Geology of the Hayward fault zone:
A digital map database

U.S. Geological Survey Open-File Report 95-597

by
R.W. Graymer
D.L. Jones
E.E. Brabb

Geologic Description

Introduction

	The Hayward is one of three major fault zones of the San 
Andreas system that have produced large historic earthquakes 
in the San Francisco Bay Area (the others being the San 
Andreas and Calaveras).  Severe earthquakes were generated by 
this fault zone in 1836 and in 1868, and several large 
earthquakes have been recorded since 1868.  The Hayward fault 
zone is considered to be the most probable source of a major 
earthquake in the San Francisco Bay Area, as much as 28% 
chance for a magnitude 7 earthquake before the year 2021 
(Working Group on California Earthquake Probabilities, 1990).
	The Hayward fault zone, as described in this work, is a 
zone of highly deformed rocks, trending north 30 degrees west 
and ranging in width from about 2 to 10 kilometers.  The 
historic earthquake generating activity has been concentrated 
in the western portion of the zone, but the zone as a whole 
reflects deformation derived from oblique right-lateral and 
compressive tectonic stress along a significant upper crustal 
discontinuity for the past 10 million or more years. 
	The Hayward fault zone is bounded on the east by a 
series of faults that demarcate the beginning of one or more 
structural blocks containing rocks and structures unrelated 
to the Hayward fault zone.  The eastern bounding faults are, 
from the south, the Calaveras, Stonybrook, Palomares, Miller 
Creek, and Moraga faults.  These faults are not considered to 
be part of the Hayward fault zone, although they are shown on 
the map to demarcate its boundary.  The western boundary of 
the zone is less clearly defined, because the alluvium of the 
San Francisco Bay and Santa Clara Valley basins obscures 
bedrock and structural relationships.  Although several of 
the westernmost faults in the zone clearly project under or 
through the alluvium, the western boundary of the fault is 
generally considered to be the westernmost mapped fault, 
which corresponds more or less with the margin of thick 
unconsolidated surficial deposits.  The Hayward fault zone is 
truncated to the south by the Calaveras fault, which trends 
about north 10 west, and so forms an oblique east and south 
boundary.  All of the faults within the southern part of the 
zone probably splay into the Calaveras fault.  The northern 
margin of the zone as dealt with herein is San Pablo Bay, but 
the zone of deformation undoubtedly continues north of the 
Bay through the area bounded by the Rodgers Creek and Tolay 
faults.
	Scientific study of the Hayward fault zone has had a 
long history.  The first systematic study of the then called 
Haywards rift was undertaken soon after the major earthquake 
that occurred on the fault in 1868.  However, this study was 
later lost or repressed by nervous local officials (Lawson, 
1908).  Following the 1906 San Francisco earthquake, the 
State Earthquake Investigation Commission, headed by A.C. 
Lawson, reinvestigated the 1868 earthquake.  Their report 
(Lawson, 1908) contains a map of the Bay Area showing the 
approximate trace of the 1868 ground rupture.  In 1914, 
Lawson published the San Francisco folio, in which he 
discussed the Hayward fault zone, although he did not show 
the Hayward fault on the accompanying maps because he could 
not locate the fault exactly.  Since 1914, many workers have 
released larger scale geologic maps of areas including parts 
of the Hayward fault zone (Crittenden, 1951, Robinson, 1956, 
Hall, 1958, Radbruch and Case, 1967, Dibblee, 1972a-c, 1973a-
b, 1980a-g, Radbruch, 1969, Graymer and others, 1994).  In 
addition, topical studies on the Hayward fault zone have 
demonstrated its accumulated right-lateral and east-up offset 
(Russell, 1926, Buwalda, 1929), the formation of a 
groundwater barrier due to fault gouge (Forbes, 1949), and 
the evidence of fault creep at several points along the fault 
zone (Radbruch and Bonilla, eds, 1966).  With the discovery 
of creep in the fault zone, and the prevailing model of 
future movement occurring where historic movement and present 
creep are known (Radbruch and Bonilla, 1966), several workers 
have produced different maps of the "active" strands within 
the fault zone based on creep, geomorphic, and trench studies 
(Radbruch-Hall, 1974, Herd, 1977, 1978, Helley and Herd, 
1977, Lienkaemper, 1992).  More recent work has focused on 
the relationship between the Hayward fault zone and the 
Calaveras fault (Andrews and others, 1993), the San Andreas 
fault (Zandt and Furlong, 1982), and the entire regional 
fault system (Jones and others, 1994).
	Current thought regarding the Hayward fault seems to be 
that it is a relatively narrow, long lived zone of 
predominantly right-lateral strike-slip deformation 
(Lienkaemper, 1992).  It is thought to connect to the 
Calaveras fault via a deep crossover structure in the region 
of Mission Peak (Andrews and others, 1993).  The creeping 
strand is considered to be the locus of stress accumulation 
and probable future hazard (Radbruch and Bonilla, 1966).  
	Geologic relationships within the Hayward fault zone (as 
described below) indicate a much more complex geologic 
history for the fault, however.  Recent observations (also 
discussed below) suggest late Holocene movement on strands 
within the zone other that the creeping strand (as most 
recently defined by Lienkaemper, 1992).  In addition, there 
is evidence that a large amount of fault normal compression 
has also been taken up within the Hayward fault zone.
	The purpose of this report is fivefold.  1) to 
demonstrate the short history of the creeping strand and to 
describe the structural history of the fault zone as a whole; 
2) to illustrate potentially active strands within the 
Hayward fault zone in addition to the creeping strand defined 
by Lienkaemper; 3) to describe structures related to the 
Hayward fault zone including compressive structures; 4) to 
show possible Hayward - Calaveras fault connections in 
addition to that proposed by Andrews and others; 5) to 
describe rock types involved in the Hayward fault zone.

