iPlex Lunch - winter-2013

Jan. 9, 2013
noon - 1 p.m.
Geology 1707

Presented By:

• Cathy Busby - UCSB

The transtensional eastern boundary of the Sierran microplate (Walker Lane rift) represents the northernmost extension of the Gulf of California rift, and it forms an onland analog in several ways. It formed at the same time (about 12 Ma), by a similar mechanism: transtension within the thermally- and structurally-weakened axis of a subduction-related arc. The two segments show similar structural trends: NE oblique slip normal faults (Walker Lane) or seafloor spreading centers (Gulf of California), connected by long NNW strike slip faults. However, the process of continental rupture has not yet been completed in the Walker Lane, so the structural controls on transtensional rift volcanism can be directly observed on land. The Walker Lane segment also differs from the Gulf of California segment by showing a northward time-transgressive transition from arc rift magmatism to continental rift magmatism, following the northward migration of the Mendocino triple junction (MTJ). The effect of MTJ migration has been previously recognized in arc to rift geochemical transitions, but not in the timing of development of large arc volcanic fields. For the past ~11-12 Ma, the biggest arc rift and continental rift volcanic centers or fields have been sited on major releasing fault stepovers on the trailing edge of the Sierran microplate. Additionally, major transtensional arc rift centers or fields appear to have progressively migrated northward with time, in advance of the TMJ, although gaps exist in detailed map and age data. These large transtensional arc volcanic fields/centers are, from south to north (oldest to youngest): (1) A ~11 – 9 Ma arc volcanic field that lies along the Sierran crest and rangefront in the Sonora Pass – Bridgeport area of the central Sierra Nevada. Its transtensional structural setting and its size (~ 50 X 50 km) had not been appreciated prior to my field efforts with students, although a modest-sized caldera in this volcanic field had long been recognized (“Little Walker caldera” of Priest, 1979). At this center, “flood andesites” were erupted from 6–8 km long fault-controlled fissures and ponded in grabens, to thicknesses of 400 m, with single flows up to 25 km3 in volume. Total volume is difficult to estimate due to Pleistocene glacial erosion, but it is >200 km3. (2) The Ebbetts Pass center, which formed at ~5–4 Ma (dating in progress with Paul Renne, BGC). This large center had not been recognized prior to our mapping; it appears to be a complex central volcano with a large footprint (>16 km diameter, glacially eroded). Its original volume may be better estimated after its collapse deposits are mapped and dated, because it appears to have repeatedly collapsed into range-front half grabens. (3) The active Lassen arc volcanic center, which formed at <3.5 Ma in a transtensional environment “favorable to the development of major volcanic centers” (Muffler et al., 2008, EOS 8-53). The active Long Valley rift volcanic field south of the MTJ also formed in a releasing bend in the Walker Lane transtensional rift (since ~4.5 Ma); the structure of this field (Jayko and Bursik, 2012; Riley et al., 2012) is remarkably similar to that of the ~11-9 Ma arc rift volcanic field at Sonora Pass. The active Coso rift volcanic center also formed in a releasing stepover (Pluhar et al, 2006).

Jan. 16, 2013
noon - 1 p.m.
Geology 1707

Presented By:

• Paul Davis - UCLA

We address the question as to why earthquakes occur in some places and not others? The depth extent of seismogenic zones ranges from about 20 km on the continents, in strike-slip or rift zones, to depths greater than 600 km in subduction zones. In ocean plates earthquakes are rarely deeper than 60 km. We show how this range is described by a thermo-mechanical model in which the transition from brittle to ductile behavior occurs when the tectonic strain-rate equals the dislocation creep-rate for material flow. The former depends on the tectonics while the latter depends on homologous temperature, i.e., the ratio of temperature to effective-melting temperature. The model describes depth of earthquakes in California, in ocean plates, subduction zones, the variable widths of double Wadati-Benioff zones worldwide, and the deeper than average events beneath outer rises and Hawaii.

Jan. 23, 2013
noon - 1 p.m.
Geology 1707

Presented By:

• Caroline Beghein Boyce - UCLA

The mantle transition zone (MTZ) is defined by discontinuities in seismic wave velocities at 410 km and 670 km depth that mostly result from phase changes in the olivine structure. Because the MTZ is believed to play an important role in the thermochemical evolution of our planet, constraining mantle flow at these depths is important for our understanding of mantle dynamics and the history of plate tectonics. Seismic anisotropy can help us in that regard since it can result from deformation by dislocation creep during convection and provides the most direct constraints on mantle flow patterns. Here we used a recent seismic dataset with much higher depth resolution than in previous work to constrain the three-dimensional pattern of seismic anisotropy in the top 1000 km of the mantle, and in particular in the MTZ. In this talk I will present our 3-D anisotropy model, how it was obtained, and describe how our new inferences on anisotropy can provide constraints on the deformation mechanisms taking place in the mantle

