iPlex Lunch - spring-2015
Syncing geological clocks
April 8, 2015
noon - 12:50 p.m.
Geology 1707
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Mountains form where the Earth’s plates collide; during this upheaval rocks are deformed by massive forces. The rates and timescales over which these deformational processes occur are determined from tiny accessory minerals that record geological time through radioactive decay. However, there remain major unresolved challenges in linking the dates yielded from these accessory phases to specific deformation events and discerning the effects of deformation on the isotopic and elemental tracers in these phases. The Himalayan orogen represents the ideal natural laboratory to decode the record of the deformational processes encrypted in the rocks. In the eastern region of Sikkim, a unique series of ‘time windows’ are exposed by doming of a major ductile fault, revealing the inner workings of one of the major mountain-building structures that accommodated the India-Asia collision.
Here, we investigate this region, using combined laser ablation (split-stream) U-Pb and REE analysis of deformed monazite along with EBSD imaging and Pressure-Temperature (P-T) phase equilibria modelling to (1) link accessory phase ‘age’ to ‘metamorphic stage’ and (2) to quantify the influence of deformation on monazite (re)crystallisation mechanisms and its subsequent effect on the crystallographic structure, ages and trace-element distribution in individual grains. These data provide links between ages and specific deformation events, thus helping further our understanding of the role of dynamic recrystallisation in producing age variation within and between crystals in a deformed rock.
This study provides new insight into how deformation is accommodated along major thrust faults during mountain building and demonstrates the importance of fully integrating the pressure-temperature-time-deformation history of accessory phases to better interpret the meaningfulness of ages yielded from deformed rocks and thus understand the deformational history of the cores of evolving mountain belts.
Sea-level change during a Snowball Earth deglaciation
April 15, 2015
noon - 12:50 p.m.
Geology 1707
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The Snowball Earth hypothesis posits that two global glaciation events in the Neoproterozoic entombed the Earth and that continental ice sheets reached paleo-equatorial latitudes (Kirschvink, 1992). Cap carbonates are thought to represent marine deposition atop formerly glaciated margins during the glacioeustatic sea-level rise accompanying a Snowball Earth deglaciation (Hoffman et al., 1998; 2007). Inferences of globally coherent sea-level rise in the vicinity of rapidly melting ice sheets conflicts with insights from geodetic and geophysical models of the Plio-Pleistocene ice age that predict that the decay of ice sheets results in a sea-level change characterized by significant geographic variability, including pronounced sea-level fall in the vicinity of rapidly melting ice sheets driven by elastic/gravitational effects and viscoelastic crustal deformation (post-glacial rebound).
In this talk, I present lithofacies observations and a sequence stratigraphic analysis of the Noonday Dolomite, Death Valley region, CA, a ~ 635 Ma Marinoan-equivalent cap carbonate. This analysis reveals complex post-Snowball sea-level change, including a large amplitude sea-level fall that punctuated the post-glacial transgression. I speculate on the mechanisms responsible for the observed sea-level change using a gravitationally self-consistent theory that accounts for the gravitational and deformational perturbations to sea level on a viscoelastic Earth model. I apply the theory to model a Marinoan Snowball deglaciation across a generalized Ediacaran paleogeography with a synthetic ice sheet distribution (Creveling and Mitrovica, 2014). I conclude that either a hiatus in melt-water flux or a rapid, local collapse of Snowball ice sheets can reconcile the sea-level fall observed in the Noonday Dolomite. Further I conclude that globally distributed syn-deglacial transgression over formerly glaciated margins provides strong evidence for ‘Snowball’-scale ice volumes that contribute to a sea-level rise that overwhelms local dynamic effects.
Reconstructing Southern California
April 22, 2015
noon - 12:50 p.m.
Geology 1707
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Southern California is a critical component in paleotectonic models for: (1) the evolution of the USA-Mexico Cordillera; (2) the interaction of continental and oceanic plates; and (3) relations between subduction and transform processes during the Mesozoic and Cenozoic. Detailed palinspastic reconstruction of both offshore and onshore components of the diverse and complex settings of southern California is essential in order to test paleotectonic models for the evolution of the broader region. The unique geologic history of southern California can be described in terms of distinct phases of tectonic development, which resulted in corresponding distinct tectonostratigraphic sequences. Steep-slab subduction of the Farallon plate characterized most of the Cretaceous history of the southwestern USA, whereas flat-slab subduction characterized the Laramide orogenic event (80-40 Ma). As the subducting Farallon plate rolled back following the Laramide orogeny, the Pacific plate first came into contact with the North American plate in southern California soon after 30 Ma. Two triple junctions then traveled in opposite directions along the continental margin. The southern triple junction had a complex history, including three distinct stages of capture of Farallon microplates and contiguous parts of the North American margin by the Pacific plate. These three microplate-capture events resulted in transrotation (18-12 Ma), transtension (12-6) and transpression (6-0 Ma) in the Los Angeles region. Reconstructing this complex history is an extraordinarily challenging and rewarding endeavor.
