Magma mix-up

Basalts from the Snake River Plain (pictured above) in Yellowstone National Park in the northern United States look like they originate from deep in Earth’s mantle. But they have the characteristic isotopic signature of rocks from shallower reaches — from the continental lithosphere. This curiosity has been explained by Barry Hanan of San Diego State University, California, and his colleagues. Using mass spectrometry on samples of volcanic rock, the authors showed that deep-mantle magma can inherit the lithosphere’s isotopic signature when it rises and picks up contamination. The findings warn geologists about assuming too much from isotopic signatures.

Source: Nature, Vol 45618/25 December 2008

Reference: Geology 36, 51–54 (2008)

New Publication - Dr. Barry Hanan

Geochemical stages at Jasper Seamount and the origin of intraplate volcanoes

Geochemical stages at Jasper Seamount and the origin of intraplate volcanoes
J. G. Konter
Scripps Institution of Oceanography, University of California, San Diego, 9500 Gilman Drive, La Jolla, California 92093-0225, USA

H. Staudigel
Scripps Institution of Oceanography, University of California, San Diego, 9500 Gilman Drive, La Jolla, California 92093-0225, USA

J. Blichert-Toft
Laboratoire des Sciences de la Terre, Ecole Normale Supérieure de Lyon, F-69364 Lyon CEDEX 7, France

B. B. Hanan
Department of Geological Sciences, San Diego State University, San Diego, California 92182-1020, USA

M. Polvé
Observatoire Midi-Pyrénées, Université Paul Sabatier, F-31400 Toulouse, France

G. R. Davies
Faculty of Earth and Life Sciences, Vrije Universiteit Amsterdam, 1081 HV Amsterdam, Netherlands

N. Shimizu
Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543-1541, USA

P. Schiffman
Department of Geology, University of California, Davis, California 95616, USA

Ocean intraplate volcanoes (OIVs) are formed in a sequence of stages, from large to small, that involve a systematic progression in mantle melting in terms of volumes and melt fractions with concomitant distinct mantle source signatures. The Hawaiian volcanoes are the best-known example of this type of evolution, even though they are extraordinarily large. We explore the Pb-Sr-Nd-Hf isotopic evolution of much smaller OIVs in the Fieberling-Guadalupe Seamount Trail (FGST) and small, near-ridge generated seamounts in the same region. In particular, we investigate whether we can extend the Hawaiian models to Jasper Seamount in the FGST, which displays three distinct volcanic stages. Each stage has characteristic variations in Pb-Sr-Nd-Hf isotopic composition and trace element enrichment that are remarkably similar to the systematics observed in Hawaii: (1) The most voluminous, basal “shield building” stage, the Flank Transitional Series (FTS), displays slightly isotopically enriched compositions compared to the common component C and the least enriched trace elements (143Nd/144Nd: 0.512866–0.512909, 206Pb/204Pb: 18.904–19.054; La/Sm: 3.71–4.82). (2) The younger and substantially less voluminous Flank Alkalic Series (FAS) is comparatively depleted in Sr, Nd, and Hf isotope compositions plotting on the side of C, near the least extreme values for the Austral Islands and St. Helena. Trace elements are highly enriched (143Nd/144Nd: 0.512912–0.512948, 206Pb/204Pb: 19.959–20.185; La/Sm: 9.24). (3) The Summit Alkalic Series (SAS) displays the most depleted Sr, Nd, and Hf isotope ratios and is very close in isotopic composition to the nearby near-ridge seamounts but with highly enriched trace elements (143Nd/144Nd: 0.512999–0.513050, 206Pb/204Pb: 19.080–19.237; La/Sm: 5.73–8.61). These data fit well with proposed multicomponent melting models for Hawaii, where source lithology controls melt productivity. We examine the effect of melting a source with dry peridotite, wet peridotite, and pyroxenite, calculating melt productivity functions with depth to evaluate the effect of potential temperature and lithospheric thickness. This type of melting model appears to explain the isotopic variation in a range of small to large OIVs, in particular for OIVs occurring far from the complicating effects of plate boundaries and continental crust, constraining their geodynamic origin.

