Bulletin of the Seismological Society of America, Vol. 78, No. 2, pp. 956-978, April 1988
FREQUENCY AND SIZE OF QUATERNARY SURFACE RUPTURES OF
THE PITAYCACHI FAULT, NORTHEASTERN SONORA, MEXICO
BY WILLIAM B. BULL AND PHILIP A. PEARTHREE
ABSTRACT
Movements along the Pitaycachi fault since the Miocene juxtaposed different
alluvial units and created 2- to 45-m-high fault scarps downslope from a pedimented mountain front prior to 1887. In 1887, a major earthquake formed a 75km-long, ~- to 4-m-high scarp along the trace of prehistoric surface ruptures.
Diverse evidence from many study sites indicates that about 200,000 yr elapsed
between the prior (youngest Pleistocene) event and the 1887 surface rupture.
Cumulative disPlacements of Pliocene(?) to mid-Pleistocene alluvial fans and
stream terraces decrease with decreasing age. The trace of the prior rupture
was largely buried by sheets of late Pleistocene and Holocene piedmont alluvium.
Late Pleistocene soils are offset about the same amount as the height of the
1887 scarp. Valleys that are as much as 40 m deep and 0.5 to 0.9 km wide have
been eroded since the prior event; they contain multiple late Pleistocene and
Holocene stream terraces that were not faulted until 1887. Pre-1887 alluvial fault
scarps were degraded to 2 ° to 9 ° slopes before the 1887 event, even in resistant
materials such as clay-rich soil horizons with unweathered rhyolite cobbles and
calcrete. Scarp height-maximum slope regressions and diffusion-equation analyses for reconstructed pre-1887 scarp profiles indicate that the prior event
occurred more than 100,000 yr ago. Acceleration of scarp degradation rates
during the Holocene, and/or relatively resistant materials exposed in the scarps,
would increase the age estimates to 200,000 yr or more.
Very long recurrence intervals are the characteristic style of movement on the
Pitaycachi fault. At one site, six ages of diverse valley fills were inset into
pedimented granodiorite upslope from the fault between the prior and 1887
events. Only 3 m of relief remained before the 1887 rupture increased the scarp
height from 3 to 6 m. Some hillslopes have triangular talus facets of carbonatecemented colluvium that resulted from infrequent fault movements and intervening periods of erosion. Smooth hillsides of resistant volcanic rocks between the
facets show that virtually all of the prior surface-rupture event scarps had been
removed by prolonged slope degradation.
INTRODUCTION
The unusual Pitaycachi fault is interesting to earth scientists and planners. It is
unusual because the great Sonoran earthquake of 1887 occurred in a region otherwise
devoid of historical surface-rupturing earthquakes, and interesting because the 75
km rupture may be the maximum length known for historic normal faulting events.
This major seismic event demonstrates the potential for large earthquakes on
similar Basin and Range faults in adjacent areas of Sonora, Chihuahua, New Mexico,
and Arizona. Unlike many historical ruptures in the Western United States, there
is no evidence along the Pitaycachi fault for either Holocene or late Pleistocene
surface ruptures and, thus, large magnitude earthquakes. How does one assess the
potential for future seismic hazards in a region with such faults? One approach is
to determine the frequency and sizes of prehistoric surface ruptures.
We used standard techniques such as age estimates based on: (1) soil-chronosequences developed on flights of piedmont alluvial fans and terraces and (2) analyses
of systematic degradation of fault scarps in alluvium since their time of formation.
956
QUATERNARY SURFACE RUPTURES OF THE PITAYCACHI FAULT
957
We also described qualitative characteristics of landscapes along the Pitaycachi
fault that indicate a great antiquity for the surface rupture that preceded the historic
earthquake and very slow, long-term uplift rates. Our studies document the stratigraphic displacements and scarp heights associated with the 1887 and prehistorical
surface-ruptures, and indicate that the 1887 rupture trace has been the site of
multiple--but very infrequent--large fault movements during the Quaternary. We
use the following temporal terms: 1 ky = 1,000 yr; 1 ka = 1 ky before present; 1
my = 1,000,00 yr; 1 Ma = 1 my before present (North American Commission on
Stratigraphic Nomenclature, 1983, Article 13; Colman et al., 1987). Subdivisions of
the Quaternary are assigned the following ages in ka: Holocene, 10 to 0; late
Pleistocene, 125 to 10; middle Pleistocene, 790 (Johnson, 1982) to 125; and early
Pleistocene, 1800 to 790.
GEOGRAPHIC, GEOLOGIC, AND SEISMIC SETTINGS
The 1887 surface rupture occurred in the San Bernardino Valley of northeasternmost Sonora, Mexico, which lies on the western fringe of the Chihuahuan Desert.
The southern 25 km of the 1887 scarp is within the Sierra Pilares de Teras
(Goodfellow, 1888); the northern 15 km cuts across piedmont alluvial surfaces
(Figures 1 and 8) to within 12 km of the Arizona border, and the central 35 km is
at the western base of the Sierra de San Luis. Thick sequences of rhyolitic and
other volcanic rocks are common in the mountains, and granitic rocks and limestone
occur in the Javelina and Capadero areas, respectively.
The Mw 7.2 to 7.4 earthquake of 1887 (DuBois and Sbar, 1981; Herd and
McMasters, 1982; Natali and Sbar, 1982) occurred along a Basin and Range fault
in a region of historically low seismicity. DuBois and Smith (1980) analyzed the
intensity patterns and summarized the historical accounts of the 1887 earthquake.
Data collected in the San Bernardino Valley in 1978 to 1979 show continuing
microseismic activity in the vicinity of the 1887 event (Natali and Sbar, 1982).
Quaternary fault scarps occur in adjacent portions of Arizona (Pearthree and Calvo,
1987) and New Mexico (Machette and McGimsey, 1982; Machette, 1986; Machette
et al., 1986).