Description of rocks and map units in the Hayward fault zone

Overview

The rocks within the Hayward fault zone include sedimentary, 
igneous, and metamorphic rocks that range in age from 
Jurassic through Quaternary.  The pre-Tertiary rocks are 
divided into two groups:  the strongly to slightly 
metamorphosed igneous and sedimentary rocks of the Franciscan 
complex, and the relatively unmetamorphosed rocks of the 
Coast Range ophiolite and Great Valley sequence.  The contact 
between these two pre-Tertiary groups is everywhere a fault.  
Throughout most of the mapped zone, the base of the Tertiary 
strata is also faulted, but Paleocene rocks are in 
unconformable contact with Great Valley sequence rocks in the 
Niles area, and middle Miocene rocks unconformably overlie 
Great Valley Sequence rocks in the Milpitas area.
	Pre-Tertiary rocks are composed of two separate 
displaced terranes that were amalgamated during Late 
Cretaceous to early Tertiary time and subsequently underwent 
complex deformation, and were unconformably overlain by 
Tertiary strata.  One of the terranes is the Franciscan 
complex, and the other is composed of the Coast Range 
ophiolite and Great Valley sequence.  Both terranes contain 
bedded rocks as young as Late Cretaceous (Campanian), so 
amalgamation must post-date Campanian deposition.
	Following is a description of the rocks and map units 
contained in the Hayward fault zone.  For more complete 
descriptions of some of the units see Graymer and others 
(1994), Hall (1958), Radbruch (1969), Crittenden (1951), 
Graymer (1995), Bailey and others (1964), and Savage (1951)

Franciscan Complex

JKfm	Franciscan melange (Late Jurassic and Early 
Cretaceous).  Sheared black argillite, graywacke, 
and meta-graywacke containing blocks and slabs of 
meta-graywacke and shale (fs), chert and metachert 
(fc), serpentinite (sp), greenstone (gs), 
amphibolite, tuff, eclogite, quartz schist, 
greenschist, basalt, marble, conglomerate, and 
blueschist.  Blocks range in size from a few 
millimeters to several hundred meters in length.  
Only some of the largest blocks and slabs are shown 
on the map.

JKfn	Sandstone of Novato Quarry (Late Cretaceous).  
Distinctly bedded to massive, fine- to coarse- 
grained, mica bearing, lithic wacke.  Where 
distinctly bedded, sandstone beds are about one 
meter thick, and siltstone interbeds are a few 
centimeters thick.  Sedimentary structures are well 
preserved.  In the type area in Marin County, about 
20 km northwest of the map area, fossils of 
Campanian age have been discovered (Bailey and 
others, 1964).  In north Oakland, the sandstone is 
associated with a large body of fine-grained quartz 
diorite (JKfgm).  Although the margins of the 
intrusive body are pervasively sheared, the magma 
was probably originally intruded into the 
sandstone.  This relationship is demonstrated by 
hydrothermal alteration that has changed the non-
quartz grains in the sandstone into white powder, 
probably clay minerals, in many parts of the 
sandstone outcrop area.

JKf	Undivided rocks of the Franciscan Complex (Late 
Jurassic to Late Cretaceous).  

Coast Range Ophiolite and Great Valley Sequence 

Coast Range Ophiolite (Jurassic).  Consists of:

sp	Serpentinite.  Mainly sheared serpentinite, but 
also includes massive serpentinized harzburgite.
Jgb	Gabbro 
Jb	Massive basalt and diabase.
Jpb	Pillow basalt, basalt breccia, and minor diabase.

Overlying Volcanics

Jsv	Keratophyre and quartz keratophyre (Late Jurassic).  
Highly altered intermediate and silicic volcanic 
and hypabyssal rocks.  Feldspars are almost all 
replaced by albite.  In some places, closely 
associated with (intruded into?) basalt.  This unit 
includes rocks previously mapped as Leona and 
Northbrae rhyolite, erroneously considered to be 
Tertiary (Dibblee, 1980, 1981, Radbruch and Case, 
1967, Robinson, 1956).  Recent biostratigraphic and 
isotopic analyses have revealed the Jurassic age of 
these rocks (Jones and Curtis, 1991).  These rocks 
are probably the altered remnants of a volcanic arc 
deposited on ophiolite during the Jurassic.

Great Valley Sequence (Late Jurassic to Late Cretaceous).  
Consists of:

JKk	Knoxville Formation (Late Jurassic and Early 
Cretaceous).  Mainly dark, greenish-gray silt or 
clay shale with thin sandstone interbeds.  Locally 
includes thick pebble to cobble conglomerate beds 
in its lower part (JKkc).  Locally at the base 
includes beds of angular, volcanoclastic breccia 
(JKkv) derived from underlying ophiolite and 
silicic volcanic rocks.  The depositional contact 
of Knoxville formation on ophiolite and silicic 
volcanic rocks is preserved in several locations in 
the Hayward fault zone.