Received 10 September 2008; accepted 26 November 2008; published 3 February 2009.

Citation: Konter, J. G., H. Staudigel, J. Blichert-Toft, B. B. Hanan, M. Polvé, G. R. Davies, N. Shimizu, and P. Schiffman (2009), Geochemical stages at Jasper Seamount and the origin of intraplate volcanoes, Geochem. Geophys. Geosyst., 10, Q02001, doi:10.1029/2008GC002236.

A plume-triggered delamination origin for the Columbia River Basalt Group

Victor E. Camp
Barry B. Hanan
Department of Geological Sciences, San Diego State University, 5500 Campanile Drive, San Diego, California 92182-1020, USA

ABSTRACT
The Columbia River Basalt Group reveals a complete and detailed stratigraphic succession to assess the interplay of lithospheric and asthenospheric processes. This record of chemical change through time is used to evaluate genetic models for Columbia River Basalt volcanism. We recognize four primary constraints on source melting: (1) a plume component appears to be the dominant source of Imnaha Basalt; (2) Grande Ronde Basalt is best interpreted as being derived from a mafi c pyroxenite or eclogite source; (3) the sequence of source melting must correspond with the stratigraphic record; and (4) working models must explain a stepfunction chemical change at the Imnaha– Grande Ronde stratigraphic boundary. We can envision only three potential models to satisfy these primary constraints: (1) melting of a mantle plume entrained with eclogite, (2) plume interaction with the Juan de Fuca plate, and (3) delamination triggered by plume emplacement. The fi rst two of these are inconsistent with the time-stratigraphic sequence of melting and cannot satisfy all four primary constraints. In contrast, a model of plume-triggered delamination accurately predicts a progressive sequence of melting that satisfi es each of the primary constraints. Such a model is consistent with recent numerical experiments demonstrating that delamination is the expected result of plume emplacement beneath thin Mesozoic lithosphere lying adjacent to a thick cratonic boundary. We test this model by comparing the observed history of uplift and tectonism in eastern Oregon and adjacent Washington to that predicted by the numerical models to reveal consistent stress regimes and strikingly similar topographic and structural profiles.

Geosphere; June 2008; v. 4; no. 3; p. 480–495; doi: 10.1130/GES00175.1

Seismogenic, Electrically Conductive, and Fluid Zones at Continental

George R. Jiracek, Victor M. Gonzalez, T. Grant Caldwell,
Philip E. Wannamaker, and Debi Kilb

We explore the idea that fluid occurrence below the seismogenic zone plays an active role in the rupture process by examining how fluids spatially relate to seismicity at three continental plate boundaries: South Island of New Zealand, the Himalaya, and San Andreas fault, USA. With this objective, we project earthquake hypocenters onto magnetotelluric (MT) electrical resistivity cross-sections. MT detection of conductive zones in the crust containing low fractions of fluids (less than 1%) requires an interconnected network of fluid-filled porosity facilitated by shearing, fracturing, and/or grain-edge wetting. Mechanisms promoting fluid reservoirs in the ductile crust include: 1) stalling of upward propagating porosity waves, 2) tectonically induced neutral buoyancy, and 3) development of ductile shear zones. Distinct conductive horizons are detected at depth in the ductile crust in New Zealand and the Himalaya where the tectonic convergence is high. In the Parkfield segment of the San Andreas fault, where convergence is low, there is high conductivity in the ductile crust but it forms a sub-vertical corridor to the surface with no distinct top. The tops of sub-horizontal conductive zones are ~20 km depth in New Zealand and ~25–40 km in the Himalaya where the seismogenic crust extends only to 12 and 25 km depth, respectively. The deep conductive layer in New Zealand may have originated as a “water sill” facilitating water-weakening, localized deformation, and eventually becoming a water-rich, anisotropic, mylonized, ductile shear zone. Fluid exchange through the active Alpine fault may initiate or be initiated by fault rupture. Localized, unstable flow in deep fluidized zones detected by MT may trigger earthquakes above.