Soil-profile characteristics were used to estimate ages of alluvial geomorphic
surfaces. The Las Cruces, New Mexico, soils study of the U.S. Soil Conservation
Service and soils studies of similar regions (Bachman and Machette, 1977; Gile and
Grossman, 1979; Gile et al., 1981) are relevant to this study because the climate,
parent materials, topography, and biota are similar to those of the Pitaycachi fault
area. Las Cruces is about 250 km northeast of the Pitaycachi area. The altitudes
and climate for these two areas and Douglas, Arizona (located 50 km to the
northwest of the fault), are compared in Table 1. Two-thirds of the precipitation
occurs as summer rains in both the Pitaycachi and Las Cruces areas. Rates of input
of atmospheric dust are important in the genesis of argillic and calcic soil horizons
(Machette, 1985); Quaternary rates appear to be temporally variable, but large, for
both study areas. At higher altitudes in both areas, conditions were marginal for
net accumulation of soil carbonate during some Pleistocene wet intervals. The
similarities in factors that affect rates of soil formation between the two study areas
allow tentative correlation of soils of the Las Cruces and Pitaycachi areas.
THE PITAYCACHI FAULT
The potential for large earthquakes in the southern Basin and Range Province
indicated by the 1887 earthquake stimulated studies of historical and Quaternary
958
WILLIAM B. BULL AND PHILIP A. PEARTHREE
N
31 °
KiLometers
o~=-=- =--=~,o
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EXPLANATIOI
Quaternary and
Tertiary Sedi
Undifferentiated
1887 Surface RL
109 °
FIG. 1. Map showing place names and 1887 surface ruptures. Base map showing the main 1887
surface ruptures was providedcourtesyof D. G. Herd, U.S. GeologicalSurvey.
surface ruptures along the Pitaycachi lhult. T he fault was named after a prominent
local peak at the suggestion of Douglas Shakel (Pima Community College, Tucson,
Arizona, oral communication, 1977). Studies of the 1887 rupture were made by
Aguilera (1888, 1920), Gianella (1960), and Sumner (1977). Detailed maps that
show the distribution of fault scarps in the San Bernardino Valley have been made
by D. G. Herd (U.S. Geolological Survey, written communication, 1981).
QUATERNARY SURFACE RUPTURES OF THE PITAYCACHI FAULT
959
The central two-thirds of the 1887 scarp is 2~ to 4 m high, but south of Rio de
Bavispe and north of Arroyo de los Embudos the scarp typically is only ½to 1½ m
high. The trace of the 1887 faulting is especially prominent where graben formation
or subsidence due to fissuring of the downthrown block has concentrated local
runoff, which has promoted dense vegetation. Fault displacements can be determined at many localities by measuring the vertical offset of soil-horizon boundaries
or bedded alluvium across the fault. In addition, scarp heights approximate maximum displacements on steep slopes. Representative scarp heights are noted in
Figure 2, but net tectonic displacements may be less than scarp heights because of
local subsidence or warping of the downdropped block materials.
The morphologies of the 1887 scarps are highly dependent on the materials that
have been ruptured as well as on height. The scarp shown in Figure 3 consists
primarily of a free face in carbonate-cemented gravel, but has a well-defined debris
TABLE 1
COMPARISONS OF ALTITUDE AND PRESENT CLIMATE AT
DOUGLAS, ARIZONA, LAS CRUCES, NEW MEXICO, AND ALONG
THE PITAYCACHI FAULT
Mean Annual
Altitude
Douglas
Las Cruces
Pitaycachi (estimated)
."
(m)
Temperature
(°C)
Precipitation
(ram)
1200
1100-1500
1100-1300
17.3
15.6-14
18-16
300
200-350
250-300
(4A . . . . " J ....
i
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_.' ~.~',"(9)
-
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~
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Escarpa Vleja
~?~
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:
Escarpa
"I (5~
Antigua /
Javelina
....
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-
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.......
12%
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f
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~"' / ~''''0-'''~'''''''-'--,.''~~ heightinmeters
i/ '
4J (4B)Figurelocatiors
FIG. 2. Map showing informally named study sites (bold lettering) and heights of 1887 scarps in
meters; locations of illustrations are shown by figure number in parentheses.
960
WILLIAM B. BULL AND PHILIP A. PEARTHREE
:
4m
FIG. 3. The 4-m-high scarp in massive carbonate-cemented gravel south of Arroyo Capadero is
dominated by a large free-facetopographic element. Blocks of cemented gravel lie on the debris-slope
topographic element.
slope. Debris slopes are better developed On scarps of unconsolidated gravel or
sandy gravel.
Three representative profiles of scarps formed by the 1887 event are shown in
Figure 4. Subsidence immediately downslope from the scarp (Figures 4b and 5) may
be the result of graben formation, fissuring, or both. In Figure 4b, the prefaulting
slope is 2 ° downslope from the fault zone, but is 9 ° upslope from the zone. This
convex 9 ° slope is the remnant of a 2-m-high, gentle hillslope--a much degraded
scarp resulting from a prior and much older surface rupture than the 1887 rupture.
EVIDENCE FOR RECURRENT FAULT RUPTURES
Fault-zone characteristics
Observations of fault-zone characteristics demonstrate the occurrence of multiple
ruptures, give clues regarding frequency of rupture and cumulative displacement,
and provide important information about tectonic deformation of materials as they
influence fault-scarp morphology. The 60 ° to 70 ° west-dipping fault zone is exposed
in a 6-m-high bank of Arroyo Hondo; a cross-section of the stratigraphy and
structure is shown in Figure 6. Two markedly different alluvial sequences with
different deformational styles have been juxtaposed by faulting and are capped by
a.) Bolsa site
/
~
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15
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b.) Javelina site
10
E 4
2 °
0.2 m 1887 ~ p t u r e ~ 2 . 4 o
1.4°
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2
3.6°
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40
.
110
.
80
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5°
.
.
.
.
.
120
2j
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30
40
50
.
160
Distance,
240
200
rn
Distance, m
c.) Facet site
20E
15-
&5o / ~ . ~ ' ~ "
26°
10-
/
,1-
5
0
0
20
4.5o
~o
/o
i
i
i
i
40
60
80
100
Distance,
i
120
i
140
160
m
FIG. 4. Topographic profiles of the 1887 fault scarp at sites with multiple ruptures. (a) 0.2 m scarp
in 3.2-m-high scarp superposed on clayey gravels at the Bolsa site. (b) 3.9 m scarp in carbonate-cemente?
gravels at the Javelina site. (c) 3.5 m scarp in clayey gravels at the Facet site.
FIG. 5. Aerial view southeast of the 1887 scarp north of Arroyo Capadero. Concentration of local
runoff has promoted a dense growth of vegetation in trough of local subsidence immediately downslope
from the scarp. Such subsidence may be associated with fissuring such as illustrated in Figure 6 or may
reflect the presence of antithetic faults and local graben formation.
