U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Map 3395

Sheet 2 of 2

Pamphlet accompanies map

Framework Geologic Map and Structure Sections along the Bartlett Springs Fault Zone

and Adjacent Area from Round Valley to Wilbur Springs, Northern Coast Ranges, California

By

 Robert J. McLaughlin1

, Barry C. Moring1, Christopher S. Hitchcock2, and Zenon C. Valin1

2018 1

U.S. Geological Survey;

2

InfraTerra, Inc.

Any use of trade, product, or firm names in this publication is for descriptive 

purposes only and does not imply endorsement by the U.S. Government

This plate is offered as an online-only, digital publication. Users should be 

aware that, because of differences in rendering processes and pixel 

resolution, some slight distortion of scale may occur when viewing it on a 

computer screen or when printing it on an electronic plotter, even when it is 

viewed or printed at its intended publication scale

Digital files available at https://doi.org/10.3133/sim3395

Suggested citation: McLaughlin, R.J., Moring, B.C., Hitchcock, C.S., and 

Valin, Z.N., 2018, Framework geologic map and structure sections along the 

Bartlett Springs Fault Zone and adjacent area from Round Valley to Wilbur 

Springs, northern Coast Ranges, California: U.S. Geological Survey 

Scientific Investigations Map 3395, pamphlet 60 p., 2 sheets, scale 1:100,000, 

https://doi.org/10.3133/sim3395.

ISSN 2329-132X (online)

https://doi.org/10.3133/sim3395

KJs

Jgb

KJom

omv

oms

omd

omg

omc

omb

omun

Joc

Jov

Jod

Jog

Josp

fcm

ss

c

v

cgl

ms

b

un

spo

vpo

vsm

dsm

psm

ssm

fym

by

cgy

cy

gy

lsy

vy

uny

mspy

fys

cy

cpp

vpp

fmp

cmp

vy

fpp

miy

db

vmg

cmg

cgmg

smg

af

Qls

Qal

Qoa

Qt

Qty

Qto

Qtvo

Qf

Qfy

Qfo

Qfvo

Qgd

Qsn

Qtrv

Qv

QTc

Tm

Te

Tep

Tcg

Ku

Kuls

Kul

Kl

Klcg

Klls

Kssp

LIST OF MAP UNITS

See Description of Map Units (in pamphlet) for complete unit descriptions. 

Some unit exposures on the printed or plotted map are too small to distinguish 

the color for unit identification. These units are labeled where possible, and 

unlabeled units are attributed in the database. Note that many tiny units, primarily blocks, are covered by symbols.

UNCONSOLIDATED DEPOSITS

Artificial fill (Holocene)

Landslide deposits (Holocene and Pleistocene)

ALLUVIAL DEPOSITS

Alluvial deposits (Holocene and Pleistocene)

Older alluvial deposits (Holocene and Pleistocene)

TERRACE DEPOSITS

Alluvial terrace deposits (Holocene and Pleistocene)

Younger alluvial terrace deposits and surfaces (Holocene?)

Older alluvial terrace deposits and surfaces (Pleistocene)

Very old alluvial terrace deposits and surfaces (Pleistocene)

FAN DEPOSITS

Alluvial fan deposits (Holocene and Pleistocene)

Younger alluvial fan deposits and surfaces (Holocene?)

Older alluvial fan deposits and surfaces (Pleistocene)

Very old alluvial fan deposits and surfaces (Pleistocene)

GLACIAL DEPOSITS

Glacial deposits, present locally (Pleistocene)

SPRING DEPOSITS

Hydrothermal deposits (Pleistocene and younger)

Travertine (Pleistocene and younger)

VOLCANIC ROCKS

Clear Lake Volcanic rocks (Pleistocene)

FLUVIAL AND LACUSTRINE FILL

Cache Formation (Pleistocene and upper Pliocene)

MARINE OVERLAP DEPOSITS

Sandstone and lignitic shale of the Covelo area (middle Miocene)

Marine sandstone and shale (Eocene)

Sandstone, shale, and conglomerate (Eocene and upper Paleocene)

Conglomerate (Eocene and upper Paleocene)

GREAT VALLEY COMPLEX 

Sandstone, siltstone, and mudstone (Upper Cretaceous, 

Cenomanian and younger)

Limestone (Upper Cretaceous)

Sandstone, siltstone, and mudstone with concretionary limestone 

and conglomerate (Upper and Lower Cretaceous, Cenomanian to Hauterivian)

ELDER CREEK TERRANE

Siltstone, mudstone, minor sandstone, conglomerate, and 

sedimentary serpentinite (Lower Cretaceous, Albian to 

Hauterivian)

Conglomerate (Lower Cretaceous, Albian to Hauterivian)

Bioclastic limestone, (Lower Cretaceous, Hauterivian)

Sedimentary serpentinite (Lower Cretaceous)