Seismic Hazard Analysis using Seismic Noise
April 29, 2015
noon - 12:50 p.m.
Geology 1707
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Rapid growth of populated urban environments increases exposure to earthquake hazard. In particular, major metropolitan area such as Los Angeles and Tokyo, sit on deep sedimentary basin and are subject to high seismic hazard with the nearby active plate boundaries. Ground motion prediction is central to seismic hazard analysis. Two approaches currently exist to predict ground motion; one is purely empirical and the other is purely physics-based. To validate and constrain both approaches, we propose a third strategy that respects the physics and that takes advantage of large amount of seismic data. The continuously recorded ambient seismic field carries information on three-dimensional wave propagation. We develop a new method that extracts the coherent wavefield buried in seismic noise and that constructs long-period seismograms from virtual earthquakes. In the Los Angeles sedimentary basin, we validate it against past M5 earthquakes in southern California and predict ground motion for a M7+ virtual earthquake on the southern San Andreas Fault. We find that strong seismic amplification patterns concord with basin shape and that finite source directivity couples with three-dimensional structure to intensify seismic amplification far from the fault. In the Kanto sedimentary basin that underlies Tokyo, we take advantage of dense seismic arrays to unravel the effects of focusing and reflection within the basin. We find the resonance frequency of the Kanto Basin, build linear relations between peak ground motion and basin depth, and highlight the variability in amplification pattern with the origin virtual source.
Three Eocene rivers in Sonora, Mexico
May 6, 2015
noon - 12:50 p.m.
Geology 1707
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The Eocene was an interesting and dynamic time in Sonora, Mexico. The effects of “flat-slab subduction” of the Farallon Plate beneath North America were especially pronounced. Uplift and exhumation accompanied by pronounced crustal contraction drove two significant tectonic imprints that are well-preserved in the geologic record: 1) a system of three rivers drained westward from Sonora/Chihuahua highlands to the Pacific Ocean, and 2) development of the Sonoran orogenic gold belt. Besides the Ballenas River and related Poway conglomerate originally described by Abbott and Smith (1978), two additional Eocene river drainages are identified at more northerly and southerly latitudes. Each of the three rivers tapped a different provenance area as recorded by distinctive conglomerate clast assemblages and detrital zircon populations. Because these rivers flowed orthogonally across the future trace of the San Andreas transform system, they provide important constraints on the total plate-boundary slip since Eocene time. Disposition of certain fault strands of the transform system are deduced from offset channels and sedimentary packages. Evolution of the river system coincided with a crustal shortening event manifested by a northwest-trending fold-thrust belt and widespread Eocene cooling ages. Orogenic gold is localized in sheared quartz veins along thrust faults that preserve Eocene 40Ar/39Ar ages on syntectonic white mica (Iriondo, 2008). The source of the gold-bearing fluids is still controversial but spatial association with Late Jurassic faults in Sonora appears to be a common link.
Transpression in the Southern California Borderland
May 13, 2015
noon - 12:50 p.m.
Geology 1707
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New mapping of two active transpressional fault zones in the California Continental Borderland, the Santa Cruz-Catalina Ridge fault and the Ferrelo fault, was carried out to characterize their geometries, using over 4500 line-km of new multibeam bathymetry data collected in 2010 combined with existing data. Faults identified from seafloor morphology were verified in the subsurface using existing Borderland seismic reflection data including single-channel and multi-channel seismic profiles compiled over the past three decades. The two fault systems are parallel and are capable of large lateral offsets and reverse-slip during earthquake faulting. The geometry of the fault systems shows evidence of multiple segments that could experience through-going rupture over distances exceeding 100 km. Published earthquake hypocenters from regional seismicity studies further define the lateral and depth extent of the historic fault ruptures. Historical and recent focal mechanisms obtained from first-motion and moment tensor studies confirm regional strain partitioning dominated by right-slip on major through-going faults with reverse-oblique mechanisms on adjacent structures. The two systems resemble onshore active fault zones and provide additional information for understanding the full Pacific-North America plate boundary evolution, as well as transpressional fault systems along the plate boundary. Because of their potential for dip-slip rupture, they may also be capable of generating local tsunamis that would impact southern California coastlines, including populated regions in the Channel Islands.
Large vertical motions and basin evolution in the Outer California Borderland
May 20, 2015
noon - 12:50 p.m.