961
962
WILLIAM B. BULL AND PHILIP A. PEARTHREE
Coarse gravel
~
Mediumgrainedwater laid gravel
Pleistocene
Medium gravel
~
Debris-flowdeposit
[
1
2
~'~,,
~
Fault
Fracture,open or
partly filled with
organic matter
~
Boulder
!
Fine gravel
Pebbly sand
/
~
FI~. 6. Cross-sectionof Pitaycachifault and alluvialdepositsexposedin southbank of ArroyoHondo.
The 1887 rupture was along the left fault. The other two faults and greatlydifferingalluvialsequences
on opposite sides of the fault zone are evidence for multiple fault ruptures and large cumulative
displacement. Stream erosion has removedmuch of 1887 scarp alongline of section.
bouldery Holocene gravel. Gray-brown (7.5YR 7/2) indurated water-laid alluvium
of Pleistocene age comprises the upthrown block. Drag-folding has not occurred,
and closed fractures parallel the 1887 rupture plane. Red-brown (5YR 5/6) alluvium
of Pleistocene age exposed in the downthrown block consists of water-laid gravels
and debris-flow deposits. A sample of the debris-flow matrix, which is plastic when
wet, contained 20 per cent silt and 22 per cent clay. Two pre-1887 rupture planes
do not penetrate the capping Holocene alluvium, and drag-folded beds abut the
faults. The downthrown block also has numerous vertical fissures, largely filled
with loose sand and fresh-appearing organic debris, that do not extend from the
cohesive reddish clayey Pleistocene alluvium into the overlying noncohesive, sandy
Holocene gravel.
The structures associated with the faulting shown in Figure 6 represent three
different styles of deformation near a major fault. Minor fractures in the upthrown
block that parallel the main 1887 rupture plane were probably generated by tensional
stresses during the 1887 event, as the basinward block moved away and down from
the relatively stable "upthrown block." The drag-folding associated with the prehis-
QUATERNARY SURFACE RUPTURES OF THE PITAYCACHI FAULT
963
toric faults in the downthrown block may represent soft-sediment deformation of
saturated deposits during Pleistocene fault movement(s). The open fissures (shown
in black in Figure 6) on the downthrown block represent a more brittle style of
deformation during the 1887 event that affected cohesive dry deposits well above
the water table. The orientations of these fissures are appropriate for extensional
fractures (Davis, 1984, p. 272) if the dip of the fault becomes slightly less at depth.
Pervasive fissuring during the 1887 event, and soft sediment deformation during
the prior event, reflect maximal extension of materials close to the fault zone on
the downthrown side; they may be major causes of surface subsidence (commonly
referred to as backtilting) near normal faults which generally are not exposed as
well as at the Arroyo Hondo site. At the Arroyo Hondo site, the two different styles
of deformation of the downthrown block, faults of different ages, and different
juxtaposed stratigraphies all indicate multiple prehistoric ruptures and substantial
cumulative displacement by the Pitaycachi fault.
Pre-1887 fault scarps
Our studies of the pre-1887 fault scarps demonstrate a long history of infrequent
prehistorical movements of the Pitaycachi fault, The time that has elapsed since
the surface rupture that preceded the 1887 event (hereafter referred to as the prior
surface-rupture event) is evaluated in three ways: (1) estimated ages and fault
displacements of soils formed on alluvial geomorphic surfaces; (2) analyses of
topographic profiles of pre-1887 fault scarps; and (3) genera! appraisals of the time
needed to form erosional and depositional elements of faulted landscapes.
Fault scarp height-surface age relationships. Single-rupture-event scarps are simple compared to multiple-rupture event scarps, which are a composite of two or
more times of surface rupture; both are common along the trace of the Pitaycachi
fault. Single-rupture-event scarps are characterized by extensive preserved original
surfaces adjacent to the fault zone, with tectonically induced erosion or deposition
restricted to the immediate vicnity of the 1887 rupture trace. Displaced remnants
of original piedmont surfaces commonly have the same slope.
Multiple-rupture-event scarps typically have steeper slopes on the upthrown than
on the downthrown sides of the 1887 trace (Figure 4a), which generally occurs near
the bases of preexisting subdued scarps (Figure 4, a and b). Previous surface ruptures
induced erosional retreat of the scarp crests into the original surfaces upslope from
the fault, resulting in convex scarp slopes that are steeper than the original surfaces.
This erosion was accompanied by depositional steepening downslope from the fault
as scarp-derived colluvium adjacent to the fault and a wash slope of alluvium that
accumulated as a new alluvial surface on the downslope original surface. Thus,
remnants of preserved original surfaces that are unaffected by erosion or deposition,
if still present, are relatively distant from the fault zone (scarp-crest widths are 60
to 200 m) compared with distances to original surfaces associated with singlerupture-event scarps (scarp-crest widths are 2 to 5 m). When viewed normal to the
fault trace, multiple-rupture-event scarps appear as gentle slopes, with abrupt basal
steepening caused by the 1887 displacements. The historical surface rupture caused
a segmented scarp profile. Repeated episodes of tectonically induced scarp erosion
tend to result in multiple segments of topographic profiles of scarp crests, new
alluvial slopes on the downthrown side of the fault, and renewed aggradation on
alluvial fans deposited by small streams that breach the scarp.
Soil-profile characteristics can be used to estimate ages of alluvial surfaces faulted
by single or multiple events and provide a way of bracketing the general times of
964
WILLIAM B. BULL AND PHILIP A. PEARTHREE
the prior rupture event. Calcium carbonate, iron oxides, and clay accumulation
within soil profiles of semi-arid regions (Figure 7) are particularly useful indicators
of soil age.
The extensively studied and relatively well-dated soils near Las Cruces, New
Mexico, have parent materials and profile characteristics similar to those of the
Pitaycachi area. Holocene alluvium in both study areas tends to be less gravelly
than the Pleistocene alluvium. Brownish gray B horizons characterize the Holocene
soils in contrast to the bright reddish-brown of the Pleistocene B horizons which
FIO. 7. Late Pleistocene typic haplargid paleosol capped by Holocene deposits in the north bank of
the Huella de los Caballos. The upper part of the calcic horizon (IIBkm) has a stage III carbonate
morphology. The overlying truncated argillic horizon (IIBt) has a strong angular blocky structure and
thick peal argillans; it is about 30 cm thick. A layer of lag gravel is present at the base of the Holocenc
sands (ICk). Survey rod is numbered in decimeters (10 cm).