Argillite, siltstone, and sandstone (Lower Cretaceous, upper 

Valanginian, to Upper Jurassic, Tithonian)

Megabreccia (Upper Jurassic, Tithonian to Kimmeridgian)

Ophiolitic mélange (Lower Cretaceous)

Mafic volcanic rocks, including pillow basalt, diabase, and 

noncumulate gabbroic intrusive rocks

Undivided marine clastic rocks, commonly argillitic (Lower 

Cretaceous and Upper Jurassic)

Diabase, mapped locally

Gabbro, mapped locally

Radiolarian chert, mapped locally (Upper Jurassic, Kimmeridgian)

Blueschist, mapped locally

Blocks of unknown lithology

COAST RANGE OPHIOLITE

Pelagic chert (Upper to Middle Jurassic, Tithonian to Bajocian)

Basaltic pillowed flows and flow breccias (Middle Jurassic)

Diabase dikes and sills (Middle Jurassic)

Gabbro and ultramafic rocks (Middle Jurassic)

Serpentinized ultramafic rocks (Middle Jurassic)

FRANCISCAN COMPLEX

CENTRAL BELT

Central Belt Mélange terrane (lower Tertiary and Upper 

Cretaceous)

Undivided mélange (lower Tertiary and Upper Cretaceous)

Sandstone and argillite (in part, Upper Cretaceous, Cenomanian or younger; middle Cretaceous, Aptian or younger; 

and possibly, Upper Jurassic, Tithonian or younger)

Radiolarian chert (Upper Cretaceous to Middle Jurassic)

Basaltic volcanic rocks (Jurassic)

Conglomerate (Lower Cretaceous and (or) Upper Jurassic 

or younger)

Diabase (Cretaceous or Jurassic?)

Metasedimentary rocks (Lower Cretaceous? or younger)

Blueschist blocks (Jurassic)

Blocks of unknown lithology

Pomo terrane (Upper Cretaceous to Upper Jurassic)

Basaltic breccia, sandstone, and shale (Upper Cretaceous, 

Maastrichtean)

Alkalic pillow basalt, pillow-breccias, and minor diabase 

and chert (Jurassic)

Marin Headlands-Geysers terrane (Upper Cretaceous to 

Lower Jurassic)

Sandstone and argillite (Upper Cretaceous, Cenomanian? or 

younger)

Conglomerate (Upper Cretaceous, Cenomanian? or 

younger)

Radiolarian chert (Upper Cretaceous, lower Cenomanian to 

Lower Jurassic, Pliensbachian)

Basaltic volcanic rocks (Lower Jurassic)

Snow Mountain volcanic terrane (Lower Cretaceous, 

Valanginian or younger?)

Basaltic to rhyolitic volcanics and intrusive rocks

Diabasic intrusive rocks, largely sill-like

Porphyritic basaltic rocks

Metasandstone and argillite

EASTERN BELT

Yolla Bolly terrane (Upper and Lower Cretaceous to Upper 

Jurassic, Tithonian)

Mélange of the Yolla Bolly terrane (Upper Cretaceous)

Blueschist

Conglomerate

Metachert (Early Cretaceous, Albian or Aptian, to Middle 

Jurassic, Aelenian)

Gabbro

Limey mudstone

Metavolcanic rocks (Middle Jurassic, Aelenian or older)

Metaserpentinite blocks or lenses

Unidentified blocks

Metasandstone and argillite, locally conglomeratic (Upper and 

Lower Cretaceous, Cenomanian to Aptian, to Late Jurassic, 

Tithonian)

Metachert (Lower Cretaceous, Albian to Aptian, to Middle 

Jurassic, Aalenian)

Metavolcanic rocks (Middle Jurassic, Aalenian or older)

Mafic intrusive rocks (Early Cretaceous)

Pickett Peak terrane (Lower Cretaceous, Aptian to Barremian)

Undivided metasedimentary and metavolcanic rocks and 

metachert

Metachert

Basaltic metavolcanic rocks

Mendocino Pass terrane (Lower Cretaceous, Valanginian)

Foliated metasandstone, argillite, and minor undivided 

metachert and greenstone

Metachert and greenstone blocks

EXPLANATION OF MAP SYMBOLS

Contact—Solid where location is accurate; long-dashed where 

approximate; short-dashed where inferred; dotted where 

concealed. Queried where identity or existence questionable

Fault—Solid where location is accurate; long-dashed where 

approximate; short-dashed where inferred; dotted where 

concealed. Queried where identity or existence questionable

Fault Lineament—Inferred from features on aerial photography. 