Jan. 30, 2013
noon - 1 p.m.
Geology 1707

Presented By:

• Kathleen M. Marsaglia - California State University-Northridge

The geology of North Island, New Zealand is largely a product of Paleozoic to Cenozoic plate-subduction processes with a Late Cretaceous to Oligocene pause associated with Gondwana rifting. The reinstatement of subduction in the Oligocene was accompanied by emplacement of an ophiolite-bearing allochthon. This subduction propagated eastward, evolving into the current Hikurangi convergent margin. There is evidence that since the Miocene, large-scale reentrants formed along the shelf edge in the forearc via submarine mass wasting. Some reentrants are associated with seamount subduction, a process also linked to modern slow-slip events (aseismic slip). This presentation focuses on the forearc sedimentary record of two of these re-entrants: the nuances of sediment supply from source-to-sink into the modern Poverty re-entrant and the recognition of a potential Miocene example in outcrop. The region is notable in that it was a Source-to-Sink focus site for the NSF MARGINS Program and is now an international primary site for the new NSF GeoPRISMS Subduction Cycles and Deformation Initiative. Furthermore, an Integrated Ocean Drilling Program Expedition is tentatively slated for 2015 that will drill the Poverty forearc to explore the origin of slow-slip events.

Feb. 6, 2013
noon - 1 p.m.
Geology 1707

Presented By:

• Peter Bird - UCLA

Kinematic finite-element program NeoKinema solves for long-term crustal velocities, long-term distributed strain rates (not elastic), and fault slip rates on the Earth's surface by fitting GPS velocities, stress directions, and geologic offset rates along mapped faults. Due to its 2-D domain, it is able to represent the entire western US at 10~20-km resolution. The most recent version allows for imposition of geologic limits on fault offset rates as well as best-estimates with their uncertainties. Intensive work preparing deformation models for two seismic-hazard projects (the CA UCERF3 project and the USGS NSHM2014 project) has led to close scrutiny and debate about both inputs and outputs by diverse experts. Today, I will discuss model implications for 3 old problems of CA neotectonics: (1) How does crust "get around" the left step of the San Andreas plate boundary? The old answer that it is overthrust and thickened is only half-right; while there are high heave-rates on a few thrusts (from San Fernando W to Gaviota), the famous range-front thrusts are quite slow and the total is insufficient. Instead, the need for area-reduction is cut in half by a less-appreciated right step of about 1/4 of the slip beginning in Joshua Tree NP and connecting to the Walker Lane. (2) Why is the Garlock fault active at sinistral slip rates of >5 mm/a? The old answer that it is a transform bounding extension to the N is wrong because the extension direction is NW-SE, not SW-NE. My old answer that the Mojave rotates clockwise is mostly wrong. (This rotation is only local, in the E Mojave.) Instead, the Garlock fault accomodates the NE-ward shift of crustal area away from the transpresional left step of the San Andreas, towards the transtensional right step. (Crust to the NW of the Garlock is not involved in the lateral shift.) (3) How does the Mendocino triple-junction resolve its kinematic incompatibility? Classic plate theory shows that at least one plate must deform in this region. These detailed models show that the San Andreas "grinds to a halt" with only ~8 mm/a at its NW end, while the adjacent Mendocino-Eureka region (onshore) and Gorda region (offshore) are deforming at very high rates. At a larger scale, the kinematic problem is also eased by clockwise rotation of the entire Pacific Northwest about a pole near Missoula MT, which is kinematically linked to NW-SE extension extending through central NV to the Wasatch fault zone of UT, much as was inferred by Ingersoll [1982]. This "solution" is preconditioned by the prior history (subduction, accretion, intrusion, orogeny, extension) of the deforming regions, and has only a faint resemblance to "plate tectonics".

Feb. 13, 2013
noon - 1 p.m.
Geology 1707

Presented By:

• B. C. Burchfiel - MIT
• Radoslav Nakov - nstitute of Geology, Bulgarian Academy of Sciences, Sofia

The generally east-west trending Balkan orogen consists a northern belt of folded and thrusted Mesozoic and Cenozoic strata that forms its the external fold and thrust belt of late Mesozoic and early Cenozoic age and a southern belt that consists of deformed igneous and metamorphic rocks overprinted by Cenozoic extensional basins. Unlike most foreland fold-thrust belts whose deformation commonly migrates toward its foreland, the fold thrust belt within the Balkan orogen is marginal to the Moesian platform to the north and was deformed in at least three superposed events related to changes in plate interactions. The earliest event of late Early to early Late Cretaceous deformed strata deposited on the Moesian continental margin and within a continental rifted belt containing deep water flysch of Late Jurassic-Early Cretaceous age, a probable eastward extension of oceanic troughs from the South Carpathians. The shortening was a consequence of S or SW synthetic subduction within Vardar zone along the southern margin of the Balkan orogen. In Late Cretaceous time a back arc or intra-arc rift zone developed along the southern margin of the fold belt terminating shortening. The back/ intra- arc basin closed in Late Cretaceous time deforming the fold-thrust belt for a second time antithetically to N or NE subduction in the Vardar zone. In Latest Cretaceous and Paleogene time the southern part of the Balkan orogen became extensional with abundant magmatism. The final foreland fold-thrust belt deformation was Late Eocene extending into Oligocene time and was a transfer of shortening in response to clockwise rotation within the Carpathians and southward motion of the Moesian platform north of the thrust belt. During this event southern Bulgaria was in an extensional regime, the beginning of the back arc extension that dominated the south to southwest vergent Hellenide orogen throughout the Cenozoic. Within the Balkan area during the Eocene to Present the area south of the thrust belt shows a migration of extension to the west and south in response to slab roll back at the Hellenic subduction zone.

