Tectonic geomorphology of mountains: a new approach to paleoseismology

328 Pages·2007·16.782 MB·English

by  Bull, William B

Tectonic Geomorphology
of Mountains:
A New Approach to Paleoseismology
William B. Bull

Tectonic Geomorphology of Mountains

Tectonic Geomorphology
of Mountains:
A New Approach to Paleoseismology
William B. Bull
(cid:2)2007WilliamB.Bull
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Firstpublished2007byBlackwellPublishingLtd
1 2007
LibraryofCongressCataloging-in-PublicationData
Bull,WilliamB.,1930–
Tectonicgeomorphologyofmountains:anewapproachtopaleoseismology/WilliamB.Bull.
p.cm.
Includesbibliographicalreferencesandindex.
ISBN-13:978-1-4051-5479-6(hardback:alk.paper)
ISBN-10:1-4051-5479-9(hardback:alk.paper)1.Morphotectonics.2.Paleoseismology.I.Title.
QE511.44.B852007
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2006100890
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Contents
Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . viii
1 Scrunch and Stretch Bedrock Uplift
1.1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2 Pure Uplift, Stretch andScrunch Bedrock Uplift . . . . . . . . . . . . . . . . . 6
1.2.1 Isostaticand Tectonic Uplift . . . . . . . . . . . . . . . . . . . . . 6
1.2.2 StretchandScrunch Tectonics . . . . . . . . . . . . . . . . . . . . . 12
1.3 Landscape ResponsestoRegional Uplift . . . . . . . . . . . . . . . . . . . . 23
2 Concepts for Studies of Rising Mountains
2.1 Themesand Topics . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
2.2 The Fundamental Control ofBase Level . . . . . . . . . . . . . . . . . . . . 28
2.2.1 Base Level . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
2.2.2 Base-Level Change . . . . . . . . . . . . . . . . . . . . . . . . . 28
2.2.3 The Base Level ofErosion . . . . . . . . . . . . . . . . . . . . . . 31
2.2.4 The Changing Level ofthe Sea . . . . . . . . . . . . . . . . . . . . 33
2.2.5 SpatialDecayof theEffectsofLocal Base-Level Changes . . . . . . . . . . . 37
2.3 Thresholdof Critical PowerinStreams . . . . . . . . . . . . . . . . . . . . 39
2.3.1 Relative Strengths ofStream Powerand ResistingPower. . . . . . . . . . . . 41
2.3.2 Threshold-Intersection Points . . . . . . . . . . . . . . . . . . . . . 42
2.4 Equilibrium inStreams . . . . . . . . . . . . . . . . . . . . . . . . . . 42
2.4.1 Classification ofStream Terraces . . . . . . . . . . . . . . . . . . . . 42
2.4.2 Feedback Mechanisms. . . . . . . . . . . . . . . . . . . . . . . . 45
2.4.3 DynamicandStatic Equilibrium . . . . . . . . . . . . . . . . . . . . 46
2.5 Time Lags ofResponse . . . . . . . . . . . . . . . . . . . . . . . . . . 49
2.5.1 ResponsestoPulses ofUplift . . . . . . . . . . . . . . . . . . . . . 50
2.5.2 Perturbations that Limit Continuity ofFluvialSystems . . . . . . . . . . . . 51
2.5.3 Lithologic andClimaticControls ofRelaxation Times . . . . . . . . . . . . 54
2.5.4 Time Spans Needed toErodeLandforms . . . . . . . . . . . . . . . . . 57
2.6 Tectonically-InducedDowncutting . . . . . . . . . . . . . . . . . . . . . . 58
2.6.1 Straths,Stream-Gradient Indices,and Strath Terraces. . . . . . . . . . . . . 58
2.6.2 Modulation ofStream-TerraceFormationby Pleistocene–Holocene ClimaticChanges . 65
2.7 Nontectonic Base-Level Falland Strath Terrace Formation . . . . . . . . . . . . . 66
2.8 HydraulicCoordinates . . . . . . . . . . . . . . . . . . . . . . . . . . 69
3 Mountain Fronts
3.1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
3.2 Tectonically ActiveEscarpments . . . . . . . . . . . . . . . . . . . . . . . 79
3.2.1 Faceted Spur Ridges . . . . . . . . . . . . . . . . . . . . . . . . 79
3.2.2 Mountain–PiedmontJunctions . . . . . . . . . . . . . . . . . . . . 83
3.2.3 PiedmontForelands . . . . . . . . . . . . . . . . . . . . . . . . 86
vi Contents
3.3 Fault Segmentation ofMountain Fronts. . . . . . . . . . . . . . . . . . . . 97
3.3.1 Different Ways toStudyActiveFaults. . . . . . . . . . . . . . . . . . 97
3.3.2 SegmentationConcepts and Classification . . . . . . . . . . . . . . . . 104