Dotted where projected beneath surficial deposits

Tuff of Clearlake Volcanic rocks (Qv)—Mapped in Cache 

Formation (QTc)

Active trace of Bartlet Springs Fault Zone (Lienkaemper, 

2010)—Arrows show relative movement. In cross sections: 

circled minus, movement away from observer; circled plus, 

movement towards observer

Strike and dip of beds

Inclined

Overturned

Horizontal

Vertical

Strike and dip of metamorphic fabric in sheared or foliated rocks

Inclined 

Vertical

Location of block or lens too small to map in broken formation or 

mélange

Higher-grade metamorphic rock

Serpentinite

Volcanic rock

Sandstone

Conglomerate

Chert

Block of unknown lithology

Fossil Location—Map number corresponds to table 1 

Hydrothermal bleaching (Quaternary)—Diffuse zone in 

sedimentary and metasedimentary rocks, characterized by quartz, 

carbonate, and clay mineral veins and veinlets. Associated with 

the Barttlet Springs Fault Zone in the vicinity of Neuman 

Springs

Sinter or silica carbonate rocks

• Travertine

40

40

30

58•

CENTRAL BELT

Yolla Bolly Terrane

FRANCISCAN COMPLEX 

Pickett Peak

Terrane

Mendocino 

Pass Terrane

EASTERN BELT

 MARINE OVERLAP DEPOSITS

FLUVIAL AND LACUSTRINE FILL

Pomo

Terrane

Central Belt Mélange Terrane

Marin 

HeadlandsGeysers

Terrane

TERTIARY

 COAST

RANGE

GREAT VALLEY COMPLEX OPHIOLITE

Miocene

Eocene 

Eocene and 

Paleocene

Pleistocene 

and Pliocene

MESOZOIC

CENOZOIC

Early

Jurassic

Rice Valley ELDER CREEK TERRANE

CORRELATION OF MAP UNITS

UNCONSOLIDATED DEPOSITS

CRETACEOUS

Early

Cretaceous

JURASSIC

Holocene

VOLCANIC

ROCKS

[See Description of Map Units (in pamphlet) for precise unit ages]

Late

Cretaceous

Pleistocene

TERRACE

DEPOSITS

ALLUVIAL

DEPOSITS

FAN

DEPOSITS

GLACIAL

 DEPOSITS

SPRING 

DEPOSITS

Late 

Jurassic

Middle 

Jurassic

QUATERNARY

QUATERNARY

AND TERTIARY

Snow 

Mountain 

Volcanic

Terrane

Qls 

Qfy

Qfo

Qfvo

Qal 

Qty

Qto

Qtvo

miy

Qt

Qoa 

fys 

fys 

fmp

Qsn

?

?

fpp

Qtrv

?

?

Qf

af

Qgd

Qv

Tm

Te

Tep

QTc

Tcg

fym 

by cgy cy gy lsy mspy

uny vy

?

?

?

?

?

?

Kl

Klcg

Joc

Jov

Jod

Jog

Josp

?

cmp

?

?

?

?

?

?

?

?

?

?

?

?

?

Kssp

Klls

KJom

omb 

omv 

omg omun 

?

?

?

?

?

?

oms omd 

omc

Jgb

KJs

ss 

?

c b

un

cgl 

db

ms

v

?

fcm

spo

vpo

cmg

smg 

vmg

?

?

dsm

psm

ssm

ssm

vsm cpp

vpp

Kuls

cy

vy

Kul

Ku

cgmg 

Little Indian

Valley

Qfo

QtvoQal fys Qal Qal

fcm

fcm fcm

fcm

fym

fys fym fys

fys

fym

Qal Tep KJom

KJom

Kul

fpp

Kul

Qal 4,000

2,000

SEA LEVEL SEA LEVEL

2,000

4,000

6,000

8,000

10,000

12,000

FEET

E

4,000

2,000

2,000

4,000

6,000

8,000

10,000

12,000

FEET

E'

NORTHERN BARTLETT SPRINGS FAULT ZONE

NORTHWEST SOUTHEAST

NORTHEAST

NORTHEAST

NORTHEAST

SOUTHWEST

SOUTHWEST

SOUTHWEST

SOUTHWEST NORTHEAST

SECTION

B’–B”

SECTION

C–C'

fcm

fcm

fcm

KJom?

fym

fys

fys

QfyQal

Qoa Josp KJom? Qt fys?

fpp

fcm

fcm fys

v

Qls

QtyQt

Qal

Qls Qls

c v

FEET

A

NORTHERN BARTLETT SPRINGS

FAULT ZONE 

4,000

6,000

2,000

SEA LEVEL

2,000

4,000

6,000

8,000

10,000

12,000

4,000

6,000

2,000

SEA LEVEL

2,000

4,000

6,000

8,000

10,000

12,000

FEET

A'

_ +

_

2,000

4,000

6,000

FEET FEET

SEA LEVEL

4,000

2,000

6,000

8,000

SEA LEVEL

2,000

4,000

6,000

6,000

4,000

2,000

B B''

8,000

10,000 10,000

_ +

fys fys fym

fym fym

fys

fys

Qty

fpp QalJosp

Qfo Tep

Qtvo vsm

vsm Qgd Qls Qgd

vsm

Qls Qls fys fys

fys

Josp Josp

Josp

KJs

KJs?