Knc	Sandstone and shale of Niles Canyon (Early 
Cretaceous, Albian).  Distinctly bedded, gray to 
white, well lithified, massive to cross-bedded, 
biotite-bearing, coarse- to fine-grained sandstone, 
siltstone, and shale.  Sandstone varies from 
biotite and quartz rich wacke to lithic wacke.  
Plant debris is common.  Sandstone tends to form 
discontinuous outcrops on the ridges and uplands 
and prominent resistant outcrops in canyons, 
whereas siltstone and shale are largely visible 
only in canyons.  This formation is equivalent to 
part of the Niles Canyon formation of Hall (1958)

Kjm	Joaquin Miller Formation (Late Cretaceous, 
Cenomanian).  Thinly bedded shale with minor 
sandstone.  Grades into thinly bedded, fine-grained 
sandstone near the top of the formation.  The 
contact with the overlying Oakland Sandstone is 
gradational.

Ko	Oakland Sandstone (Late Cretaceous, Cenomanian 
and/or Turonian).  Massive, medium- to coarse-
grained, biotite and quartz rich wacke.  Contains 
prominent interbedded lenses of pebble to cobble 
conglomerate.  Conglomerate clasts are 
distinguished by a large amount of silicic volcanic 
detritus, including quartz porphyry rhyolite.  
Conglomerate composes as much as fifty percent of 
the unit in the Oakland hills, but it becomes a 
progressively smaller portion of the unit to the 
south.

Kcv	Unnamed sandstone, conglomerate, and shale of the 
Castro Valley area (Late Cretaceous, Turonian and 
younger(?)).  The lower part of the unit is 
composed of distinctly bedded, mica bearing 
siltstone, fine-grained mica bearing wacke, shale, 
and, locally, one thin pebble conglomerate layer.  
The middle part of the unit is composed of 
distinct, thick beds of medium- to coarse-grained, 
mica rich wacke and pebble to cobble conglomerate.  
The middle part grades upward into distinctly to 
indistinctly bedded, medium- to fine-grained, mica 
rich wacke and siltstone.  This unit is bounded 
above and below by faults.

Ksc	Shephard Creek Formation (Late Cretaceous, 
Campanian).  Distinctly bedded mudstone and shale, 
mica rich siltstone, and thin beds of fine-grained, 
mica rich wacke.  This formation is conformably 
overlain by the Redwood Canyon Formation.

Kr	Redwood Canyon Formation (Late Cretaceous, 
Campanian).  Distinctly bedded, cross-bedded to 
massive, thick beds of fine- to coarse grained, 
biotite and quartz rich wacke and thin interbeds of 
mica rich siltstone.  This formation is conformably 
overlain by the Pinehurst Shale.

Kp	Pinehurst Shale (Late Cretaceous, Campanian).  
Siliceous shale with interbedded sandstone and 
siltstone.  Includes maroon, concretionary shale at 
base.  This formation was originally considered to 
be Paleocene, but it contains foraminifers and 
radiolarians of Campanian age in its type area and 
throughout its outcrop extent.


Ku	Undivided, unnamed sandstone and siltstone 
(Cretaceous)

Kc	Undivided, unnamed conglomerate (Cretaceous)

Tertiary strata

Tas	Unnamed glauconitic sandstone (Paleocene).  Coarse-
grained, green, glauconite rich, lithic sandstone 
with well preserved coral fossils.  Locally 
interbedded with gray mudstone and hard, fine-
grained, mica bearing quartz sandstone.  Outcrop of 
this unit is restricted to a small, fault bounded 
area in the Oakland hills.

Tps	Unnamed siltstone and sandstone (Paleocene).  Dark 
gray, indistinctly to distinctly bedded siltstone, 
claystone, and shale.  Grades downward into a 
basal, indistinctly bedded, dark brown to green, 
coarse-grained, glauconite bearing, lithic 
sandstone.  This unit rests unconformably on 
unnamed Late Cretaceous sandstone and shale east of 
Niles.  It is about 100 meters thick.  Overlying 
strata have been faulted or eroded away.  Three 
samples of the mudstone have yielded abundant 
coccolith floras of late Paleocene (CP-8) age (D. 
Bukry, U.S. Geol. Survey, written comm., 1992).

Tes	Unnamed mudstone (Eocene).  Green and maroon, 
foraminifer rich mudstone, locally interbedded with 
hard, distinctly bedded, mica bearing, quartz 
sandstone.  This unit is bounded above and below by 
faults.

	Tolman Formation of Hall (1958) (Eocene?).  Divided 
into:
Ttls	Gray, algal limestone, interbedded with white, 
calcium carbonate matrix pebble conglomerate, 
and clean, medium- to coarse-grained sandstone 
with calcite cement.  Sandstone grains include 
quartz, feldspar, and lithic fragments, and in 
places include a large percentage of algal 
debris.  Sandstone weathers to orange color.

