facing riser of Qal7 to potential Qal7 remnant across large modern wash
Measurement Name Offset (m) Unit Description
AVA2-E-3 8.3 – 19.9 Qal6 East-facing riser of Qal6 offset
AVA2-F-1 11.6 ± 2 Qal6 East-facing riser of Qal6 offset
AVA2-G-5 40.2 ± 10 Qal7 East-facing riser of Qal7 downstream offset from large modern drainage
AVA2-G-6 77 – 134 Qal8 Range of total offset possible along the western edge of Qal8 unit
AVA2-G-7 50 – 120 Qal8 Range of total offset possible along the eastern edge of Qal8 unit
Measurement Name Offset (m) Unit Description
AVA3-E-1 36 ± 5 Qal7 West-facing riser of Qal7 offset from highly eroded upstream unit
AVA3-E-2 37.4 ± 5 Qal7 East-facing riser of eroded Qal7 downstream offset from highly eroded upstream unit
AVA3-F-1 38 – 45 Qal7 East-facing riser of Qal7 offset from large riser of highly eroded upstream unit
ACKNOWLEDGEMENTS
This work was supported by SCEC awards #14217, 13073 and 12174. We thank Mike
Baskerville for assistance in coordinating access to site SRE in the China Lake Naval Air
Weapons Station. We thank Ed Rhodes, James Dolan, Robert Zinke and Nicolette Grill for
field assistance at that site.