Jov Qal

Qfo Qty

Qal Qt

Qfo vsm

Kul

KJom

B'

fcm Qfo Qfo Qal Qfvo Qfy

vpo

spo

Kl

Kl

KJom

Qls Josp

fcm?

c b

fcm

Josp

Josp

fcm

fcm

Ku Ku Tep

NORTHERN BARTLETT SPRINGS

FAULT ZONE 

Snow Mountain

BEND IN

SECTION

SECTION

E–E'

SECTION

E–E'

cmg

cmg

vmg

2,000

SEA LEVEL

4,000

6,000

2,000

SEA LEVEL

4,000

6,000

4,000

6,000

2,000

8,000

10,000

4,000

6,000

2,000

8,000

10,000

14,000

12,000

FEET

C

FEET

C'

14,000

12,000

spo Kl Qal

Ku Qls

Josp

Qal

Qty Qal

Qfy fys fym fys

fym fys

fpp

KJom

fys

fym fys vy

vy vy

Qal

fys fys

vy cy Josp

Jov Qfo QtyQfo Qfo Qto Qt QtyQalQtyJosp KJs

Qfy Qal Qal Qt v

Lower Blue 

Lake

fcm

fcm

fcm

fcm

Qal

Josp

vpo ? ?

?

? ? ? ?

NORTHERN BARTLETT

SPRINGS FAULT ZONE 

PACIFIC RIDGE

ANTIFORM

MIDDLE MOUNTAIN

SYNFORM 

_ +

smg

Qal

Qt? Qal Qal

vmg vmg

smg

smg fcm fcm fcm QfyQal Qal

Qal

Kl?

Qoa fcm

fcm

fcm

fcm

v

Qls vy fys Qal

fym fys

fys Qls Qls

fys fys fys fys Qls Qls fys

fys

Josp vy miy Josp Josp

KJs

Jov

KJs

KJs

vy

fys

fym

fys

fym

fym

Josp

Josp Josp

Qls KJom KJom

Qls

Qls fcm Josp

Josp ss

Josp

2,000

SEA LEVEL

4,000

6,000

4,000

2,000

8,000

10,000

12,000

6,000

4,000

2,000

8,000

10,000

12,000

6,000

2,000

SEA LEVEL

4,000

6,000

FEET

D

FEET

NORTHERN BARTLETT SPRINGS D'

MIDDLE MOUNTAIN FAULT ZONE

SYNFORM 

PACIFIC RIDGE

ANTIFORM

smg

_ +

+

LUDOS RIOS

JAMISON RIDGE

PLASKETT

RIDGE

THATCHER 

RIDGE

PLASKETT

MEADOWS

123°15' 123° 122°45' 122°30'

39°45'

39°30'

39°15'

39°

Figure 2. Map showing quadrangles in map area and sources of geologic data.

