An Evaluation of Proposed Mechanisms of Slab Flattening in Central Mexico

Central Mexico is the site of an enigmatic zone of flat subduction. The general geometry of the subducting slab has been known for some time and is characterized by a horizontal zone bounded on either side by two moderately dipping sections. We systematically evaluate proposed hypotheses for shallow subduction in Mexico based on the spatial and temporal evidence, and we find no simple or obvious explanation for the shallow subduction in Mexico. We are unable to locate an oceanic lithosphere impactor, or the conjugate of an impactor, that is most often called upon to explain shallow subduction zones as in South America, Japan, and Laramide deformation in the US. The only bathymetric feature that is of the right age and in the correct position on the conjugate plate is a set of unnamed seamounts that are too small to have a significant effect on the buoyancy of the slab. The only candidate that we cannot dismiss is a change in the dynamics of subduction through a change in wedge viscosity, possibly caused by water brought in by the slab

TB102: A Markov Analysis of Structural Change and Output Prediction in the New England Egg Industry

The purpose of this study is to examine changes in the size distribution of egg farms as a Markov process in the three-state region consisting of Connecticut, Massachusetts, and New Hampshire. In 1978 these three states together received cash receipts of 83 million dollars from egg sales. In addition to examining the size distribution of farms, the occurrence of a marked structural change in the egg industry during the period 1967 to 1978 is statistically tested. Also, the future size distribution of egg farms, total number of layers, and egg output are projected.https://digitalcommons.library.umaine.edu/aes_techbulletin/1086/thumbnail.jp

The lack of correlation between flat slabs and bathymetric impactors in South America

Flat slab subduction has been attributed to various causes including mantle wedge dynamics, overriding by the upper plate, age of the subducting plate, and subduction of anomalously thick oceanic crust. One often favored explanation for flat slabs is the subduction of buoyant features on the oceanic plate in the form of an aseismic-ridge or oceanic plateau. We show through plate tectonic reconstructions of the Marquesas, Tuamotu, and Austral plateau, assuming that features on the conjugate plate can be used as proxies for subducted bathymetric anomalies, that there is very little correlation between the subduction of such anomalies and historic zones of flat subduction in South America. It is apparent that subduction of a bathymetric anomaly need not lead to a flat slab and not all flat slabs are associated with the subduction of a bathymetric anomaly

Saving Puzzles and Saving Policies in the United States

In the past two decades the widely reported personal saving rate in the United States has dropped from double digits to below zero. First, we attempt to account for the decline in the National Income and Product Accounts (NIPA) saving rate. The macroeconomic literature suggests that about half of the drop since 1988 can be attributed to households spending stock market capital gains. Another thirty percent is accounting transfers from personal saving into government and corporate saving because of the way pensions and capital gains taxes are treated in the NIPA. Second, while NIPA saving measures are well suited for measuring the supply of new funds for investment and capital accumulation, it is not clear that they should be the target of government saving policies. Finally, we emphasize that the NIPA saving rate is not useful in judging whether households are preparing for retirement or other contingencies. Many households have accumulated significant wealth, primarily through retirement saving vehicles and capital gains, even as the saving rate slid. There remains a segment of the population, however, who save little and whose behavior appears untouched either by the stock market boom or the slide in personal saving. We explore reasons and policy options for their puzzlingly low saving rate.

What Accounts for the Variation in Retirement Wealth Among U.S. Households?

Even among households with similar socioeconomic characteristics, saving and wealth vary considerably. Life-cycle models attribute this variation to differences in time preference rates, risk tolerance, exposure to uncertainty, relative tastes for work and leisure at advanced ages, and income replacement rates. These factors have testable implications concerning the relation between accumulated wealth and the shape of the consumption profile. Using the Panel Study of Income Dynamics and the Consumer Expenditure Survey, we find little support for these implications. The data are instead consistent with rule of thumb, mental accounting, or hyperbolic discounting theories of wealth accumulation

Plate Tectonic Constraints on Flat Subduction and Paleomagnetic Constraints on Rifting

Plate tectonics shapes our dynamic planet through the creation and destruction of lithosphere. This work focuses on increasing our understanding of the processes at convergent and divergent boundaries through geologic and geophysical observations at modern plate boundaries. Recent work had shown that the subducting slab in central Mexico is most likely the flattest on Earth, yet there was no consensus about what caused it to originate. The first chapter of this thesis sets out to systematically test all previously proposed mechanisms for slab flattening on the Mexican case. What we have discovered is that there is only one model for which we can find no contradictory evidence. The lack of applicability of the standard mechanisms used to explain flat subduction in the Mexican example led us to question their applications globally. The second chapter expands the search for a cause of flat subduction, in both space and time. We focus on the historical record of flat slabs in South America and look for a correlation between the shallowing and steepening of slab segments with relation to the inferred thickness of the subducting oceanic crust. Using plate reconstructions and the assumption that a crustal anomaly formed on a spreading ridge will produce two conjugate features, we recreate the history of subduction along the South American margin and find that there is no correlation between the subduction of a bathymetric highs and shallow subduction. These studies have proven that a subducting crustal anomaly is neither a sufficient or necessary condition of flat slab subduction. The final chapter in this thesis looks at the divergent plate boundary in the Gulf of California. Through geologic reconnaissance mapping and an intensive paleomagnetic sampling campaign, we try to constrain the location and orientation of a widespread volcanic marker unit, the Tuff of San Felipe. Although the resolution of the applied magnetic susceptibility technique proved inadequate to contain the direction of the pyroclastic flow with high precision, we have been able to detect the tectonic rotation of coherent blocks as well as rotation within blocks