QUATERNARY SURFACE RUPTURES OF THE PITAYCACHI FAULT
965
are rich in iron oxides. We assume that the minimum time needed to attain a given
stage of carbonate morphology is about the same in the two soil study areas. Stages
of carbonate morphology are summarized in Table 1. In noncalcareous gravels at
Las Cruces, about 2 to 4 ky are needed to accrete stage I coatings on pebbles and
cobbles, 10 to 40 ky for stage II, 120 to 200 ky for stage III, and roughly 500 ky for
stage IV (Gile and Grossman, 1979; Gile et al., 1981; Machette, 1985). The times
needed to develop these stages may be less for gravel with limestone clasts.
Comparison of carbonate accumulation between the two areas is complicated by
the fact that late Pleistocene climates in the Pitaycachi area favored leaching of
carbonate from soil profiles, except where calcareous lithologies are present. Early
Pleistocene soils have stage V to stage VI petrocalcic horizons where carbonate is
present in the parent material, whereas soils in similar topographic positions in the
landscape that lack carbonate parent materials typically have minimal carbonate
accumulation, but have thick and strongly developed argillic horizons.
Approximate ages were assigned to the single- and multiple-rupture-event alluvial
geomorphic surfaces by evaluating the degrees of development of both argillic and
calcic horizons (Table 2). Holocene and late Pleistocene surfaces have been ruptured
only by the 1887 event. Surfaces of early(?) and mid-Pleistocene age also were
ruptured by the prior event at the Bolsa, Arroyo de los Embudos, Capadero, and
Javelina localities; scarp heights ranging from 2 to 8 m are sums of the 1887 and
prior rupture events. Thus, the prior surface-rupture event occured after deposition
of early and mid-Pleistocene alluvium and before deposition of late Pleistocene
alluvium. Comparison of the soils in the Pitaycachi area with the semi-arid upper
piedmont soils of the Las Cruces area indicates that the late Pleistocene surfaces
are 100 to 200 ka and that the mid-Pleistocene surfaces are about 200 to 500+ ka.
TABLE 2
STAGES IN THE MORPItOGENETIC SEQUENCE OF SOIL CARBONATE ACCUMULATION IN GRAVELLY
ALLUVIUM (AFTER MACHETTE, 1985, e. 5)
Stage
Diagnostic Morphology
Distributionof CaC03
Calcic Soils
I
Thin, discontinuous coatings on pebbles,
usually on undersides.
Coatings sparse to common.
II
Continuous, thin to thick coatings on
tops and bottoms of pebbles.
Coatings common, some carbonate in
matrix, but matrix still loose.
III
Massive accumulations between clasts;
becomes cemented in advanced form.
Essentially continuous dispersion in matrix.
Pedogenic Calcretes
IV
Thin (<0.2 cm) to moderately thick (1
cm) laminae in upper part of horizon.
Thin laminae may drape over fractured
surfaces.
Cemented platy to weak tabular structure
and indurated laminae. Horizon is 0.51 m thick.
V
Thick laminae {>1 cm) and small to
large pisolites. Vertical faces and fractures are coated with laminated carbonate (case-hardened surface).
Indurated dense, strong platy to tabular
structure. Horizon is 1-2 m thick.
VI
Multiple generations of laminae, breccia,
and pisolites; recemented. Many casehardened surfaces.
Indurated and dense, thick strong tabular
structure. Horizon is commonly >2 m
thick.
966
WILLIAM B. BULL AND PHILIP A. PEARTHREE
The estimated minimum age of the prior event is 100 ka, based on soil information
and may be as old as 500 ka. Thick, very red, Bt horizons, with >40 per cent clay
and stage IV carbonate (Table 3) are pedogenic features that develop over 200 to
500 ky time spans from sandy gravel parent materials.
The mid-Pleistocene surfaces record only one pre-1887 surface-rupture event,
and displacements calculated from ~reconstructed pre-1887 scarps (1.0 to 3.6 m)
indicate that the prior event had surface offsets similar to those of the 1887 event.
If late Cenozoic displacements along the central part of the Pitaycachi fault
characteristically were 2 to 3 m, the early Pleistocene Escarpa Vieja surface, which
is offset roughly 15 m, records as many as five surface-rupture events. The even
older 43-m-high Escarpa Antigua may have required 5 my to form if major surfacerupture events have occurred at average return periods of 300 to 500 ky
300 ky return period
x 43 m total displacement
= 4.9 Ma).
2.5 m characteristic displacement
The cumulatively greater displacement of alluvial geomorphic surfaces with increasing age demonstrated by Table 2 shows that episodic faulting has occurred during
much of the late Cenozoic, even though evidence has been preserved only at a few
localities.
Surface ruptures have repeatedly terminated at the Bolsa site, the northern end
of the 1887 scarp. An aerial view of vegetation and soil patterns associated with a
3- to 6-m-high scarp (Figure 4a) on an early(?) Pleistocene surface are shown in
Figure 8. The scarp dies out 1 km to the north. The toe of the scarp separates areas
of markedly different soil types and plant density on the downthrown and upthrown
sides. The 1887 displacement was only 0.1 to 0.3 m. Repeated infrequent displacements during the Pleistocene induced accumulations of pebbly clayey sand downslope from the fault, and the formation of a surficial lag gravel that protects clayey
gravels on the 1° to 3 ° slopes of the scarp crest.
Morphologic analysis of the pre-1887 fault scarps. Fault-scarp morphologies provide clues about the length of time that elapsed between the prior and 1887 surface
ruptures. The most commonly used methods to estimate scarp age from scarp
morphology are the relations between scarp height and maximum slope angle
(Bucknam and Anderson, 1979) and variations on diffusion models of scarp degradation (Colman and Watson, 1983; Mayer, 1984; Nash, 1984; Hanks et al., 1984).
Reasonable application of these methods to pre-1887 Pitaycachi fault scarps depends
on: (1) being able to remove the 1887 component of total surface rupture and (2)
evaluation of rates of scarp degradation in the San Bernardino Valley, compared to
those in regions where fault scarps of known age have been studied. We graphically
removed the 1887 scarp from 17 topographic profiles of multiple-rupture-event
scarps and used two quantitative methods and One qualitative method to estimate
the age of the prior surface rupture.