SALT

CANYON

WILBUR SPRINGS BENMORE 

CANYON

LODOGA

LEESVILLE

GILMORE

PEAK

HOUGH SPRINGS

POTATO 

HILL

FOUTS SPRINGS

 BARTLETT

MOUNTAIN

 BARTLETT 

SPRINGS

STONYFORD SAINT JOHN

MOUNTAIN

CROCKETT 

PEAK

FELKNER

HILL

KNEECAP RIDGE

HULL MOUNTAIN

LAKE

PILLSBURY

SANHEDRIN 

MOUNTAIN

BRUSHY

WILLIS RIDGE MOUNTAIN

FOSTER

MOUNTAIN

VAN ARSDALE

RESERVOIR

POTTER

VALLEY 

REDWOOD

VALLEY

ELK

MOUNTAIN

COW

MOUNTAIN

UKIAH UPPER

LAKE

LAKEPORT

HIGHLAND 

SPRINGS KELSEYVILLE

CLEARLAKE

HIGHLANDS

LUCERNE

CLEARLAKE 

OAKS

LOWER

LAKE

WILSON

VALLEY 

GLASCOCK

MOUNTAIN

5

5

1

1

6, 15

6, 15

5, 18

5, 27

2, 16, 

18 

21

12, 13

12, 18

8

1

1

2

18

25

12, 13

1 8

 8, 

 16

10

14, 18

7

9

2

14, 16

4, 16, 18

18

3, 16

18

7, 17, 18 

4, 16, 18 

18

19, 20

20

11, 7,

22 

11

11

11

3, 24

3

11

11

4, 16, 18

23

26

AREA OF

MAP

EXPLANATION

1 Ohlin and others, 2010

2 Berkland, 1978; CDWR, 1966, 1968, and 1969

3 McLaughlin, R.J., unpublished mapping, 1980–1981 

4 McLaughlin and others, 1990

5 Jayko and others, 1989; Clark, 1940

6 Ohlin H.N., 1989, unpublished mapping, 1985

7 Hitchcock, C.S., unpublished mapping, 2012–2013; McLaughlin, 

 R.J., and Moring, B.C., aerial reconnaissance using Google Earth 

 imagery, 2014

8 Etter, 1979; U.S. Forest Service, unpublished mapping

9 Stanford, 1991

10 Berkland, 1973

11 Hearn and others, 1995; Sims and Rymer, 1976; Rymer, 1981

12 Brown and others, 1981; MacPherson, 1983

13 Brown, 1964

14 Suppe and Foland, 1978; Etter, 1979; U.S. Forest Service, 

 unpublished mapping

15 Blake and others, 1992

16 McLaughlin, R.J. and Moring B.C., mapping 2012–2013

17 Ohlin, H.N., unpublished field work, 1980–1981

18 McLaughlin and Moring, aerial reconnaissance using Google Earth 

 imagery, 2012–2014

19 Ohlin, H.N., unpublished mapping, 1979–1981

20 McLaughlin, 1978; Ohlin H.N., and McLaughlin, R.J., unpublished 

 mapping, 1978–1981

21 Lehman, 1974

22 McLaughlin, R.J., and Moring B.C., minor revisions

23 Lawton, 1956

24 Swe and Dickinson, 1970

25 Shervais and others, 2005a, b

26 Lienkaemper, 2010

27 Clark, 1940 

1

4

4

6

6

3

2

5

39 ̊00’

122 ̊0’ 122 ̊30’ 

39 ̊30’

Figure 5. Map showing major structural features in map area and locations of structure and seismicity cross sections.

NORTHERN

BARTLETT

SPRINGS

(Hunting)

t5

t4

t3

t1

t2-6

A

A'

B

B'

B''

C'

C

E'

E

D

D'

NORTHERN

BARTLETT

SPRINGS

Logan

Springs

Kuikui

Pomo

Bad Ridge

Bear Valley Resort

Wilson Fault

Kennedy

Fault

Sulphur Bank

Konocti Bay

Borax

Lake

Collayomi

Wright

Way

Big Valley

Scotts Valley

Clover Valley

Sanhedrin Mountain

Little Indian Valley

SOUTHERN BARTLETT SPRINGS

Cross Springs

EXPLANATION

MAJOR FAULTS

Active segments of the Bartlett Springs Fault Zone—From aerially 

interpreted geomorphic features (Lienkaemper, 2010)

Other faults, undifferentiated—Includes faults mapped in field and 

faults inferred from aerial lineaments. Includes faults discussed in 

this report, though not all named faults on figure are discussed

CROSS SECTION

Line of structure section

Line of seismicity cross section—See figure 5, 12 (Sheet 2) and figure 11 

(pamphlet) for additional information

MAJOR FOLDS

Synforms

1 Bartlett Springs Synform

4 Middle Mountain Synform

5 Unnamed synform

Antiforms

2 Pacific Ridge Antiform

3 Wilbur Springs Antiform

6 Bartlett Mountain Antiform

EARTHQUAKE EPICENTERS

Magnitude 5.2 earthquake, August 10, 2016

Aftershock sequence, August 10, 2016–September 15, 2016

Epicenters 1984–2014

A A'

t1

MFZ ~5 Ma

MFZ PRESENT

Exhumed rocks of 

the Eastern Belt 

Partially

exhumed 

rocks of 

 the Central 

 Belt

 

Coastal

Belt

Coastal

Belt

Coastal

Belt

Klamath

Block

Rice Valley

Rice 

Valley

Rice 

Valley

Round Valley

Round

Valley Round 

Valley

Middle 

Mountain

Middle 

Mountain

Middle 

Mountain

Lower 

Lake

Lower 

Lake Lower 

Lake

Great Valley Complex and

Coast Range ophiolite

Neogene Eel River

forearc basin

Neogene Eel River

forearc basin

APPROXIMATE

AREA OF FIGURE 7

A PALEOCENE–EOCENE MARGIN B ~MIDDLE MIOCENE MARGIN C PRESENT MARGIN

EXPLANATION

Estuarine deposits of Round Valley area associated with former Neogene 

(Eel River) forearc basin (Middle Miocene)

Sandstone and shale, shallow to deep marine, associated with former 

Mesozoic–Paleogene Great Valley forearc basin (Eocene and 

Paleocene)

Sandstone shale and conglomerate, marine, and ophiolitic basement and 

mélange of former Great Valley forearc basin (Upper 

Cretaceous–Middle Jurassic)

Igneous and metasedimentary basement rocks of the Klamath 

Mountains (Jurassic and older)

FRANCISCAN COMPLEX

Central Belt (Upper Cretaceaous and older)

Eastern Belt (Upper Cretaceous–Upper Jurassic)

Coastal Belt (Tertiary and younger)

MAJOR FOLD AXES

Antiform

Synform

Figure 6. Geologic maps showing progressive deformation of the northern California margin during transition from subduction to right-lateral 

strike slip. The Northern Bartlett Springs Fault is viewed as having developed since the Miocene from deformation focused along the Bartlett 

Springs Synform, one of several northwest-trending folds that preceded northward encroachment of the Mendocino Triple Junction and the 

San Andreas Fault. The folding formed in response to northeast-southwest transpression across the subduction margin. 