Jiracek, G. R., Gonzalez, V. M., Caldwell, T. G., Wannamaker, P. E., and Kilb, D., 2007, Seismogenic, electrically conductive, and fluid zones at continental plate boundaries in New Zealand, Himalaya, and California, USA: in D. Okaya, T. Stern, and F. Davey, eds., A continental plate boundary: Tectonics at South Island, New Zealand, Geophys. Mono. Ser. 175, AGU., Washington, DC, 347-369.


A Continental Plate Boundary: Tectonics at South Island, New Zealand

David Okaya, Tim Stern, Fred Davey, Editors

A Continental Plate Boundary offers in one place the most comprehensive, up-to-date knowledge for researchers and students to learn about the tectonics and plate dynamics of the Pacific-Australian continental plate boundary in South Island and about the application of modern geological and geophysical methods. It examines what happens when convergence and translation occur at a plate boundary by

• describing the geological and geophysical signature of a continental transform fault;

• identifying the diverse vertical and lateral patterns of deformation at the plate boundary;

• assessing an apparent seismicity gap on the plate boundary fault and fast-moving plate motions;

• comparing this plate boundary to other global convergent continental strike-slip plate boundaries;

• documenting the utility of the double-sided onshore–offshore seismic method for exploration of a narrow continental island; and

• Providing additional papers presenting previously unpublished results.

This volume will prove invaluable for seismologists, tectonophysicists, geodesists and potential-field geophysicists, geologists, geodynamicists, and students of the deformation of tectonic plates.

Geophysical Monograph Series, Volume 175, 350 pages, hardbound, 2007, ISBN: 978-0-87590-440-5, AGU Code GM1754405

New Publication - Barry Hanan

Yellowstone plume–continental lithosphere interaction beneath the Snake River Plain

Barry B. Hanan1, John W. Shervais2, and Scott K. Vetter3

1. Department of Geological Sciences, San Diego State University, San Diego, California 92182-1020, USA,

2. Department of Geology, Utah State University, Logan, Utah 84322-4505, USA,

3. Department of Geology, Centenary College, Shreveport, Louisiana 71134, USA

ABSTRACT
The Snake River Plain represents 17 m.y. of volcanic activity that took place as the North American continent migrated over a relatively fixed magma source, or hotspot. The identification of a clear seismic image of a plume beneath Yellowstone is compelling evidence that the Miocene to recent volcanism associated with the Columbia Plateau, Oregon High Lava Plains, Snake River Plain, Northern Nevada Rift and Yellowstone Plateau represents a single magmatic system related to a mantle plume. A remaining enigma is, why do radiogenic isotope signatures from basalts erupted over the Mesozoic–Paleozoic accreted terrains suggest a plume source while basalts erupted across the Proterozoic–Archean craton margin indicate an ancient subcontinental mantle lithosphere source? We show that ancient cratonic lithosphere like that of the Wyoming province superimposes its inherent isotopic composition on sublithospheric plume and/or asthenospheric melts. The results show that Yellowstone plume could have a radiogenic isotope composition similar to the mantle source of the early Columbia River Basalt Group and that the plume source composition has persisted to the present day.

Hanan BB, Shervais JW, Vetter SK (2008) Yellowstone plume–continental lithosphere interaction beneath the Snake River Plain. Geology: Vol. 36, No. 1 pp. 51–54

New Publication - Lindsey Leighton & Stephen Schellenberg

Featured PALAIOS Article - January 2008

INFLUENCE OF SPATIOTEMPORAL SCALE ON THE INTERPRETATION OF PALEOCOMMUNITY STRUCTURE: LATERAL VARIATION IN THE IMPERIAL FORMATION OF CALIFORNIA