The relative youthfulness of the 1887 fault scarps combined with gentle slopes of
prior scarps makes removal of the 1887 segments from scarp profiles fairly straightforward. Pre-1887 fault scarps were reconstructed by projecting scarp segments
immediately above and below the 1887 scarp into the middle of the scarp and
moving the segments vertically until they were contiguous (Figure 9). Scarp heights
and vertical surface-displacement data were then obtained from the reconstructed
QUATERNARY
o
SURFACE
RUPTURES
OF
THE
PITAYCACHI
967
FAULT
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968
WILLIAM B. BULL AND PHILIP A. PEARTHREE
profiles. The validity of the prior scarp reconstructions depends on: (1) accurate
delineation and removal of the 1887 scarp from the profile and (2) preservation of
the steepest pre-1887 scarp-slope segment above or below the 1887 scarp. Post-1887
rounding of scarp crests and toes by erosion and deposition creates a small uncertainty in defining the 1887 component of multiple-rupture-event scarps, but pre1887 topographic elements were identified by segments above and below the scarp
with similar or identical slopes. If the steepest pre-1887 scarp slopes were obliterated
by the 1887 surface rupture or subsequent erosion, the maximum pre-1887 slope is
underestimated and scarp-age estimates are too young.
Assigning age estimates to fault scarps through morphologic analysis, whatever
the specific method, requires evaluation of climatic and lithologic variations between
the San Bernardino Valley and areas where dated scarps have been studied. The
most widely used dated scarps in the Western U. S. are those of the highest Lake
Bonneville shoreline of central Utah (Bucknam and Anderson, 1979). Studies o f
other dated scarps in Idaho (Nash, 1984; Pierce and Colman, 1986) and New Mexico
(Machette, 1986; also cited in Hanks et al., 1984) provide further information on
rates of scarp degradation. Although the climates of these areas are not identical,
all are semi-arid. Sonora and New Mexico receive greater amounts of intense
summer rainfall, and central Utah and Idaho winters have many more diurnal
freeze-thaw cycles. Most scarp materials in the San Bernardino Valley probably are
at least as resistant to erosion as the scarp materials in the other study areas, and
the scarps typically are underlain by cobbly and houldery gravels of unweathered
welded tuff embedded in pedogenic clay. Vegetation on the fault scarps typically
consists of low desert bushes of moderate density; grass cover is very sparse. We
tentatively conclude that rates of scarp erosion under the present climate in Sonora
are similar to, but possibly lower than, rates in Utah, Idaho, and New Mexico.
Past regional climatic changes also affected rates of scarp degradation. More
effective soil moisture during times of full-glacial climates probably sustained more
vegetation in the presently semi-arid parts of Arizona, New Mexico, and Sonora.
The Holocene has been a time of accelerated hillslope erosion in the southwest
deserts (Bull, 1979) because of less plant cover and monsoon thunderstorms (Spaulding and Graumlich, 1986). Machette (1986) shows that the rate of scarp degradation
averaged over the past 30 to 40 ky is ~ to ¼ of the average Holocene rate of
degradation. Using the relatively fast Holocene scarp-degradation rates for older
scarps will tend to underestimate true scarp age.
Three different methods were employed to estimate the age of pre-1887 scarps
along the Pitaycachi fault from their morphologies. The first is a plot of maximum
slope angle against log of scarp height (Figure 10A). Data points from individual
Pitaycachi scarp profiles are plotted, as are regression lines from Holocene or latest
Pleistocene scarps in Utah (Bucknam and Anderson, 1979) and New Mexico
(Machette and McGimsey, 1982); a regression line from about 100 ka fault scarps
along the Santa Rita fault zone of southeastern Arizona (Pearthree and Calvo,
1987) is included for comparative purposes. No significant regression line can be
obtained from the Pitaycachi data, but all but one data point near the Santa Rita
regression line, suggesting the pre-1887 Pitaycachi scarps, formed about 100 ka.
Two somewhat different dating methods based on the diffusion equation were
also used to estimate the age of pre-1887 scarps. Hillslope processes on transportlimited slopes (slopes at the angle of repose or less) change the altitude of any point
on the slope with time (~y/~t) as a product of a constant of diffusivity (c) and slope
QUATERNARY SURFACE RUPTURES OF THE PITAYCACHI FAULT
969
FIG. 8. Trace of the Pitaycachi fault across early(?) Pleistocene piedmont surface north of Arroyo
Cajon Bonito; view to south. The fault scarp appears as a dark band, and the contrast in texture and
tone is the result of differing soil types and plant densities. Gravelly soils are in the foreground and more
clay-rich soils in the middleground. Total scarp height is 3 to 6 m and appears to be the result of small
repeated fault displacements.
curvature (62y]Sx 2) (Nash, 1980)
@/~t=c
Uy/~x 2.
(1)
H a n k s et al. (1984) propose using a solution to this diffusion equation in order to
model scarp degradation after the imposition of a vertical step in t o p o g r a p h y (size
= 2a) on a pre-existing slope (b)
y(x, t) = a e r f ( x / 2 ~ f ~ ) + bx.
(2)
In equation (2), y is the altitude of a p o i n t relative to the scarp mid-point, x is the
horizontal distance f r o m the mid-point, t is the scarp age, and e r f ( x / 2 , f ~ ) is the
error function of the a r g u m e n t x/2,f~ct. T h e model scarp profile t h a t m o s t closely
resembles the actual scarp profile defines the best value for ct.
970
WILLIAM B. BULL AND PHILIP A. PEARTHREE
7
A.I
6
1981 Fault Scarp Profile
5
E
4
¢,.
.$
-r 3
2
p profile
j
1
0
---c- Projectionof
pre-1887sudace
i
0
i
20
40
60
Distance, rn
"~
B. I
ReconstructedP
r
e
~
4
E3
e-
2
/
l
~
Reconstructedprior
_fansudace _
0
•
~
•
20
i
40
•
60
Length, m
FIG. 9. Graphical reconstruction of pre-1887 Pitaycachi fault scarp south of Escarpa Vieja. (A) 1981
fault scarp profile. Lines with open circles show projection of pre-1887 fault scarp surface. Vertical
displacement represented by 1887 event is about 1.3 m. (B) Topographic profile of 1981 scarp with the
estimated 1.3 m of 1887 displacement removed. Lines with open circles illustrate about 1.4 m of vertical
surface displacement of original fan surface in the prior event.