Dominated by east-west convergence, following exhumation of Eastern and Central Belts of Franciscan 

Complex from depth of 20–30 km. Yellow circled areas delineate Paleogene deposition near former 

western margin of the Jurassic–early Cenozoic Great Valley fore-arc basin. 

Light-pink transparent overlay delineates former southeastern part of early Miocene and younger (Eel River) 

fore-arc basin based on erosional and tectonic remnants of the former Neogene fore-arc basin that includes 

yellow-circled Miocene remnant at Round Valley. The Neogene fore arc may have extended much farther to 

the southeast (Nilsen and Clarke, 1989) and deposition was on exhumed Franciscan and Paleogene rocks of 

older Great Valley fore-arc basin. Orientations of synforms and antiforms inboard of the subduction margin 

indicate major northeast-southwest transpression that accompanied or postdated Neogene fore-arc 

deformation. Slip along the Northern Bartlett Springs Fault is inferred to have right-laterally dismembered the 

Bartlett Springs Synform since middle to late Miocene formation, uplift, and erosion of the Neogene fore-arc 

basin. See figure 7 for details of slip restoration of the Bartlett Springs Synform.

Showing major strike-slip faults inboard of the San Andreas Fault, the Mendocino Triple Junction, the 

present subduction margin, and the remaining offshore and uplifted and eroded onshore Neogene (Eel 

River) fore-arc basin. The present Bartlett Springs Fault Zone extends beyond the Mendocino Triple 

Junction into the modern subduction margin and dismembers the Bartlett Springs Synform. 

Reconstruction of this dismemberment can be used to approximate total dextral displacement and 

approximate a long-term slip rate for the Northern Bartlett Springs Fault. MFZ, Mendocino Fault Zone.

0

0 5 10 15 MILES

10 20 30 KILOMETERS

Round Valley

Miocene

Lower Great Valley 

Complex

Paleogene

Circled areas include Paleogene marine strata deposited on Cretaceous and older 

Great Valley Complex or Coast Range Ophiolite, representing the former westward 

extent of the Jurassic-Eocene Great Valley forearc basin. Round Valley area also 

includes remnants of Miocene estuarine strata indicating the former southeastern 

extent of a Neogene forearc basin (Eel River basin) that formed in the Miocene 

further to the west (Nilsen and Clarke, 1989).

STRATIGRAPHIC AND STRUCTURAL RELATIONS 

CONSTRAINING RECONSTRUCTION OF NORTHERN 

BARTLETT SPRINGS FAULT ZONE DISPLACEMENT

PALEOCENE–EOCENE

CONVERGENT

M

A

R

GIN

MIOCENE–PALEOCENE

CONVERGENT

M

A

R

GIN

COASTA

L

BELT

TH

RUST

SAN

ANDREA

S

FA

ULT

S

A

N

A

N

D

R

EAS

FAULT

PRESENT

C

O

N

VER

GEN

T

MA RGIN

COASTAL 

BELT THRUST

BARTLETT

SPRINGS

FAULT

Maacama

Fault

5

6

3

2

4

1

Figure 7. Schematic reconstruction of Bartlett Springs Synform configuration prior to its dismemberment along the Northern Bartlett Springs 

Fault Zone (fig. 6 B,C). Also shown are other subparallel folds northeast and southwest of the Bartlett Springs Synform discussed in this report. 

The restoration suggests a total cumulative oblique dextral slip of approximately 38–47 km for the Northern Bartlett Springs Fault Zone since the 

middle to late Miocene, based on linking remnants of Paleogene strata deposited along the uplifted and eroded former west margin of the 

Paleogene and older Great Valley fore arc. Slip could have begun earlier than the Miocene, however, and total displacement could be >50 km. 

See discussions in pamphlet on Wilbur Springs Dextral Hook and Northern Bartlett Springs Fault.