Tts	Dark gray to dark greenish-gray, indistinctly 
bedded, glauconite bearing, medium- to coarse-
grained lithic sandstone.  Locally interbedded 
with minor amounts of fine-grained sandstone 
and siltstone.
	The Tolman Formation is bounded above by a fault.  
It overlies Redwood Creek Formation sandstone and 
Pinehurst shale along an obscured contact.

Tgs	Unnamed glauconitic mudstone (Oligocene(?) and 
Miocene).  Brown mudstone, interbedded with 
glauconite grain sandy mudstone.  Locally mudstone 
and sandy mudstone contains phosphate nodules up to 
one centimeter in diameter.  Locally interbedded 
with brown siltstone and fine-grained sandstone 
(Tgss).  This unit is bounded below and above by 
faults.  This unit is equivalent to the Sobrante(?) 
Formation of Radbruch (1969).

Tsh	Unnamed sandstone, shale, chert, and dolomite 
(early Miocene).  Sandstone is massive, medium-
grained quartz arenite which weathers orange.  The 
sandstone is interbedded with dark gray siltstone 
and claystone containing many concretions, and some 
conglomerate.  The conglomerate contains pebbles of 
varicolored chert, andesite, and quartzite in 
dolomite matrix.  This formation also includes a 
breccia at the base containing large (1-2 meters 
long) angular to subrounded blocks of dolomite, 
chert, siliceous shale, and sandstone containing 
plant fragments, in a highly sheared siltstone and 
argillite matrix.  A concretion in the shale has 
yielded Siphogenerina transversa and a rich benthic 
fauna of late Saucesian (early Miocene) age (K. 
McDougall, U.S. Geol. Survey, written comm., 1992).  
This unit is limited to a small, fault bounded lens 
near the mouth of Niles Canyon.

Ts	Sobrante Sandstone (middle Miocene).  White, fine- 
to medium-grained quartz sandstone.  Occurs in 
discontinuous outcrops below the base of the 
Claremont Formation, in fault contact with 
underlying Cretaceous strata.

	Claremont Formation (middle to late Miocene).  
Divided informally into:
Tcc	Chert and siliceous shale member.  Chert 
occurs as distinct, massive to laminated, gray 
or brown beds as much as 10 cm thick with thin 
shale partings.  Distinctive black, laminated 
chert occurs in Alum Rock Canyon and in minor 
amounts in the Mission Pass area and the 
Berkeley Hills.  Siliceous shale is dark brown 
to gray, finely laminated, with grains as 
large as silt size.  Some of the shale 
contains abundant foraminifers and fish 
scales.  This member also contains prominent 
interbedded lenses of massive, tan, 
foraminifer-bearing dolomite as much as one 
meter in length, that weathers to a 
distinctive yellowish orange color.  The 
Claremont chert and shale member is similar to 
the Tice Shale, but is distinguished by the 
presence of chert and more siliceous shale.
Tcs	Sandstone and siltstone member.  Light brown, 
gray, and white, fine-grained quartz sandstone 
and siltstone.
	The Claremont formation is fault bounded above and 
below throughout the Berkeley and Oakland hills.  
It unconformably overlies the Sobrante Sandstone in 
the Niles area, and unconformably overlies 
Cretaceous strata in the Milpitas area.

To	Oursan Sandstone (middle to late Miocene).  Brown 
to tan siltstone and fine-grained sandstone, and 
distinctly to indistinctly bedded black mudstone.  
In places the sandstone contains large amounts of 
secondary calcite.  The sandstone locally contains 
large lenses, as long as 2 meters, of tan, 
foraminifer bearing dolomite that weather to a 
distinctive yellowish orange color, similar to 
those found in the Claremont Formation.  Poorly 
preserved foraminifers are common in the siltstone 
and mudstone beds.  This unit only occurs in the 
Mission Pass area, where it lies conformably on the 
Claremont Formation.

Tt	Tice Shale (late Miocene).  Distinctly bedded, dark 
brown, gray, and tan, siltstone, mudstone, and 
siliceous shale.  Tice Shale weathers locally to a 
reddish brown color.  The shale contains numerous 
lenses of massive, tan dolomite, as much as two 
meters in length, that weather to a characteristic 
yellowish orange color, similar to those found in 
the Oursan and Claremont Formations.  Locally, the 
shale also contains abundant fish scales.  This 
unit only occurs in the Mission Pass area, where it 
lies conformably on the Oursan Sandstone.

Tbr	Briones Formation (late Miocene).  The basal part 
of this Formation consists of distinctly bedded, 
gray to white, fine-grained sandstone and 
siltstone.  Sandstone beds are as thin as 5 to 10 
cm, with 2 to 10 cm thick shale interbeds.  These 
are interbedded with massive fine-grained sandstone 
beds as much as five meters thick.  The middle part 
of the Formation consists of indistinctly bedded, 
white, fine- to coarse-grained sandstone, 
conglomeratic sandstone, and massive shell-hash 
conglomerate (shell beds).  Shell-hash conglomerate 
is made up of interlocking mollusk and barnacle 
shells and shell fragments in a white calcareous 
sandstone matrix.  The upper portion of the 
Formation consists of distinctly to indistinctly 
bedded, massive to cross-bedded, fine- to coarse-
grained white sandstone.  The Briones Formation is 
restricted to the southern part of the Hayward 
fault zone.  It lies for the most part 
unconformably over the Claremont Formation, but 
lies conformably on the Tice Shale where that unit 
is present.