Nash (1984) proposed another method for estimating scarp ages based on the
diffusion equation, which assumes a finite initial slope equal to the angle of repose
of the scarp-forming materials. He develops a relationship between (tc/H2)tan2~
and tan s / t a n ~, where H is the initial fault offset, ~ is the angle of repose minus
the slope of the surface prior to faulting (0), and/~ is the steepest present scarp
slope less 0. This relationship defines a value for tc for each scarp profile.
These two methods deliver unique products of scarp age and the diffusivity
constant for each scarp profile, but estimates of scarp age depend on calibrating the
diffusivity constant. Spatial variations in scarp materials (and perhaps temporal
variations in climate) cause uncertainty in determining appropriate values of c for
the San Bernardino Valley. We have acknowledged this uncertainty by using
maximum and minimum values of c determined from studies of the Bonneville
shoreline scarps and late Pleistocene fault scarps in New Mexico (1.0 to 0.3 × 10 -3
mZ/yr; Hanks et al., 1984; Machette, 1986), thereby obtaining minimum and maximum age estimates for each scarp profile. These age-range estimates were then
compiled in histograms to obtain modal estimates for the age of the prior event and
to give a sense of the variability in the age estimates (Figure 10, B and C).
30
o I n d i v i d u a l s c a r p profiles,
Pitaycachi fault
O}
IA' I
25
'10
20
.......
New Mexico
.........
Bonneville
..................... S o u t h e a s t e r n
&
O
15
o
10
E
E
5
Arizona /
o
o
o
-~'""
o
8
°o
o
.......
o
o
0
10
Scarp height, m
12-
B.I
rdF/~l
10.
"~ = 101
n=17
c~ = 1.0 m2/ky
c2= 0.3 rn2/ky
~2=335
E 8'~
6-
J~
E 4
z
0-49
50 - 99
100 - 149
Age,
150 - 199
200 - 250
>250
ka
c.I
~/,~
c1= 1.0 m2/ky
I ~:~ ::::f c 2 = 0 . 3 n12/ky
iii!i!i!iiiii!!il
iii!ii!i
6
= 94 ~ =
n=17
245
~:ii:ii~:~iiili!:::iiii::
E
l~:ili!i:i~i:!i:i:-!
!,:'i::~i~:ii~::i!!i
;!i
ILl
4
~:~i:ii!i!i!!:i!:::i!:i
i~i:::i!iiii~/
.,Q
i:ii~ii~i,i'~:i!i:i!i!i
' !i!:iiii~ii:i:i~i~i:~
l
E
2:2
•
0-49
50 - 9 9
100- 149
150-199
ii~::i¸i~ :i:::~:
200-250
>250
Age, ka
FIG. 10. Age estimates of the time of prior surface rupture along Pitaycachi fault based on scarp
morphology. (A) Maximum scarp-slope angle versus scarp height using regression lines from 5 ka fault
scarps in New Mexico (Machette, 1986), 15 ka Bonneville shoreline scarps from Utah (Bucknam and
Anderson, 1979), and about 100 ka fault scarps from southeastern Arizona (Pearthree and Calvo, 1987).
(B). Estimates of time of the prior surface-rupture event using a finite initial scarp slope equal to the
angle of repose 33 ° (Nash, 1984). Using a value of cl = 1.0 m2/ky (from Hanks et al., 1984), the mean
age estimate (~) is 101 ka; using a value of c2 = 0.3 m~/ky (based on Machette, 1986), the mean estimate
is 335 ka. (C) Estimates of time of the prior surface-rupture event using an initial vertical step in
topography and modeling of the whole scarp profile (Hanks et al., 1984). Maximum and minimum values
for c are the same as in (]3).
972
WILLIAM B. BULL AND PHILIP A. PEARTHREE
A pre-Holocene age for pre-1887 scarps is clearly indicated by both methods.
Using a finite initial scarp slope, a scarp age of 20 to 60 ka is suggested if the
maximum of c is correct, while if the minimum value of c is a better approximation
of the diffusivity, the scarp age is apparently 100+ ka. Profile modeling (Hanks et
al., 1984) suggests an age of between 20 and 80 ka for the prior event using the
higher value for c, while the lower c value indicates an age of 100 to 200 ka (Figure
10C). If late Pleistocene rates of scarp degradation were lower than Holocene rates,
and/or are lower in the San Bernardino Valley than in other parts of the Western
United States because of more resistant scarp materials, then applying the lower
value for the average diffusivity is more likely to be correct, and an age of 100 to
200 ka is indicated for the prior surface-rupture event.
Faulted landform assemblages
A third way of estimating the return period since the prior surface-rupture event
involves qualitative evaluations of the time needed to degrade hillslopes and erode
valleys.
Hillslopes. Some hillslope morphologies reflect the interactions between continuous degradation and very infrequent episodes of uplift. The Pitaycachi fault
separates basaltic bedrock hillslopes from basin alluvium about 6 to 10 km eastnortheast of Colonia Morelos. Repeated uplifts have steepened the footslope and
thereby have created conditions favorable for continued erosion of colluvium and
bedrock upslope from the fault. Rates of hillslope denudation vary along the fault.
Triangular facets that are remnants of former higher levels of footslopes are
preserved locally (Figure 11), where denudation rates are relatively low. The
substantial scarp heights at the downslope margins of the triangular facets suggest
they are the product of many surface-rupture events. Stage V and locally stage VI
pedogenic carbonate in the surficial materials of erosional remnants suggest that
they are early Pleistocene or older. Such hillslope remnants have resisted erosion
in part because cementation by carbonate has strengthened the volcanic rock and
colluvium. Rillwash and sheetflow have been diverted to the flanks of the triangular
outcrops where denudation is relatively rapid. The intervening lower footslopes
have been inset since the prior surface-rupture event and provide a clue as to the
time that has elapsed since the prior event. Erosion has obliterated the prior event
fault scarp in basalt between the facets. Such virtually complete scarp degradation
is suggestive of several hundred ky between surface-rupture events.