EXPLANATION

Nonmarine basin deposits, undifferentiated (Quaternary)

Volcanics and deposits of Clear Lake area (Holocene–Pliocene)

Estuarine deposits of Round Valley area (Middle Miocene)

Sandstone and shale, shallow to deep marine (Eocene and Paleocene)

Sandstone and shale, moderately to deep marine, turbiditic (Upper 

Cretaceous–Upper Jurassic)

Ultramafic and mafic rocks of Coast Range ophiolite and ophiolitic 

mélange (Lower Cretaceous and Upper–Middle Jurassic)

FRANCISCAN COMPLEX

Central Belt (Upper Cretaceous and older)

Mélange

Snow Mountain volcanic terrane

Eastern Belt (Upper Cretaceous–Upper Jurassic)

Yolla Bolly terrane (Upper Cretaceous–Upper Jurassic)

Metasandstone and slaty argillite

Mélange

Pickett Peak terrane (Lower Cretaceous–Upper Jurassic)

MAJOR FOLDS

Synform

Restored Bartlett Springs Synform

Middle Mountain Synform

Unnamed synform northeast of Rice Valley

Antiform

Pacific Ridge Antiform

Wilbur Springs Antiform

Bartlett Mountain Antiform

OTHER SYMBOLS

Faults

Points used to illustrate range in minimum strike–slip displacement for 

restoration of Bartlett Springs synform configuration

6

6

1

1

1

3

3

2

5

4

4

Clear

Lake

~47 Km

~38 Km

Approximate range in the minimum 

amount of slip necessary to restore 

hypothetical Bartlett Springs synform

configuration 

Potter 

Valley

Little Indian 

Valley 

Reservoir

Eden 

Valley

Lake

Pillsbury

Lake 

Pillsbury

Round 

Valley

?

SNOW MOUNTAIN

VOLCANIC TERRANE

Middle 

Mountain

Lower

Lake 

Rice 

Valley

Round 

Valley

FUTURE

NORTHERN

BARTLETT

SPRINGS

FAULT

0

0 5 10 15 MILES

10 20 30 KILOMETERS

Round Valley

Miocene

Lower Great Valley 

Complex

Paleogene

Circled areas include Paleogene marine strata deposited on Cretaceous and older 

Great Valley Complex or Coast Range Ophiolite, representing the former westward 

extent of the Jurassic-Eocene Great Valley forearc basin. Round Valley area also 

includes remnants of Miocene estuarine strata indicating the former southeastern 

extent of a Neogene forearc basin (Eel River basin) that formed in the Miocene 

farther to the west (Nilsen and Clarke, 1989).

STRATIGRAPHIC AND STRUCTURAL RELATIONS 

CONSTRAINING RECONSTRUCTION OF NORTHERN 

BARTLETT SPRINGS FAULT ZONE DISPLACEMENT

A B

31-90°

Figure 8. Map showing Wilbur Springs Dextral Hook in context of regional structure 

and long-term transpressional deformation along the Northern Bartlett Springs Fault 

Zone. A, Partial map of California, showing major fault blocks associated with 

Miocene and younger development of the San Andreas Fault, northwestward 

translation of Salinian block, clockwise rotation of Western Transverse Ranges block 

(Colgan and others, 2012), and Upper Cretaceous–Neogene clockwise rotation of 

southern Sierra Nevada /Tehachapi block (Chapman and others, 2010). White box 

outlines Wilbur Springs Dextral Hook. B, An expanded view of Wilbur Springs Dextral 

Hook derived from figures 6 and 7, showing reconstructed Bartlett Springs Synform 

and other folds of the dextral hook. Orientations of folds suggest maximum 

compression was ~N. 41° E. to S. 41° W., ±16°. Geometry and approximate timing of 

Wilbur Springs Dextral Hook formation is similar to larger-scale dextral hook formed 

by northward translation of the Salinian block since the early to middle Miocene. 

Upper Cretaceous–Paleogene rotations in southern Sierra Nevada-Tehachapi block, 

east-vergent crustal wedging, and unroofing of the Franciscan Complex 

accompanied shallow subduction of overthickened Farallon Plate and was followed 

by northward translation of the Salinian block during the Miocene (Chapman and 

others, 2010, 2012; Saleeby, 2003; Ernst and McLaughlin, 2012). See text for further 

discussion. Fault abbreviations: CASSZ, Cascadia Subduction Zone; GARFZ, Garlock 

Fault Zone; HAY-CAFZ, Hayward-Calaveras Fault Zone; MAAFZ, Maacama Fault 

Zone; MFZ, Mendocino Fault Zone; NBSFZ, Northern Bartlett Springs Fault Zone; 

NSAFZ, Northern San Andreas Fault Zone; RC-HLDFZ, Rodgers Creek-Healdsburg 

Fault Zone; SAFZ, San Andreas Fault Zone; SG-HFZ, San Gregorio-Hosgri Fault Zone; 

WW-KCFZ, White Wolf-Kern Canyon Fault Zone.