Tor	Orinda Formation (late Miocene).  Distinctly to 
indistinctly bedded, non-marine, pebble to boulder 
conglomerate, conglomeratic sandstone, and coarse- 
to medium-grained lithic sandstone.  Clasts are 
sub-angular to well rounded, and contain a high 
percentage of detritus derived from the Franciscan 
complex.  The Formation locally includes 
interlayered bright green porphyritic dacite sills 
(Torv) with phenocrysts of quartz and plagioclase 
and large xenoliths of pink and red chert.  Orinda 
Formation lies unconformably over Briones Formation 
in the southern part of the Hayward fault zone, but 
is faulted at the base in the northern part.

Tm	Moraga Formation (late Miocene).  Basalt and 
andesite flows, with minor rhyolite tuff.  Ar/Ar 
ages ranging from 9.0+-0.3 to 10.2+-0.5 Ma have 
been reported from the Moraga Formation (Curtis, 
1989).  Interflow sedimentary rocks (Tms) are 
mapped locally.  The Moraga Formation conformably 
overlies the Orinda Formation in the northern part 
of the Hayward fault zone.

Tst	Siesta Formation (late Miocene).  Non-marine 
siltstone, claystone, sandstone, and minor 
conglomerate.  The Siesta Formation conformably 
overlies the Moraga formation.

Tbp	Bald Peak Basalt (late Miocene).  Massive basalt 
flows.  Ar/Ar ages ranging from 8.37+-0.2 to 8.46+-
0.2 Ma have been reported from this unit (Curtis, 
1989).  Bald Peak Basalt conformably overlies the 
Siesta Formation.

Tn	Neroly Sandstone (late Miocene).  Blue or white, 
volcanic-rich, shallow marine sandstone.  This unit 
only occurs within the Hayward fault zone as a 
small fault lens east of Oakland.

Tv	Unnamed volcanic rocks (late Miocene or early 
Pliocene).  This unit shows remarkable variation in 
lithology, although it occurs in only a small area 
northwest of Niles.  In the southern part of the 
outcrop area it consists of white to gray, rhyolite 
to andesite, interbedded pyroclastic flows, tuff, 
volcanic breccia, and volcanoclastic conglomerate.  
In the northern part, the rocks are mainly black or 
red porphyritic basalt.  Phenocrysts include 
plagioclase, pyroxene, and olivine.  The volcanic 
rocks unconformably overlie the Orinda formation.

Tss	Unnamed sandstone and conglomerate (Tertiary).  
Red, coarse-grained, glauconite bearing lithic 
sandstone and pebble conglomerate.  Contains many 
small pectin and other invertebrate fossils 
locally.  Occurs within the Hayward fault zone only 
as two small fault lenses east of Berkeley.

Pliocene and Pleistocene Gravels

Tsk	Silver Creek Gravels of Graymer and DeVito (1993) 
(Pliocene).  Interbedded conglomerate, sandstone, 
siltstone, tuffaceous sediments, tuff and basalt.  
This unit is distinguished from other gravels by 
the presence of interbedded volcanic rocks, the 
interbeds of nonmarine green and red mudstone, its 
relatively well consolidated nature, and by clast 
composition.  About 75% of the detritus is derived 
from Franciscan rocks, whereas the other 25% is 
derived from volcanic rocks, chert from the 
Claremont Formation, and other Cenozoic rocks.  An 
interbedded basalt within this unit has been dated 
at 2.6 Ma (Nakata and others, 1993), and a 
interbedded tuff within Silver Creek Valley has 
been dated at about 3 to 4 Ma (M. Wills, Cal. 
State. Univ. San Jose, personal comm., 1995).  The 
base of this unit is nowhere exposed, and the top 
is cut by the Silver Creek thrust.
QTp	Packwood Gravels of Crittenden (1951) (Pliocene and 
Pleistocene(?)).   This rock unit generally 
consists of gravel to cobble (rare) silty and fine 
sandy conglomerate, fine silty sandstone, gravely 
to fine sandy siltstone, and minor olive-green 
claystone beds.  Noteworthy is the presence of 
numerous nonmarine red beds in the unit.  This unit 
differs from all other gravels in that the clasts 
are composed almost entirely of detritus derived 
from conglomerates and sandstone of the Cretaceous 
Great Valley Sequence.  The base of this unit is 
interbedded with and coeval to the Silver Creek 
gravels (M. Wills and D. Andersen, Cal. State Univ. 
San Jose, personal comm., 1995).  However, the top 
of the unit postdates and overlaps the Silver Creek 
thrust, which postdates the deposition of the 
Silver Creek Gravels

QTl	Livermore Gravels (Pliocene and/or Pleistocene).  
Poorly to moderately consolidated, indistinctly 
bedded, cobble conglomerate, gray conglomeratic 
sandstone, and gray coarse-grained sandstone.  Also 
includes some siltstone and claystone.  Clasts 
contain mostly graywacke, chert, and metamorphic 
detritus probably derived from the Franciscan 
complex.  There are no known age diagnostic fossils 
or volcanic rocks from the Livermore Gravels within 
the Hayward fault zone.  The unit is, however, very 
similar, and may be equivalent to, the Irvington 
Gravels.