Hillslope elements that were ruptured by pre-1887 events are even more difficult
to discern where less resistant materials are present. Bouldery Tertiary alluvium
underlies ridge-and-ravine topography about 1 to 2 km southeast of the Facet study
locality (Figure 2). Erosional remnants that record former hillslope topographies
are not present. Instead, subtle slope steepening and subdued triangular facets are
present upslope from the 1887 scarp, as is the case for the Facet locality (Figure
4c). Such virtual obliteration of topographic effects of the prior displacement
probably requires at least 100 ky.
Valleys. Regional downcutting along the Rio San Bernardino has caused extensive piedmont dissection of basin fill along the east side of the San Bernardino
Valley. Erosion has created 40- to 50-m-deep valleys since the prior surface-rupture
event along Arroyo de los Embudos and Arroyo Hondo. Arroyo de los Embudos is
a 1-km-wide valley cut into the "Stock Tank" alluvial geomorphic surface of midPleistocene age; this surface has been ruptured by multiple events. Valley downcutting began after deposition of extensive mid-Pleistocene piedmont alluvium. The
QUATERNARY SURFACE RUPTURES OF THE PITAYCACHI FAULT
973
FIG. 11. Aerial view along the northwest side of faulted bedrock hills 7 km east-northeast of Colonia
Morelos. Erosional wedges of lava flows (10 to 35 m high) and carbonate-cemented colluvium immediately
upslope from the 2 m high 1887 scarp are indicative of large amounts of cumulative late Cenozoic uplift
caused by displacements along the same fault. The lack of pre-1887 fault scarps between these isolated
remnants of former hillslopes is indicative of the long time span and extensive erosion that occurred
between the prior and 1887 surface-rupture events.
valley contains three mid(?) to late Pleistocene cut terraces with well-developed
argillic soil horizons and a late Holocene fill terrace; none of these terraces were
ruptured prior to 1887. The topographic cross-valley profile of Figure 12 depicts
Pleistocene terraces as high as 30 m above the channel of Arroyo de los Embudos
that were not ruptured by the prior event.
Aggradation of valleys cut into a dissected pediment has occurred repeatedly at
the Javelina site (Figure 13 and Table 4). Correlation of the Pleistocene-Holocene
stratigraphic contact exposed in the scarp face with that exposed in the fanhead
trench downslope from the fault indicates an 1887 displacement of 3.1 m. Before
1887, the fault scarp was only 2½ to 3½ m high.
About 2 to 4 m of downcutting and subsequent backfilling is represented by five
valley fills exposed in the scarp face. The fills have a great variety of lithologies,
colors, and sedimentary structures, which reflect different combinations of climatecontrolled changes in weathering and erosion of sediments from the hillslopes, and
in discharges of water and sediment. The older fills have redder hues, whereas the
younger ones have gray hues.
Not all of the six valley fills are associated with the stream system of the modern
channel in Figure 13. By viewing the planimetric trends from the top of the scarp,
it is readily apparent that the valley fills were derived from three different stream
974
WILLIAM B. BULL AND PHILIP A. PEARTHREE
' a.) Arroyo de los Embudos
50
1'
/Mu/tlpIs-rupture SfocMa•k surface
40 ~ / e - r l
0
5o
4o
G/ngle-ruptur5 Plei~tocene terraces
[00
~00
300
400
500
600
700
3O
2O
I0
0
9OO
Boo
meters
/ b,) Arroyo Hondo
50
m
-
/oo
/
30
site
3,O
'01
O
i ~ v a l i n a
50
__Singleruptur~Pleistocene terrace
~
0
",,.______V"
'
~r~
,
--
1
O0
20b
meters
t 'o
,
'
300
' '
0
o r
o
l
i
fo
i
i
20
il
/
i
iI , i
30
4o
8
6~
4
2
I,
O
50
meters
FIG. 12. Profiles across valleys that have been eroded since the prior surface-rupture event (a) Arroyo
de los Embudos. (b) Arroyo Hondo. (c) Small wash at the Javelina site.
systems, This accounts, in part, for the diversity of alluviums. Fill units 2, 5, and 6
were derived from the fluvial system north of the cross~section channel, and fill
1 at the right side of the section was derived from the stream system to the south.
Fill units 3 and 4 were derived from the stream system of the present valley. Such
shifting of streams from different source areas indicates that a lack of faulting
resulted in relief that was sufficiently low over long periods of time to preclude
permanent stream-channel entrenchment. Fill unit 1 may have been a sheet of
piedmont alluvium deposited on pedimented bedrock. The time span required for
the numerous climatic changes associated with the six periods of valley degradation
and aggradation, and the subsequent diagenetic reddening and cobble weathering,
is estimated to exceed 0.5 my, and includes the 1887 and only one prior surfacerupture event.
Pediments. The erosional backwearing of mountain fronts to form the gently
sloping bedrock plains--pediments--requires the passage of long time spans, during
which there has been relative tectonic stability. Pediments are formed through
processes of hillslope retreat; therefore, rates of pedimentation are comparable to
rates of hillslope denudation. Even in rocks that are moderately susceptible to
weathering and erosion, such as granitic rocks, the rates of denudation within the
semi-arid study area may be less than 200 m/my, based on regional studies of
denudation made by Schumm (1963), Judson and Ritter (1964), and Mayer (1982).
The pediment immediately upslope from the Pitaycachi fault at the Javelina site is
½to 1 km wide, and the pediment embayment formed on carbonate rocks farther
north (Figure 5) is several kilometers wide. Such extensive pediments, locally
common upslope from the Pitaycachi fault, have been deeply incised by valleys.
Thus, it appears that several million years of tectonic stability occurred in the
late(?) Tertiary, during which pediments were formed. Renewal of displacement
along the Pitaycachi fault and regional valley downcutting occurred in the latest
Cenozoic. The pediments appear to have been formed during the Tertiary when
denudation rates greatly exceeded low rates of uplift and before the reestablishment
or strengthening of the extensional stresses responsible for the Plio-Pleistocene
975
QUATERNARY SURFACE RUPTURES OF THE PITAYCACHI FAULT
8
NW
Mid-Pleistocene
6
~.,~, bedrock
Holocene
~
H01ocene
~
I
SE
- - ~
a) 2
0
t
t
2
J
6
i
i
10
i
i
t
14
J
t
18
J
22
t
i
26
i
J
,
30
Meters
FIG. 13. Section of valley fill units exposed in the fault scarp at the Javelina site. See Table 4 for
descriptions of Holocene, late Pleistocene, and mid-Pleistocene deposits. Comparison of the maximum
hues of soils and fill units listed in Tables 1 and 3 suggests that fill 6 is of Holocene age, fill 5 was
deposited during the late Pleistocene, and fills 1 to 4 are early to mid-Pleistocene in age.
