The Effects of Noise Due to Random Undetected Tilts and Paleosecular Variation on Regional Paleomagnetic Directions

Random tilting of a single paleomagnetic vector produces a distribution of vectors which is not rotationally symmetric about the original vector and therefore not Fisherian. Monte Carlo simulations were performed on two types of vector distributions: (1) distributions of vectors formed by perturbing a single original vector with a Fisher distribution of bedding poles (each defining a tilt correction) and (2) standard Fisher distributions. These simulations demonstrate that inclinations of vectors drawn from both distributions are biased toward shallow inclinations. There is a greater likelihood of statistically “drawing” a vector shallower than the true mean vector than of drawing one that is steeper. The estimated probability increases as a function of angular dispersion and inclination of the true mean vector. Consequently, the interpretation of inclination-only data from either type of distribution is not straightforward, especially when the expected paleolatitude is greater than about 50°. Because of the symmetry of the two distributions, declinations of vectors in each distribution are unbiased. The Fisher mean direction of the distribution of vectors formed by perturbing a single vector with random undetected tilts is biased toward shallow inclinations, but this bias is insignificant for angular dispersions of bedding poles less than 20°. This observation implies that the mean pole calculated from a large set of paleomagnetic directions obtained for coeval rocks over a region will be effectively unbiased by random undetected tilts of those rocks provided the angular dispersion of the undetected tilts is less than about 20°. However, the bias of the mean can be significant for large (\u3e20°) angular dispersion of tilts. The amount of bias of the mean direction maximizes at about 10°–12° in mid-latitude regions but is usually less than 8°. Consequently, large (\u3e12°) inclination discordances are probably not the result of random undetected tilts, even if the angular dispersion of the tilts exceeds 20°

Paleomagnetism of Middle Miocene Volcanic Rocks in the Mojave-Sonora Desert Region of Western Arizona and Southeastern California

Paleomagnetic directions have been obtained from 190 early to middle Miocene (12–20 Ma) mafic volcanic flows in 16 mountain ranges in the Mojave-Sonora desert region of western Arizona and southeastern California. These flows generally postdate early Miocene tectonic deformation accommodated by low-angle normal faults but predate high-angle normal faulting in the region. After detailed demagnetization experiments, 179 flows yielded characteristic directions interpreted as original thermal remanent magnetizations (TRM). Because of the episodic nature of basaltic volcanism in this region, the 179 flows yielded only 65 time-distinct virtual geomagnetic poles (VGPs). The angular dispersion of the 65 VGPs is consistent with the angular dispersion expected for a data set that has adequately averaged geomagnetic secular variation. The paleomagnetic pole calculated from the 65 cooling unit VGPs is located at 85.5°N, 108.9° within a 4.4° circle of 95% confidence. This pole is statistically indistinguishable (at 95% confidence) from reference poles calculated from rocks of similar age in stable North America and from a paleomagnetic pole calculated from rocks of similar age in Baja California. The coincidence of paleomagnetic poles from the Mojave-Sonora desert region with reference poles from the stable continental interior indicates that (1) significant vertical axis net tectonic rotations have not accompanied post-middle Miocene high-angle normal faulting in this region; (2) there has been no detectable post-middle Miocene latitudinal transport of the region; and (3) long-term nondipole components of the middle Miocene geomagnetic field probably were no larger than those of the recent (0–5 Ma) geomagnetic field. In contrast, paleomagnetic data indicate vertical axis rotations of similar age rocks in the Transverse Ranges, the Eastern Transverse Ranges, and the Mojave Block. We speculate that a major structural discontinuity in the vicinity of the southeastward projection of the Death Valley fault zone separates western areas affected by vertical axis rotations from eastern areas that have not experienced such rotations

Strain and Magnetic Fabric in Santa Catalina and Pinaleno Mountains Metamorphic- Core-Complexes Mylonite Zones, Arizona

Anisotropy of magnetic susceptibility (AMS) is capable of recording finite strain in weakly magnetized rocks. AMS was measured for 228 samples from 20 sites in two mylonite zones with the same deformational history. AMS measurements were compared with finite strains determined from dike rotations and from foliation orientations. In one zone (the Santa Catalina Mountains) the orientations of susceptibility and finite strain ellipsoids are in excellent agreement, and there is a logarithmic relationship between susceptibility difference (ΔKi = [Ki-K]/K) and finite strain magnitude. In the second zone (the Pinaleno Mountains) minimum susceptibility is perpendicular to the finite flattening plane, but the maximum susceptibility does not parallel the maximum extension direction, and there is no systematic relationship between susceptibility magnitude and strain magnitude. Oriented polished thin sections indicate that magnetite in the protolith of the Santa Catalina mylonite occurs as randomly oriented, elongate grains. With subsequent deformation, the long axes are rotated into the maximum extension direction. In the Pinaleno mylonites, both equant and elongate magnetite grains are present. With deformation, the elongate magnetite grains are rotated into the maximum flattening plane but show no preferred orientation within this plane. AMS in the two mylonite zones appears to be predominantly controlled by the orientation of elongate magnetite grains with respect to the megascopic fabric. The final orientation of the elongate grains is a function of their initial orientation as well as the finite strain. Therefore, despite similar deformational histories, the two zones display different AMS patterns due to the differences in occurrence, initial orientation, and shape of ferromagnetic grains

Matrix Metalloproteinase-1 Expression in Fibroblasts Accelerates Dermal Aging and Promotes Papilloma Development in Mouse Skin.

Fragmentation, disorganization, and depletion of the collagen-rich dermal extracellular matrix (ECM) are hallmarks of aged human skin. These deleterious alterations are thought to critically mediate many of the prominent clinical attributes of aged skin including thinning, fragility, impaired wound healing, and propensity for carcinoma. Matrix metalloproteinase-1 (MMP1), initiates cleavage of collagen fibrils and is significantly increased in dermal fibroblasts in aged human skin. To investigate the role of elevated MMP1 in skin aging, we generated a conditional bitransgenic mouse (Col1a2;hMMP1) that expresses full-length, catalytically-active human MMP1 (hMMP1) in dermal fibroblasts. hMMP1 expression is activated by a tamoxifen-inducible Cre recombinase that is driven by the collagen1A2 (Col1a2) promoter and upstream enhancer. Tamoxifen induced hMMP1 expression and activity throughout the dermis in Col1a2 in hMMP1 mice. At six months of age, Col1a2;hMMP1 mice displayed loss and fragmentation of dermal collagen fibrils, which was accompanied by many of the features of aged human skin, such as contracted fibroblast morphology, reduced collagen production, increased expression of multiple endogenous MMPs and proinflammatory mediators. Interestingly, Col1a2;hMMP1 mice displayed substantially increased susceptibility to skin papilloma development. These data demonstrate that fibroblast expression of hMMP1 is a critical mediator of dermal aging and creates a dermal microenvironment that promotes keratinocyte tumor development