Gulf of California - Eastern California - Walker Lane:Two major transtensional belts in the

Pacific - North America plate boundary

Paul Umhoefer, Northern Arizona University

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Linking MARGINS and EarthScope;

lead into discussion…

SIMILARITIES IN BOTH BELTS:• Major part of plate boundary {but 90-95% vs 20-25% of r.p.m.}• Active since 14-12 Ma• Formed between batholiths & calderas

edge PC crust; on arc• Western edge of Basin & Range

• Transtensional (throughout history?)• Strain partitioning - across plate boundary & within belts {margin vs center of belts}

• Localization of strain over time

DIFFERENCES • stages of development

• rates of motion across belts• different total offsets (opening)

• pre-rift crust (Mojave)

NATURE OF PRE-RIFTED CRUST:

Do older batholith/caldera beltscontrol location-processes of belts?Laramide (Mojave only) effects?Did mid-Tertiary caldera beltseffect rifting process?

Thermal state of crust?

WEST EDGE OF BASIN & RANGE:

Do differences in previous extensioneffect rifting processes?

OVERALL STRAIN RATES:

Is there a threshold for fundamentallydifferent behavior related to strainrates?

Science objectives:

• how previous crustal structure effects oblique rifting

• New crust and nature of lower crust in N Gulf of Cal & Salton

• Kinematic/opening budget across belts = strain rates through time

• age/nature of oldest rift basins and especially marine initiation

• failed rifts and jumping of rift centers (really strain localization)

• obliquity and style of rifting (LANFs)

• formation of strike-slip and transform faults

• strain partitioning and fault mechanics

• role of mantle lithosphere??

MARGINS funding in Salton trough - NW Mexico onshore:

None?

• A few NSF-Tectonics grants recently• Many studies of neotectonics

• Fair number of proposals• NO proposals for large passive-active seismic transects ala

Lizarralde PESCADOR in southern Gulf of California{Fuis et al study - late 1970s}

• Northern Gulf of Cal will remain a hole for seismic, except CORTEZ

Can we merge MARGINS & EarthScope?? {& Continental Dynamics & Core??}1) {USGS - new projects…}2) Salton trough - Mojave ECSZ as link between belts

3) Comparative studies S to N

4) Different stages of rifting S to N

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Lizarralde et al 2003 EwingPESCADERO cruise

CORTEZ cruise

PESCADEROcruise