QTi	Irvington Gravels of Savage (1951) (Pliocene(?) and 
Pleistocene).  Poorly to well consolidated, 
distinctly bedded cobble conglomerate, gray 
conglomeratic sandstone, and gray, coarse-grained, 
cross-bedded sandstone.  Clasts consist of about 
one-half micaceous sandstone derived from both the 
Great Valley Sequence and the Franciscan complex, 
and one-half chert, metamorphic and volcanic rocks 
derived from the Franciscan complex..  The gravels 
also include rare but distinctive clasts of 
laminated black chert from the Claremont Formation.  
A large number of Pleistocene (Irvingtonian) 
vertebrate fossils has been collected from this 
unit (Savage, 1951).  The fossils are restricted to 
a relatively narrow horizon, however, so it is 
possible that this unit could contain strata as old 
as Pliocene.

Quaternary Deposits

Qoa	Unnamed gravels and terrace deposits (Pleistocene).  
Poorly to moderately consolidated, boulder to 
pebble conglomerate and coarse-grained lithic 
sandstone.  Clasts composition varies greatly.  
Late Pleistocene (Rancholabrean) age fossils have 
been reported from two localities in this unit (UCV 
5928 and UCV 6304 in Hayward quadrangle, UCV 5834 
in Oakland East quadrangle).  

Qls	Landslide deposits (Pleistocene and/or Holocene).  
Poorly sorted clay, silt, sand, and gravel.  Only a 
few very large landslides have been mapped.  For a 
more complete map of landslide deposits, see Nilsen 
and others (1979).

Qu	Undivided surficial deposits (Pleistocene or 
Holocene).  Includes alluvial fan, levee, stream 
channel, bay mud, floodplain, and floodbasin 
deposits.

Qm	Manmade deposits (Holocene).  Landfill and dams.

Structures within the Hayward fault zone

Overview

The Hayward fault zone, as described in this paper, is a 
broad zone of deformation 2 km to 10 km wide, trending 
approximately north 30 west from an area east of San Jose to 
San Pablo Bay (and perhaps beyond).  The zone is bounded on 
the east by a series of faults that are the result of 
deformation not related to the Hayward fault zone, including 
the Calaveras, Stonybrook, Palomares, Miller Creek, and 
Moraga faults.  These are shown as Eastern boundary faults on 
sheet 3.  The western boundary of the fault zone is not as 
well defined because it is obscured by Quaternary deposits of 
the San Francisco Bay basin.  Several mapped faults must cut 
or extend beneath these deposits, but not enough data are 
available to portray the location of these faults accurately.  
Therefore the western margin of the Hayward fault zone is 
provisionally regarded as the westernmost fault trace shown 
on our map, which corresponds more or less with the onlap of 
thick Quaternary deposits. 
	Within the zone, structures reflect compressional, 
extensional, and right-lateral strike-slip deformation.  The 
zone can be arbitrarily divided into four subzones, areas 
within which the dominant structural style is relatively 
uniform.  The segregation of structural style into these 
areas is a result of changing location and style of 
deformation during the development of the Hayward fault zone 
through geologic history.