CORY M. REDMAN1, LINDSEY R. LEIGHTON*,2, STEPHEN A. SCHELLENBERG2, CHRISTOPHER N. GALE2, JENNIFER L. NIELSEN2, DONALD L. DRESSLER2 and MARY K. KLINGER2

1 Department of Biology, San Diego State University, San Diego, California 92182-4614, USA; Current address: Department of Geology and Geophysics, Texas A&M University, 3115 TAMU, College Station, Texas 77843-3115, USA
2 Department of Geological Science, Allison Center for Earth System History, San Diego State University, San Diego, California 92182-1020, USA

Numerous paleocommunity studies of marine ecosystems have demonstrated that water depth was the primary factor structuring paleocommunities. In contrast, many ecological studies find other factors play a greater role in delineating communities; this difference in results may be owing to the spatiotemporal scale at which the study is performed. To explore this hypothesis, the present study examines a set of Imperial Formation (Pliocene, California) paleocommunities at a scale potentially fine enough to exclude depth as a control over the communities, thus facilitating recognition of fine-scale ecological and environmental processes operating at this scale. Twenty-six taxa from 21 samples were collected in situ from an 8.5-m-thick interval within a 0.32 km2 area. Cluster, Bray-Curtis (polar) ordination, and detrended correspondence analyses were used to infer community structure. Cluster and ordination analyses produced similar results independent of choice of distance measure. To test whether depth would be the primary control, even at fine scales, the scores from the first ordination axis for each taxon were compared with their modern mean depth; no strong correlation exists between depth and ordination scores. Cluster and ordination results indicate that life mode (as determined by modern relatives) and to a lesser extent, grain size, were the primary factors influencing paleocommunity structure at this scale. Bivalve taxa were grouped by life mode: quasi-infaunal, shallow-burrowing infaunal, byssate, and cemented. Relative to paleoecological studies conducted on much broader spatiotemporal scales, this study highlights the potential utility of embedding fine-scale studies within broader-scale studies to capture and investigate additional sources of ecological and environmental variation.

Abstract . Full Text . PDF (1.31M)

PALAIOS; December 2007; v. 22; no. 6; p. 630-641; DOI: 10.2110/palo.2006.p06-044r

New Publication - Pat Abbott

Natural Disasters, 6th Edition

Patrick Leon Abbott
©2008, ISBN-13 9780073292328
MHID 007329232X

Description
This book focuses on natural disasters: how the normal processes of the Earth concentrate their energies and deal heavy blows to humans and their structures. It is concerned with how the natural world operates and, in so doing, kills and maims humans and destroys their works. Throughout the book, certain themes are maintained: * energy sources underlying disasters * plate tectonics and climate change * earth processes operating in rock, water, and atmosphere * significance of geologic time * complexities of multiple variables operating simultaneously * detailed and readable case studies.

Table of Contents
1 Natural Disasters and The Human Population
2 Energy Flows in Earth History and Natural Disasters
3 Plate Tectonics and Earthquakes
4 Earthquake Geology and Seismology
5 Tsunami
6 Earthquakes in Western North America
7 More U.S. and Canadian Earthquakes
8 Volcanic Eruptions: Plate Tectonics and Magmas
9 Volcano Case Histories: Killer Events
10 Mass Movements
11 Atmosphere, Oceans, and Long-Term Climate Change
12 Short-Term Climate Change and Severe Weather
13 Hurricanes and the Coastline
14 Floods
15 Fire
16 The Great Dyings
17 Impacts with Space Objects

About the Author
Pat Abbott is a professor at San Diego State University

Features

• New feature: Entire text has been revised to feature 4-color line drawings and photos.
  Case Histories: The text aims to explain important principles about the Earth and then develop further understanding through numerous case studies.


• End-of-Chapter Learning Aids: Learning aids at the conclusion of every chapter include Terms to Remember, Questions for Review and Questions for Further Thought.