3.3.3 Fault-Segment Boundaries . . . . . . . . . . . . . . . . . . . . . . 105
3.3.4 Normal Fault Surface Ruptures . . . . . . . . . . . . . . . . . . . . 106
3.3.5 Strike-Slip FaultSurface Ruptures . . . . . . . . . . . . . . . . . . . 113
3.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115
4 Tectonic Activity Classes of Mountain Fronts
4.1 Tectonic Settingofthe NorthAmerica–Pacific Plate Boundary. . . . . . . . . . . . 117
4.2 Appraisalof Regional Mountain Front TectonicActivity . . . . . . . . . . . . . . 119
4.2.1 Geomorphic Tools For Describing RelativeUplift Rates . . . . . . . . . . . 119
4.2.1.1 Mountain-Front Sinuosity . . . . . . . . . . . . . . . . . . 122
4.2.1.2 Widthsof Valleys. . . . . . . . . . . . . . . . . . . . . . 124
4.2.1.3 TriangularFacets . . . . . . . . . . . . . . . . . . . . . . 127
4.2.2 Diagnostic Landscape Classes ofRelativeTectonic Activity . . . . . . . . . . 128
4.2.3 RegionalAssessmentsofRelativeTectonicActivity . . . . . . . . . . . . . 141
4.2.3.1 Response Time Complications andStrike-SlipFaulting . . . . . . . . 141
4.2.3.2 MapsofRelativeUplift . . . . . . . . . . . . . . . . . . . 145
4.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164
5 Fault Scarps
5.1 GeneralFeatures . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165
5.2 ScarpMorphologyChanges with Time . . . . . . . . . . . . . . . . . . . . 172
5.2.1 ChangesinScarpHeight . . . . . . . . . . . . . . . . . . . . . . 173
5.2.2 Decreases inMaximum Scarp Slope . . . . . . . . . . . . . . . . . . 174
5.2.3 Diffusion-EquationModeling. . . . . . . . . . . . . . . . . . . . . 175
5.3 ClimaticControls ofFault-Scarp Morphology . . . . . . . . . . . . . . . . . 181
5.4 Lithologic Controlsof Fault-ScarpMorphology . . . . . . . . . . . . . . . . . 184
5.4.1 FaultRuptureofDifferentMaterials . . . . . . . . . . . . . . . . . . 185
5.4.2 Lithologic Controls on an 1887FaultScarp. . . . . . . . . . . . . . . . 187
5.4.2.1 Geomorphic Processes . . . . . . . . . . . . . . . . . . . . 190
5.4.2.2 ScarpMaterials . . . . . . . . . . . . . . . . . . . . . . 193
5.4.2.3 ScarpMorphology . . . . . . . . . . . . . . . . . . . . . 194
5.5 LaserSwathDigitalElevationModels . . . . . . . . . . . . . . . . . . . . 196
5.6 Dating FaultScarps with TerrestrialCosmogenic Nuclides . . . . . . . . . . . . . 201
5.6.1 Alluvium . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201
5.6.2 Bedrock . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204
5.7 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207
6 Analyses of Prehistorical Seismic Shaking
6.1 PaleoseismologyGoals . . . . . . . . . . . . . . . . . . . . . . . . . . 209
6.2 Earthquake-Generated Regional RockfallEvents. . . . . . . . . . . . . . . . . 212
Contents vii
6.2.1 New Zealand Earthquakes . . . . . . . . . . . . . . . . . . . . . . 212
6.2.1.1 Tectonic Setting . . . . . . . . . . . . . . . . . . . . . . 212
6.2.1.2 Background andProcedures . . . . . . . . . . . . . . . . . . 215
6.2.1.3 Diagnostic Lichen-SizePeaks . . . . . . . . . . . . . . . . . . 225
6.2.1.4 Tree-Ring Analyses . . . . . . . . . . . . . . . . . . . . . 227
6.2.1.5 AlpineFault Earthquakes . . . . . . . . . . . . . . . . . . . 241
6.2.1.6 RecentMarlboroughEarthquakes . . . . . . . . . . . . . . . . 246
6.2.2 CaliforniaEarthquakes . . . . . . . . . . . . . . . . . . . . . . . 255
6.2.2.1 Calibration of Lichen GrowthRates . . . . . . . . . . . . . . . 257
6.2.2.2 RecentCliff Collapse. . . . . . . . . . . . . . . . . . . . . 258
6.2.2.3 RockfallProcesses inGlaciatedValleys. . . . . . . . . . . . . . . 262
6.2.2.4 SanAndreas FaultEarthquakes . . . . . . . . . . . . . . . . . 265
6.2.2.5 Lichenometry and Precise RadiocarbonDating Methods . . . . . . . . 270
6.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 273
References Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 275
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 305
Preface
Uplift by mountain-building forces changes fluvial New Zealand and the northern Basin and Range
landscapes. Pulsatory tectonic activity on a range- Provinceinthe UnitedStates.