Structural Subzones

The four structural subzones are named for their relative 
geographic position within the Hayward fault zone: San Pablo, 
San Leandro, Castro Valley, and Fremont.  Each contains a 
distinct structural style.
	The San Pablo subzone is dominated by east vergent 
reverse faults.  In this subzone, bedrock is close to the 
surface throughout and west of the fault zone, as opposed to 
the San Leandro subzone and most of the Fremont subzone where 
depth to bedrock increases rapidly west of the alluvial 
boundary.  The youngest rocks offset by the structures in 
this subzone are late Miocene volcanic rocks.  The Wildcat 
fault, however, has geomorphic features that indicate 
Quaternary offset.
	The San Leandro subzone is bounded by the Chabot fault 
and the alluvial flat of the Bay.  It is characterized by the 
presence of the oldest rocks in the fault zone (Franciscan, 
ophiolite, keratophyre, and Knoxville), and closely spaced, 
imbricate, west vergent thrust and reverse faults.  Several 
of the faults in this subzone cut late Pleistocene 
(Rancholabrean) or younger alluvial deposits.  Many of the 
faults are confined to the Jurassic and Cretaceous rocks, 
however, and probably are related to Campanian or younger 
amalgamation of Franciscan and Coast Range Ophiolite/Great 
Valley sequence rocks.  The Pleistocene or younger faults may 
be reactivated strands of the older fault system.
	The Castro Valley subzone lies east of the Chabot fault.  
It is bounded on the south by the Sheridan Creek fault.  The 
structures in the Castro Valley subzone may be related in 
part to those in the San Pablo subzone.  They are broadly 
spaced, east-vergent reverse and thrust faults.  One fault is 
synformal, creating a detached plate of Late Cretaceous 
strata (Graymer, Jones, and Brabb, 1995b).  The youngest 
rocks cut by these faults are middle to late Miocene 
(Claremont Formation), and there is no evidence of Quaternary 
deformation in the Castro Valley subzone.
	The Fremont subzone includes and lies south of the 
Sheridan Creek fault.  A fault within this subzone, the 
Mission fault, truncates and offsets the Chabot fault in the 
vicinity of Niles, where the Chabot fault forms the boundary 
between Knoxville formation on the west and Miocene strata on 
the east.  The Mission fault cuts and offsets this boundary 
approximately 7.5 km in a right lateral sense.  These offset 
points are labled A and B on sheet 2 (hfsplt.ps).  This 
subzone contains broadly to moderately closely spaced, west-
vergent reverse and thrust faults.  Faults in this subzone 
cut Pleistocene (Irvingtonian) gravels.  In addition, the 
offset of the Chabot fault by the Mission fault indicates 
late Pleistocene or younger offset on the Mission fault.  
Active seismicity has been recorded on several faults within 
this subzone.
	The Hayward fault zone connects to the Calaveras fault 
zone via the faults in the Fremont subzone.  Andrews and 
others (1993) emphasized the trend of deep seismic activity 
in the Mission Peak region as the connecting link.  However, 
active seismicity also occurs along faults in the Fremont 
subzone south of Mission Peak, such as the Warm Springs fault 
(Walter and others, in press).  In addition, the 1868 Hayward 
fault rupture extended as far south as Milpitas, about 16 
kilometers south of the proposed connection (Lawson, 1908).  
Average creep rate along the creeping strand south of the 
proposed connection (8 mm/yr.) is higher than the average 
rate north of the connection (6 mm/yr., Lienkaemper, 1992).  
And finally, there is geomorphic evidence for late Quaternary 
offset on the Clayton fault in the area east of San Jose 
(Dibblee, 1972c).  These data suggest that rather than a 
single connection, the Hayward fault zone connection to the 
Calaveras fault zone is distributed through many of the 
faults in the Fremont subzone, of which the Mission crossover 
is only one example.
	The amount of right-lateral offset of late Miocene or 
older rocks along the Hayward fault zone remains 
controversial.  Estimates range from 7 to 27 km based on 
identification of possible source terranes for the Orinda 
Formation (Graham and others, 1984), to as much as 190 km, 
based on matching the Tolay volcanics north of the map area 
in Sonoma County with the Quien Sabe volcanics south of the 
map area in Santa Clara County (although an undetermined 
amount of that 190 km is also apportioned to the Calaveras 
fault zone, Jones and Curtis, 1991).  Linieki-Laporte and 
Andersen (1988) estimated 38 km of latest Miocene (late 
Hemphilian or about 7.7 to 6.5 Ma) Hayward fault offset based 
on matching the lower part of the Mulholland Formation east 
of the Hayward fault zone in Contra Costa County with the 
Petaluma Formation north of the study area in Sonoma County.  
The Mulholland is not cut by the Hayward fault zone, however, 
but by the Moraga-Miller Creek faults, so the 38 km can only 
be considered an estimate of post-late Miocene offset on 
those faults.  An intermediate estimate of post-late Miocene 
offset on the Hayward fault zone is 43 km based on matching 
the Tolay volcanics with the Moraga volcanics (Fox and 
others, 1985, Curtis, 1989, Jones and Curtis, 1991) because 
of the presence of coeval (about 8 Ma) titanaugite bearing 
basalt flows in each.  Strong lithic contrast between the 
units as a whole casts doubt on the correlation, however 
(Jones and Curtis, 1991).  In addition, the pre-Moraga and 
pre-Tolay units do not match, requiring pre-late Miocene 
offset along the fault zone.  The evidence of 30 km or more 
of post-late Pliocene right-lateral offset within this study 
area (see below) is inconsistent with the small amounts of 
offset (7-27 km) proposed by Graham and others (1984).  The 
evidence is consistent with the intermediate estimate of 43 
km, but suggests an acceleration of offset in the last 2 Ma 
from a long term rate of 13 km/ 6 Ma to 30 km/ 2 Ma.  Given 
the uncertainty of the correlation of the Tolay and Moraga 
volcanics, we provisionally accept the estimate of 43 km 
post-late Miocene offset as a minimum figure.  It is 
important to note that very little of this offset has 
occurred on the creeping strand.  Based on offset Jurassic 
and Cretaceous units between Niles and Berkeley, as little as 
3 km of offset has occurred along the creeping strand.  An 
example of this small amount of offset is indicated by the 
apparent 3 km displacement of a body of keratophyre cut by 
the creeping strand in east Oakland.  This offset  is denoted 
by points C and D on sheet 1 (hfnplt.ps).