• Organization: The primary organization of the book is based on an energy theme. Chapter 1 leads off with data describing natural disasters and the human population. Chapter 2 examines the energy sources underlying disasters. Disasters fueled by Earth’s internal energy are addressed in Chapters 3 through 9 and are organized on a plate-tectonics theme, with the new Chapter 5 discussing tsunami. Disasters powered primarily by gravity are covered in Chapter 10 on mass movements. Disasters fueled by the external energy of the Sun are examined in chapters 11 through 15. Chapter 16 examines the great dyings encased in the fossil record. Chapter 17 examines impact mechanisms in greater detail and includes plans to protect Earth from future impacts.


• Comprehensiveness: The text covers the most significant natural hazards, from geologic to oceanographic to atmospheric to astronomic to demographic, and their associated phenomena. The broad range of natural disasters topics allows each instructor to select those chapters that cover their interests and local hazards.
  New chapter exclusively covering tsunami.

New Publication - Jared Morrow

Physical and chemical evidence of the 1850 Ma Sudbury impact event in the Baraga Group, Michigan

Peir K. Pufahl*,1, Eric E. Hiatt2, Clifford R. Stanley3, Jared R. Morrow4, Gabriel J. Nelson5 and Cole T. Edwards6

1 Department of Earth and Environmental Science, Acadia University, Wolfville, Nova Scotia B4P 2R6, Canada
2 Department of Geology, University of Wisconsin, Oshkosh, Wisconsin 54901, USA
3 Department of Earth and Environmental Science, Acadia University, Wolfville, Nova Scotia B4P 2R6, Canada
4 Department of Geological Sciences, San Diego State University, San Diego, California 92182-1020, USA
5 Department of Earth and Environmental Science, Acadia University, Wolfville, Nova Scotia B4P 2R6, Canada
6 Department of Geology, University of Wisconsin, Oshkosh, Wisconsin 54901, USA

ABSTRACT

An ejecta layer produced by the Sudbury impact event ca. 1850 Ma occurs within the Baraga Group of northern Michigan and provides an excellent record of impact-related depositional processes. This newly discovered, 2–4-m-thick horizon accumulated in a peritidal environment during a minor sea-level lowstand that punctuated a period of marine transgression. Common ejecta clasts include shock-metamorphosed quartz grains, splash-form melt spherules and tektites, accretionary lapilli, and glassy shards, suggesting sedimentation near the terminus of the continuous ejecta blanket. Sedimentologic and geochemical data indicate that primary fallout from a turbulent ejecta cloud was reworked to varying degrees by an impact-generated tsunami wave train. Observed platinum group element anomalies (Ir, Rh, and Ru) within the Sudbury ejecta horizon are sufficient to suggest that the impactor was a meteorite. Documenting and interpreting the detailed characteristics of the Sudbury ejecta horizon in Michigan have yielded a fingerprint to identify this chronostratigraphic marker in other Paleoproterozoic basins. For the first time a foundation exists to assess the consequences of the Sudbury impact on Precambrian ocean chemistry and early life.

Geology; September 2007; v. 35; no. 9; p. 827-830; DOI: 10.1130/G23751A.1

New Publication - Aaron Pietruzska

Rapid passage of a small-scale mantle heterogeneity through the melting regions of Kilauea and Mauna Loa Volcanoes

Jared P. Marskee , Aaron J. Pietruszkaa, Dominique Weisb,c, Michael O. Garciad and J. Michael Rhodese

aDepartment of Geological Sciences, San Diego State University, 5500 Campanile Dr., San Diego, CA 92182-1020, USA
bFNRS Research Director, Department of Earth and Environmental Sciences, CP 160/02-Université Libre de Bruxelles, Avenue F. D. Roosevelt, 50, B-1050 Brussels, Belgium
cPacific Centre for Isotopic and Geochemical Research, Department of Earth and Ocean Sciences, University of British Columbia, 6339 Stores Rd., Vancouver, Canada BC V6T-1Z4
dDepartment of Geology and Geophysics, University of Hawaii, 1680 East West Rd., Honolulu, HI 96822, USA
eDepartment of Geosciences, Morrill Science Center, University of Massachusetts, 611 North Pleasant St., Amherst, MA 01003-9297, USA Received 27 September 2006; revised 12 April 2007; accepted 16 April 2007. Editor: R.W. Carlson. Available online 22 April 2007.