TABLE 4
STRATIGRAPHY OF VALLEY FILLS AT THE JAVELINA SITE*
Numbered
Aggradation
Event of
Figure 13
Lithology
Dry Color of
Weathered Alluvium
Weathering of
Granodiorite Cobbles
6
Sand, silty
Brown (7.5YR 4/3)
Unweathered cobbles to iron
oxide stains along fractures in solid cobbles
5
Gravel, clayey
Bright reddish-brown (5YR
5/8) to bright brown
(2.5YR 5/8)
Iron oxide stains along fractures in solid to punky
cobbles
4
Sand, clayey,
cross-bedded
Reddish-brown (2.5YR 4/8)
No cobbles present
3
Gravel, clayey
Reddish-brown (5YR 4/8) to
bright brown (2.SYR 4/8)
Punky to grussy cobbles
2
Gravel, sandy
Dull orange (7.5YR 7/4)
Punky to grussy cobbles
1
Gravel, clayey
Bright reddish-brown (5YR
5/6) to reddish-brown
(2.5YR 4/6)
Grussy remnants of cobbles
* For interrelations between these valley fills and estimated ages, see Figure 13.
normal faulting. Similar histories of landforms and geomorphic processes have been
noted in Arizona (Pearthree and Calvo, 1987) and New Mexico (Machette, 1986).
CONCLUSIONS
The Pitaycachi fault of northeastern Sonora, Mexico, has been the site of
infrequent but large surface ruptures. The 75-km-long, ½- to 4-m-high 1887 scarp
may be the longest known historical surface rupture of a normal fault in western
North America. Evidence from segmented topographic profiles of multiple-ruptureevent fault scarps along the trace of the 1887 rupture indicates that surface
displacement of the prior event also was about 2 to 3 m and probably occurred in
the late mid-Pleistocene.
Estimates of the time span between the prior surface-rupture event and the 1887
event involve morphologic analyses of scarp profiles, characteristics of soil profiles
on alluvial geomorphic surfaces that were unfaulted until 1887, extent of hillslope
topographic changes caused by erosion since the prior event, and histories of valley
976
WILLIAM B. BULL AND PHILIP A. PEARTHREE
erosion and alluviation between the prior and 1887 events. The following conclusions
are the basis for estimates of the time elapsed between the prior and 1887 events.
1. Late Pleistocene alluvial geomorphic surfaces were not ruptured until 1887,
but late mid-Pleistocene surfaces were ruptured by both the prior and 1887
events.
2. The upper half of multiple-rupture-event scarps on mid-Pleistocene surfaces
are 2 to 4 m higher than the 1887 increment of scarp height, by typical pre1887 scarp segments slope only 3½° to 9°. A plot of maximum scarp slope angle
against log of scarp height for 17 pre-1887 scarp profiles was compared with
regressions for 5 to 100 ka scarps in New Mexico, Utah, and southeastern
Arizona. The position of the Pitaycachi data points indicates that the prior
rupture occurred at least 100 ka.
3. Two types of diffusion-equation analyses indicate a substantial age for the
prior event. An age of 100 to 200 ka is probable if late Pleistocene rates of
scarp degradation were lower than Holocene rates, and/or a lower value for
the diffusivity constant is used because scarp materials along the Pitaycachi
fault are more resistant to erosion than materials in the New Mexico and Utah
scarps.
4. Hillslope fault Scarps in volcanic rocks that were formed as a result of the
prior surface rupture have been obliterated by erosion.
5. Valleys that are 30 to 45 m deep and as much as 1 km wide, with flights of
late Pleistocene terraces, have been formed entirely since the prior event.
6. Deposition of six climate-controlled Quaternary valley fills has occurred immediately upslope from the Pitaycachi fault. Streams were able to repeatedly
shift laterally across low drainage divides because of the lack of uplift and
relief provided by recurrent movement along the fault.
All of the aforementioned lines of evidence support our contention that about 100
to 200 ky elapsed between the prior and 1887 events. Early Pleistocene surfaces
have been offset a total of 9 to 13 m, which suggests that the average return period
between major surface ruptures (of 2 to 3 m) during the Quaternary may have been
about 0.3 to 0.5 my.
Scarps that are as high as 43 m are indicative of recurrent late Cenozoic
displacements along the Pitaycachi fault. However, the total amount of late Quaternary uplift appears to be at least an order of magnitude less than that for normal
faults in the more tectonically active parts of the Basin and Range province in
Utah, Nevada, and California. Approximate vertical-slip rates during the Pleistocene are only 0.1 m/ky for the Pitaycachi fault, whereas slip rates in the more active
parts of the province are approximately 0.1 to 0.5 m/ky.
ACKNOWLEDGMENTS
This work was supported by the National Science Foundation Grants EAR 78-03648 and EAR 8008212 ("Seismic and Geomorphic Studies of Quaternary Tectonism in the Arizona Seismic Belt"). We
greatly appreciate the friendly and whole-hearted cooperation of Ramon Salcedo of Rancho Chirriones,
Augustin Trevino of Rancho Javelina, and Manuel Gomez of Rancho Jucural. We are indebted to our
many graduate student colleagues at the University of Arizona for manuscript review, assistance in the
field, and for sharing many a campfire. Steven Natali was generous and thoughtful in his capacity as a
field guide and geophysicist, and we appreciate the continued support and exchange of viewpoints
provided by Marc Sbar. Larry Mayer and Robert Bucknam shared their scarp-profile data, and Darrell
Herd mapped the 1887 rupture. Michael Machette's many thoughtful review suggestions contributed
greatly to the coherence and completeness of the final manuscript. Roger Bull drafted the figures.
QUATERNARY SURFACE RUPTURES OF THE PITAYCACHI FAULT
977
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GEOSCIENCESDEPARTMENT
UNIVERSITYOF ARIZONA
TUCSON, ARIZONA85721
Manuscript received 11 March 1987