Geology 1707
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The Continental Borderland offshore southern California occupies a strategic position along
the continental margin. It was the locus of ~75% of Pacific-North America displacement history,
it helped accommodate the large-scale (>90°) tectonic rotation of the Western Transverse Ranges
province, and is still accommodating potentially 20% of PAC-NAM plate motion today. As
such, it represents an ideal natural laboratory to investigate plate boundary evolution and basin
development associated with transform initiation, oblique continental rifting, transrotation and
transpression. We have been using newly released grids of high-quality industry multichannel
seismic (MCS) reflection data, combined with multibeam bathymetry and offshore well data to
map and construct digital 3D fault surfaces and stratigraphic reference horizons over large parts
of the Outer Continental Borderland. These 3D surfaces of structure and stratigraphy can be
used to better understand and evaluate regional patterns of uplift, subsidence, fault interaction
and other aspects of plate boundary deformation.
In the northern Outer Borderland, mapping in Santa Cruz basin, and across both Santa Rosa
and Santa Cruz-Catalina ridges reveals a pattern of interacting high-and low-angle faults, fault
reactivation, basin subsidence, folding, and basin inversion. Subsidence since early-Miocene
time is significant (up to 4 km) and is much larger than predicted by simple thermal cooling
models of continental rifting. This requires additional tectonic components to drive this regional
subsidence and subsequent basin inversion. Farther south, a more en echelon pattern of ridges
and basins suggests a distributed component of right-lateral shear also contributed to much of
the modern Borderland seafloor topography, including major Borderland basins.
Vertical motions of uplift and subsidence can be estimated from a prominent early-Miocene
unconformity that likely represents a regional, paleo-horizontal, near-paleo-sea-level erosional
surface. As such, this paleo-reference datum can be used to reconstruct Borderland forearc basin
geometry prior to rifting, subsidence and subsequent basin inversion. Although not well
resolved, the age of the regional unconformity appears to be time transgressive, and tends to
young to the east and south. This progression may thus correlate with the oblique subduction of
the Pacific-Arguello spreading ridge, rather than the onset of later continental rifting, as rifting
in the Borderland typically progressed to the north and west following each jump in the triple
junction farther south. This sequence of 1) a regional unconformity requiring uplift, 2) followed
by subsidence, and 3) later basin inversion to form ridges thus documents an unusual and
unexpected pattern of vertical motion reversal associated with the initiation of a predominantly
strike-slip PAC-NAM plate boundary.
Oligocene-Miocene Sedimentary and Volcanic Strata of the Vincent Gap Region
June 3, 2015
noon - 12:50 p.m.
Geology 1707
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The Vincent Gap region of the eastern San Gabriel Mountains in southern California is a small but important piece of an originally continuous terrane separated into the Tejon, Soledad and Orocopia regions by the San Andreas fault system. The middle-upper Miocene Punchbowl Formation has been considered the oldest Neogene strata of the Vincent Gap region. The present study documents that strata southeast of the main exposure of the Punchbowl Formation, though aerially restricted, are temporally extensive; together with the Punchbowl Formation, they comprise a sedimentary record that spans from ~25 to ~6 Ma, includes equivalents of the Vasquez, Tick Canyon and Mint Canyon formations of the Soledad region, and relates to three sequential tectonic stages in southern California. Uppermost Oligocene-lower Miocene strata are closely correlative with the Plush Ranch, Vasquez and Diligencia formations of the Tejon, Soledad and Orocopia regions, respectively; they formed during extension induced by triple-junction instability. Interbedded 25 Ma volcanics near the base of these strata are chemically and chronologically similar to those of the Plush Ranch, Vasquez and Diligencia formations. Middle Miocene strata beneath the Punchbowl Formation are equivalent to the Tick Canyon Formation of the Soledad region, and document exhumation of the Pelona Schist. Sandstone petrofacies, conglomerate composition and detrital-zircon age data provide compatible but distinct provenance information; using all three in combination results in a more complete understanding of the provenance of each of these units's provenance. The results of this study imply that transrotation of the western Transverse Ranges and accompanying basement exhumation extended farther inboard than generally thought, adjacent to if not across the future trace of the San Andreas fault. Data from the Vincent Gap also reveal that the middle-upper Miocene Punchbowl Formation was likely part of a large drainage system, with the Mint Canyon Formation of the Soledad region representing a tributary of this system that joined downstream, and the Caliente Formation of the Tejon region representing the confluence of the two.
3 Dimensional Crustal Tomography of the Peruvian Andes
June 3, 2015
noon - 12:50 p.m.
Geology 1707
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It has been thought since the 1970s that the Andes lie over a region of thickened crust with a thickening profile roughly proportional to local elevation and a peak Moho offset 30-35km below the IASPI Moho. Recent data from PERUSE has been used in transmission tomography to independently establish the parameters of the crustal thickening under the Peruvian Andes. We have made our own 3D forward ray and inversion models in order to balance between accuracy and speed for this problem. The crustal thickening was modeled using an asymmetric Gaussian basis function. Both local and teleseismic data was used in the inversions. Current results are preliminary and indicate some evidence of a crustal root using local event inversions and teleseismic event forward models. Teleseismic and combined local+teleseismic inversions are pending shortly.