bounding fault increases relief, changes rates of geo- Thisbookapplies avariety ofgeomorphic con-
morphic processes, and modifies the shapes of hills cepts to tectonics and paleoseismology. Don’t expect
and streams. Landscape responses to uplift occupy a landscape summaries for all major mountain ranges.
critical time frame for studies of past earthquakes Repetitive descriptions would dilute explanation and
between the brevity of instrumental seismic data applicationofbasicprinciples.Doexpectessentialcon-
and long-term geologic crustal shifts. The appealing cepts that should help you better understand the land-
challenge for us is to determine how and when nearby scape evolution of your favorite mountains. Mountain
anddistantpartsofthelandscapechangeinconsecutive fronttectonicgeomorphology studiescan determine:
reachesupstreamfromatectonicallyactiverangefront. 1) Which faults areactive [Holocene ruptures],
Each climatic and lithologic setting has a characteristic 2)Faultslipratesforshorttimespans[offsetlandforms]
style and rate of erosion, which adds spice to the andlong time spans [landscapeevolution],
scientific challenge. Landscape analyses include the 3) Time of most recent surface rupture and degree of
geomorphic consequences of seismic shaking and irregularity ofearthquake recurrence interval, and
surface rupture and their associated hazards to human- 4) Intensity and extent ofseismicshaking.
kind.Tectonicgeomorphologyisessentialforcomplete Theamountofrelatedliteraturecitedbordersonbeing
paleoseismology investigations. Locations, sizes, times, unwieldy because of topic diversity of and the rapidly
and patterns of seismic shaking by prehistorical earth- increasing interest of earth scientists in these subjects.
quakes can be described and surface rupture and I had to pick and choose so as to not overwhelm
seismic-shaking hazardsevaluated. the content with citations of relevant literature. My
This book explores tectonic geomorphology of citations are merely agatewayto related literature.
mountain fronts on many temporal and spatial scales Dating times of prehistoric earthquakes and
to encourage expansion of paleoseismology inquiries estimating rates of tectonic and geomorphic processes
from the present emphasis on stratigraphic investiga- continue to be of paramount importance. Study
tions in trench exposures. Evaluating earthquake haz- methods are changing, and precision and accuracy are
ards is in part a study of mountain-front segments. improving.Diffusion-equationmodelingoffaultscarps
Cumulative displacements over late Quaternary time and stratigraphic radiocarbon dates on pre- or post
spans create landscape assemblages with distinctive earthquake material collected from trenches have long
signatures that are functions of uplift rate, rock mass been bastionsfor approximate age estimates. Sykes and
strength, and the geomorphic processes of erosion and Nishenkomadeapleain1984forbetterwaysofdating
deposition. Such interactions define classes of relative frequent earthquakes along plate boundary fault zones
uplift. Tectonic activity class maps define tectonically whose earthquake recurrence intervals may be shorter
inactiveregionsaswellasfrontsofslowtorapidlyrising than the intervals defined by groups of overlapping
mountains. Fault scarps focus our attention on recent radiocarbon age estimates. The rapid development of
surfacerupturesandpropagationofactivefaults.Dating terrestrialcosmogenicnuclidesbroadensdatingperspec-
and describing the characteristics of single prehistoric tivesbyestimatingagesbeyondthereachofradiocarbon
surface-ruptures is important. But now we can link analyses and by making surface-exposure dating a cor-
sequences of events and depict sequences of prehistori- nerstone for studies of geomorphic processes. Tree-ring
cal earthquakes along complex plate boundary fault analyses and lichenometry have potential for dating
zones. Examples here include the Alpine fault in prehistorical earthquakes with a precision of (cid:2)5 years.