Structural History

	The oldest structures within the Hayward fault zone 
involve the Jurassic and Early Cretaceous rocks in the San 
Leandro subzone.  The rocks are imbricated by closely spaced 
thrust faults.  Some of the interleaving is probably related 
to initial amalgamation of Franciscan complex and Coast Range 
ophiolite/Great Valley sequence terranes.  As shown by the 
presence of Campanian strata in both terranes, amalgamation 
occurred after Campanian deposition in the latest Cretaceous 
or early Tertiary.  This juxtaposition must have been largely 
completed by Miocene time because the same Miocene rocks 
(including Claremont and Sobrante Formations) were deposited 
on both terranes.  The depositional contact of Miocene rocks 
on Franciscan rocks is not preserved within the Hayward fault 
zone, but is just east of the zone in the area of Sunol 
Regional Park.
	During the late Miocene to early Pliocene, deformation 
was dominated by broadly spaced, east-vergent thrust faults.  
These structures are preserved in the San Pablo and Castro 
Valley subzones.  Although many of the structures in the 
Castro Valley subzone cut predominantly Late Cretaceous 
rocks, fault slices of middle to late Miocene rocks in the 
vicinity of Upper San Leandro Reservoir demonstrate that 
these structures are late Miocene or younger.  Even though 
the faults at this time were more widely spaced than earlier 
faults, the amount of compressive deformation remains very 
large.  One unnamed, synformal, folded thrust fault in the 
Castro Valley subzone doubles the Cretaceous section by 
stacking Joaquin Miller Formation and Oakland Sandstone on 
top of Shephard Creek and Redwood Canyon Formations and 
Pinehurst Shale (Graymer, Jones, and Brabb, 1995b).  Large 
scale right-lateral offset was also occurring in late Miocene 
to early Pliocene time.  As discussed above, late Miocene 
rocks have been offset at least 43 km.  At least 13 km of 
offset probably took place during late Miocene and early 
Pliocene time.
	During the late Pliocene to early Pleistocene, 
deformation seems to have been concentrated along the Chabot 
fault.  The Chabot fault underwent 23 km of right lateral 
offset, based on correlation of Pleistocene gravels (Graymer, 
Jones, and Brabb, 1995a).  In addition, the Chabot fault 
appears to be an east-dipping oblique normal fault that has 
placed Late Cretaceous and younger strata (doubled by the 
earlier thrusting) in the hanging wall down onto Jurassic and 
Early Cretaceous rocks in the footwall.  Although, Crane 
(1995) has suggested that the Chabot fault is an overturned, 
east vergent thrust fault, we have found no evidence to 
support this interpretation.
	Late Pleistocene and early Holocene deformation was 
predominantly west directed oblique thrusting, which is 
evident in the Fremont subzone.  The Mission fault has 7.5 km 
of late Pleistocene and younger right-lateral offset where it 
cuts the Chabot fault north of Niles, and 1500 meters of 
vertical offset at Mission Peak based on projected offset of 
the Briones-Orinda contact.  Farther north, the late 
Pleistocene transpressional regime reactivated old faults or 
created new faults in the San Pablo and San Leandro subzones.  
This is demonstrated by the cutting of late Pleistocene 
(Rancholabrean age) gravels in the Oakland hills as well as 
the geomorphic features of the Wildcat and other unnamed 
faults.  Additionally, recent observations by us and others 
have documented at least one parallel strand 400 meters west 
of the creeping strand in the San Leandro area that cuts the 
most recent soil horizon along a west vergent oblique reverse 
fault.  
	Most of the historic offsets within the Hayward fault 
zone are well documented by Lienkaemper (1992) in his map of 
the creeping strand.    In addition, several faults in the 
Fremont subzone are seismically active.  Others in the San 
Pablo and San Leandro subzones may also be seismically active 
but are too close to the creeping strand to be 
distinguishable in the seismic record.  More trenching and 
other studies are needed to determine the full extent of 
deformation along the Hayward fault zone, and where and how 
much ground rupture may take place in the future.

Summary

	The Hayward fault zone is a broad zone of deformation, 
within which the style and location of deformation has 
changed through geologic time.  This change in location and 
style has led to the development of different structures in 
different parts of the fault zone, herein called subzones.  
Faulting in historic time is concentrated near the western 
margin of the fault zone, except in the Fremont subzone where 
it diffuses into several sub-parallel faults as it joins the 
Calaveras fault.    The zone as a whole has undergone at 
least 43 km of right-lateral offset and a large but still 
unmeasured amount of compressional deformation.  As little as 
3 km of the right-lateral offset has occurred on the creeping 
strand.   More research is required to determine whether or 
not faults outside the creeping strand are as dangerous or 
more dangerous than the creeping strand itself.  However, 
given the active seismicity and offset of recent soils along 
strands other than the creeping strand, it is probable that 
the hazard zone will contain more than just the creeping 
strand.

Sources of data

All of the geologic mapping for the Hayward fault zone is 
new, but we acknowledge our debt to previous workers who 
established the geologic framework.  Data sources for 
geologic mapping in the fault zone within Contra Costa County 
are provided by Graymer, Jones, and Brabb (1994).  In Alameda 
and Santa Clara Counties, the geologic reports by Radbruch 
(1969), Case (1968), Graymer and others (1994), Robinson 
(1956), Hall (1958), and Crittenden (1951) are especially 
relevant.  Preliminary geologic maps by T.W. Dibblee of most 
of the 1:24,000 quadrangles are available in Open-File 
Reports of the U.S. Geological Survey (see references).  The 
map and report by Lienkaemper (1992) provided some of the 
lines for the creeping strand of the Hayward fault, and they 
contain many references to previous work.  Several students 
have done theses in the area; the ones by Haltenhoff (1978), 
Whiteley (1978), and Prowell (1974) are especially pertinent.

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