Feb. 20, 2013
noon - 1 p.m.
Geology 1707

Presented By:

• Sinan Akciz - UCLA

A significant part of the post-collisional northward indentation of India into Asia has been accommodated by the Tertiary Gaoligong Shan and Chong Shan shear zones (CSSZ), as well as by the left-lateral displacement along the better known Ailao Shan-Dianchang Shan shear zones that lie farther E in Yunnan, China. Penetrative ductile deformation marked by steep foliation and subhorizontal lineation affects all rock types within both shear zones and kinematic indicators show right-lateral and left-lateral shear along the GSSZ and CSSZ, respectively. . Boudinage structures, indicating both horizontal and vertical stretching, affected compositional bands in mylonitic rocks formed during earlier stages of deformation and leucogranite sills emplaced during later stages of deformation. U/Pb ages on several monazite grains and grain fractions from mylonitic to non mylonitic leucogranites within the GSSZ shear zone range from ca. 80-18 Ma. The youngest ages come from undeformed leucogranites cross-cutting the foliation, which indicate that right-lateral movement ended ~18 Ma, when right-lateral shear between India-Sibumasu switched to the Sagaing fault zone in Burma. High-temperature sinistral strike-slip shearing along the CSSZ was active at ca. 28 Ma, and it terminated before ca.17 Ma, based on U/Pb ages of monazite grains from deformed and foliation cross-cutting leucogranites. The documentation of these structures indicate that crustal fragments in SE extruded as at least two crustal fragments with considerable internal deformation, bounded by three large-scale intra-continental shear zones. The continuation of these strike-slip shear ones to the north, the total amount of offset along them, and the reasons why the motion along them terminated at about 17 Ma and created new structures are some of the questions that remain to be answered.

March 6, 2013
noon - 1 p.m.
Geology 1707

Presented By:

• Doug Yule - CSUN

Seminar Description coming soon.

March 13, 2013
noon - 1 p.m.
Geology 1707

Presented By:

• Ray Ingersoll - UCLA

Central to the debate about the age, origin and evolution of Grand Canyon is the history of the Colorado River and its precursors. Reversal of dextral slip along the San Andreas fault system restores southern California to a position at the downstream end of the Early Pliocene course of the Colorado River. If the Colorado River flowed across southern California to the Pacific prior to 6 Ma, then sand deposited by it would have distinctive detrital-zircon age distributions reflecting erosion of upper Paleozoic and lower Mesozoic strata of the Colorado Plateau. The latter contain 300-1100 Ma zircon that was originally transported from orogenic belts along southeastern Laurentia. Lower Paleozoic, upper Paleozoic, Triassic, Lower to Middle Jurassic, and Upper Jurassic to Cretaceous sandstone of the Colorado Plateau contains average concentrations of 300-1100 Ma zircon of 4%, 32%, 46%, 44% and 30%, respectively. Not surprisingly, Plateau-derived sand in the modern Colorado River averages 29% 300-1100 Ma zircon. Triassic to lowest Cretaceous metasedimentary wallrocks of the mid-to-Late Cretaceous southern California magmatic arc contain 34% 300-1100 Ma zircon, indicating that sand similar to that on the Plateau reached the Pacific Ocean. In contrast, only trace amounts of 300-1100 Ma zircon occur in younger southern California sandstone. Stratigraphic groupings of age distributions of 6662 detrital zircons from 167 broadly distributed sandstone samples from coastal deposits of southern California average 44-88% Cretaceous, but only 0.4-1.3% 300-1100 Ma grains, most of which can be attributed to recycling from older deposits. No individual Upper Cretaceous to Pliocene sandstone sample contains greater than 3% 300-1100 Ma zircon. Although Paleogene headwaters of southern California rivers extended into the eastern Mojave Desert, Sonora and Mogollon Highlands, our observations indicate that these headwaters did not extend as far inland as the Colorado Plateau. This conclusion conflicts with the model of a SW-flowing Arizona River during the Paleogene, but supports rapid Late Miocene-Pliocene drainage reorganization and integration of the Colorado River coincident with development of the Salton Trough and Gulf of California.