Abstract

Recent Kilauea and Mauna Loa lavas provide a snapshot of the size, shape, and distribution of compositional heterogeneities within the Hawaiian mantle plume. Here we present a study of the Pb, Sr, and Nd isotope ratios of two suites of young prehistoric lavas from these volcanoes: (1) Kilauea summit lavas erupted from AD 900 to 1400, and (2) 14C-dated Mauna Loa flows erupted from 2580–140 yr before present (relative to AD 1950). These lavas display systematic isotopic fluctuations, and the Kilauea lavas span the Pb isotopic divide that was previously thought to exist between these two volcanoes. For a brief period from AD 250 to 1400, the 206Pb/204Pb and 87Sr/86Sr isotope ratios and εNd values of Kilauea and Mauna Loa lavas departed from values typical for each volcano (based on historical and other young prehistoric lavas), moved towards an intermediate composition, and subsequently returned to typical values. This is the only known period in the eruptive history of these volcanoes when such a simultaneous convergence of Pb, Sr, and Nd isotope ratios has occurred. The common isotopic composition of lavas erupted from both Kilauea and Mauna Loa during this transient magmatic event was probably caused by the rapid passage of a small-scale compositional heterogeneity through the melting regions of both volcanoes. This heterogeneity is thought to have been either a single body ( 35 km long based on the distance between the summits of these volcanoes) or the plume matrix itself (which would be expected to be present beneath both volcanoes). The time scale of this event (centuries) is much shorter than previously noted for variations in the isotopic composition of Hawaiian lavas due to the upwelling of heterogeneities within the plume (thousands to tens of thousands of years). Calculations based on the timing of the isotopic convergence suggest a maximum thickness for the melting region (and thus, the heterogeneity) of 5–10 km. The small size of the heterogeneity indicates that melt can be extracted from small regions within the Hawaiian plume with minimal subsequent chemical modification (beyond the effects of crystal fractionation). This would be most effective if melt transport in the mantle beneath Hawaiian shield volcanoes occurs mostly in chemically isolated channels.

Earth and Planetary Science Letters Volume 259, Issues 1-2, 15 July 2007, Pages 34-50

New Publication - Jared Morrow

Shock-metamorphic petrography and microRaman spectroscopy of quartz in upper impactite interval, ICDP drill core LB-07A, Bosumtwi impact crater, Ghana

Jared R. MORROW
Department of Geological Sciences, San Diego State University, 5500 Campanile Drive, San Diego, California 92182–1020, USA

Abstract–Standard and universal stage optical microscope and microRaman spectroscopic
examination of quartz from the upper impactite interval of the International Continental Scientific Drilling Program (ICDP) Lake Bosumtwi crater drill core LB-07A demonstrates widespread but heterogeneous evidence of shock metamorphism. In the upper impactite, which comprises interbedded polymict lithic breccia and suevite from a drilling depth of 333.4–415.7 m, quartz occurs as a major component within metasedimentary lithic clasts and as abundant, isolated, single-crystal grains within matrix. The noted quartz shock-metamorphic features include phenomena related to a) deformation, such as abundant planar microstructures, grain mosaicism, and reduced birefringence; b) phase transformations, such as rare diaplectic quartz glass and very rare coesite; c) melting, such as isolated, colorless to dark, glassy and devitrified vesicular melt grains; and d) secondary, post-shock features such as abundant, variable decoration of planar microstructures and patchy grain toasting. Common to abundant planar deformation features (PDFs) in quartz are dominated by -equivalent crystallographic planes, although significant percentages of and other higher index orientations also occur; notably, c(0001) planes are rare. Significantly, the quartz PDF orientations match most closely those reported elsewhere from strongly shocked, crystalline-target impactites. Barometry estimates based on quartz alteration in the upper impactite indicate that shock pressures in excess of 20 GPa were widely reached; pressures exceeding 40–45 GPa were more rare. The relatively high abundances of decorated planar microstructures and grain toasting in shocked quartz, together with the nature and distribution of melt within suevite, suggest a water- or volatile-rich target for the Bosumtwi impact event.