5 Ma

10 Ma

15 Ma

Western Transverse 

Ranges block (present)

Wilber Springs 

Dextral Hook

SAFZ

NSAFZ

NSAFZ

SAFZ

SG-HFZ

GARFZ

WW-KCFZ

Mojave block

Southern Sierra 

Nevada/Tehachapi

block

Northern Sierra Nevada block

NBSFZ

NBSFZ

MAAFZ

MFZ

CASSZ

HAY-CAFZ

RC-HLDFZ

Salinian block

Coastal Belt

Salinian block

San Francisco

Eureka

Bakersfield

Monterey

San Gabriel

Mountains

block

Santa 

Rosa

Klamath 

block

Round 

Valley

FUTURE NBSFZ

FUTURE NBSFZ

Reconstructed 

Bartlett Springs 

synform

0

0 25 50 75 MILES

50 100 150 KILOMETERS

0

0 5 10 15 MILES

10 20 30 KILOMETERS

EXPLANATION

Nonmarine basin deposits (Quaternary)

Volcanics and deposits of Clear Lake area 

(Holocene–Pliocene)

Eel River forarc basin (Neogene)

Crystalline basement rock

Western Transverse Ranges block

Central Belt (Upper Cretaceaous and older)

Eastern Belt (Upper Cretaceous–Upper Jurassic)

Coastal Belt (Tertiary and younger)

Coast Range ophiolite and ophiolitic mélange 

(Lower Cretaceous and Upper–Middle 

Jurassic)

SYMBOLS

Upper Cretaceous—Neogene clockwise rotation in 

southern Sierra Nevada Batholith and 

Tehachapi Mountains block (Chapman and 

others, 2010)

Approximate location of buried tip of Franciscan 

tectonic wedge—Barbs on roof thrust side of 

wedge

Fault—Arrows show relative movement

Thrust fault

Vectors of maximum compression—From 

orientation of fold axes

MAJOR FOLD AXES

Antiform

Synform

31-90°

PACIFIC RIDGE ANTIFORM

PACIFIC RIDGE ANTIFORM

8/10/16 M 5.1 

main shock

8/10/16 M 5.1 

main shock

8/10/16 M 5.1 

8/10/16 M 5.1 main shock

main shock

8/10/16 M 5.1

main shock

BARTLETT SPRINGS 

 FAULT ZONE

BARTLETT SPRINGS

 FAULT ZONE

BARTLETT SPRINGS

FAULT ZONE

 Cross Section B–B’–B’’

BARTLETT SPRINGS 

FAULT ZONE

Snow Mountain

+

+

+

+

+ +

0

-5

-10

-15

-20

t4

t2-6

Depth (km)

0

-5

-10

-15

-20

Depth (km)

0

-5

-10

-15

-20

Depth (km)

Distance (km) Distance (km)

Distance (km)

Distance (km)

0

-5

-10

-15

-20

Depth (km)

0

-5

-10

-15

-20

Depth (km)

t1 t3t2-6 t4

t5

t1

B' Part of structure section B–B’–B“ Part of structure section C–C’

Structure section E–E’

t3

Figure 12. Seismicity cross sections showing hypocentral depth distribution of earthquakes along 

sections t1, t2-6, t3, t4, and t5 (see fig. 5 for locations). Hypocenters are projected into seismicity cross 

sections along lines perpendicular to the sections from ~5 km north and south of the section lines. Depths 

of the 8/10/16 main shock and its aftershock sequence are, respectively, the large pink and small orange 

triangles. In t1, t2-6, t3, and t4, post-1984 hypocenters are plotted as small green dots. In section t5, red 

dots are hypocenters projected into the section from southwest of the plane of the cross section; green 

dots are hypocenters projected into the section from northeast of the cross-section plane. Intersection of 

the Northern Bartlett Springs Fault with plane of t5 is approximated from intersection of the fault’s 

projection in t2-6, t3, and t4 with plane of t5 (magenta hexagon symbol). In seismicity cross sections t1, t3, 

and t5, we superpose parts of structure sections B–B'–B", C–C', and E–E', respectively, that correspond 

with the seismicity cross sections. See the structure sections for explanation of lithologic units. Note that 

the axis of a major antiform (Pacific Ridge Antiform) in Franciscan rocks northeast of the Northern Bartlett 

Springs Fault Zone forms prominent high topography in the hanging-wall block of the Northern Bartlett 

Springs Fault (sections t3 and t5). 

EXPLANATION

EARTHQUAKE HYPOCENTERS

8/10/16 M 5.1 main shock

8/10/16 through 9/16/16 aftershock sequence

Microseismicity 1984–2011

Fault block motion toward observer

Fault block motion away from observer

Bartlett Springs Fault Zone—Arrows show relative 

strike-slip motion across fault

CROSS SECTION T5 (E–E‘) ONLY

Intersections of Northern Bartlett Springs Fault crossing 

northeast–southwest cross sections t1 (B–B‘–B‘‘), 

t2-6, t3 (C–C‘), and t4, with the plane of 

northwest–southwest section t5 (E–E‘)

Hypocenters located behind (northeast) of plane of E–E‘

Hypocenters located in front (southwest) of plane of E–E‘

BEND IN

SECTION

SECTION

E–E’

SECTION

B–B’–B“

SECTION

C–C’

SECTION

E–E’

SECTION

t5

SECTION

E–E’

Distance (km)