Morrow, J.R., 2007, Shock-metamorphic petrography and micro-Raman spectroscopy of quartz in upper impactite interval, ICDP drill core LB-07A, Bosumtwi impact crater, Ghana: Meteoritics & Planetary Science, v. 42, p. 591-609.

New Publication - Aaron Meltzner & Tom Rockwell

Bulletin of the Seismological Society of America, December 2006, Vol. 96, Issue 6, pp. 2304-2328

Recent and long-term behavior of the Brawley fault zone, Imperial Valley, California; an escalation in slip rate?
Meltzner, Aron J. (San Diego State University, Department of Geological Sciences, San Diego, CA, United States); Rockwell, Thomas K.; Owen, Lewis A.

ABSTRACT

The Brawley fault zone (BFZ) and the Brawley Seismic Zone constitute the principal transfer zone accommodating strain between the San Andreas and Imperial faults in southernmost California. The BFZ ruptured along with the Imperial fault in the 1940 M (sub w) 6.9 and the 1979 M (sub w) 6.4 earthquakes, although in each case only minor slip apparently occurred on the BFZ; several other episodes of slip and creep have been documented on the BFZ historically. Until this study, it has been unclear whether the past few decades reflect average behavior of the fault. Two trenches were opened and a series of auger holes were bored across three strands of the BFZ at Harris Road to compare the amount of slip observed historically with the displacements observed in the paleoseismic record. Evidence is presented, across the westernmost strand of the BFZ and across the entire BFZ at Harris Road, to show that both the average vertical slip rate observed in modern times (since 1970) and the vertical creep rate (excluding coseismic slip) observed during the 1970s are significantly higher than the long-term average. Across the westernmost strand, the long- term vertical rate is 1.2 (+1.5/-0.5) mm/yr, and the average rate since about A.D. 1710 is determined to be no greater than 2.0 mm/yr; in contrast, the average vertical rate between 1970 and 2004 across that strand was at least 4.3 mm/yr, and the 1970s vertical aseismic creep rate was 10 mm/yr. Likewise, across the entire BFZ, the long-term vertical rate is 2.8 (+4.1/-1.4) mm/yr, whereas the rate between 1970 and 2004 was at least 7.2 mm/yr, and the 1970s aseismic creep rate was 10 mm/yr. The long-term strike-slip rate cannot be determined across any strands of the BFZ but may be significant. In contrast to the commonly accepted higher sedimentation rates inferred for the entire Imperial Valley, we find that the average sedimentation rate on the downthrown side of the BFZ adjacent to Mesquite Basin, in the millennium preceding the onset of agricultural influences, was at most 3.5 mm/yr. Finally, a creep event occurred on the BFZ during our study in 2002 and is documented herein.

Southern Salton Trough deltaic system, based on DEM imagery. Color contours indicate elevation; each color band represents a 5- to 10-m change in elevation. Sections of some contours are highlighted with thin black lines for improved visibility. The band labeled “12m” is the contour at an elevation of 12 m above mean sea level, which represents the highstand shoreline of Lake Cahuilla. Note the location of the modern delta, which is interpreted to have formed initially in response to the 1905– 1907 filling of the Salton Sea; slow retreat of the lake combined with regular flow of the New and Alamo Rivers has built this modern delta that is prograding into the Salton Sea. In addition to the modern delta, four prehistoric delta lobes have been interpreted on this DEM: deltas N1 and N2 on the New River, and deltas A1 and A2 on the Alamo River. Main faults are mapped in black. SMF, Superstition Mountain fault; SHF, Superstition Hills fault. Modified from Ragona (2003). ( E A color version of this figure is available in the electronic edition of BSSA.)

Full Text (pdf)