Postorogenie magmatic suites are common to many orogens, in many eases apparently just postdating the cessation of deformation. They differ from preceding orogenie suites in that they have higher temperatures, more primitive isotopic signatures, and bimodal na• tures, and thus are compositionally similar to suites found in extensional regimes. We propose that thinning of the lithospheric mantle, which may be an automatic response to orogenie lithospheric thicken• ing, is responsible for these magmatic suites. Mantle lithospheric thinning moves the asthenospheric-lithospheric thermal boundary higher in the lithospheric column, thereby raising the overall thermal budget of the orogen and the likelihood of basaltic magmatism. This removal of much of the dense, Iithospheric mantle root of the orogen also invokes uplift capable of producing horizontal orogenie buoyancy forces that will oppose and potentially terminate deformation. Extreme fractionation of the magmas is promoted by high temperatures and a noncompressional or tensional Iithospheric stress regime to produce accompanying felsic (A-type) magmas. This model explains why the cessation of deformation is coincident with high-temperature bimodal magmatism and inferred uplift, as well as the long-noted similarity and confusion between "postorogenie" and extensional magmatic suites. This arises because both reflect thinning of the Iithospheric mantle, not because both reflect extensional tectonics. We suggest that these magmatic episodes may be important in transferring Iithospheric mantle material and its compositional signatures into the crust. CHARACTERISTICS OF POSTOROGENIC MAGMATlSM Many orogenie belts contain two distinguishable magmatic suites, a series of orogenie I• and S-type granites bearing structural fabrics and an undeformed bimodal suite emplaced following the cessation of convergent

Earthquake moment tensors in eastern Pacific (ePac) slabs typically show downdip tensional (DT) axes, whereas in the western Pacific (wPac), they typically show downdip compressional (DC) axes or have mixed orientations indicative of unbending. Prevailing conceptual models emphasize uniform stress/deformation modes, that is, bulk slab stretching or shortening, as the dominant control on intermediate depth seismic expression. In contrast, we propose that a diversity of seismic expression, including DT‐ and DC‐dominated regions, is consistent with expectations of flexural strain accumulation, based on systemic differences in slab geometry. Our analysis reveals two largely unrecognized features of ePac intraslab seismicity. First, earthquake clusters consistent with slab unbending are present in ePac slabs, albeit at much shallower depths than typical of wPac slabs. Second, intermediate depth ePac DT seismicity is strongly localized to the upper half of zones undergoing curvature increase, such as flat slab segments. Our study highlights how the seismic expression of slab flexure is impacted by the relative contribution of brittle and ductile deformation. The strongly asymmetric temperature structure that is preserved in sinking slabs means that seismicity disproportionately records the deformation regime in the colder part of the slab, above the neutral plane of bending. The expression of in‐plane stress may be discernible in terms of a systematic modifying effect on the seismic expression of flexure.

The Mount Painter Region in the Northern Flinders Ranges, South Australia, contains a Mesoproterozoic gneissic complex characterized by extraordinary heat production (∼16 μW m−3), resulting in the development of elevated middle‐upper crustal thermal gradients through much of the Paleozoic. Early Paleozoic deformation and metamorphism attained amphibolite facies (>500°C) in the deepest parts of the metamorphic pile (∼10–12 km) during the ∼500 Ma Cambro‐Ordovician Delamerian orogeny. The subsequent thermal history of these rocks is assessed through new K/Ar and 40Ar/39Ar age measurements on amphiboles and micas and multiple‐diffusion‐domain thermal modeling of K‐feldspar 40Ar/39Ar data. The preferred interpretation of these data is that the deepest rocks were at ∼500°C until around 430 Ma, requiring average upper crustal thermal regimes of the order of 40°C km−1 for at least 70 million years. At around 430 Ma, and again at 400 Ma, the terrane underwent periods of moderately fast cooling, possibly separated by a period of isothermal residence. Following cooling at 400 Ma, the terrane entered a second period of relative tectonic quiescence remaining essentially isothermal until ∼330 Ma. This long residence near the closure temperature of biotite resulted in variable argon loss from biotites in the 400 Ma to 330 Ma interval. Tectonic and thermal stability was terminated by a further period of moderately fast cooling (∼4°–8°C Ma−1) in the interval between 330 and 320 Ma. The three cooling episodes at around 430 Ma, 400 Ma, and 330 Ma are interpreted to be the result of exhumation resulting in a combined minimum of 6–7 km of denudation. We attribute this exhumation to the Alice Springs orogeny, a major intraplate tectonothermal event known throughout central Australia but not previously recognized as a significant tectonic event in the Adelaide Fold Belt. These new data provide compelling evidence that thermally modulated variations in lithospheric strength control the distribution of intraplate deformation at the continental scale.

We use the fact that geoid anomalies are directly related to the local dipole moment of the densitydepth distribution to help constrain density variations within the lithosphere and the associated tectonic stresses. The main challenge with this approach is isolating the upper mantle geoid contribution from the full geoid (which is dominated by sources in the lower mantle). We address this issue by using a high-pass spherical harmonic filtering of the EGM2008-WGS84 geoid to produce an "upper mantle" geoid. The tectonic implications of the upper mantle are discussed in terms of plate tectonics and intraplate stresses. We find that globally there is about a 9 meter geoid step associated with the cooling oceanic lithosphere that imparts a net force of ~2.5x10 12 N/m in the form of "ridge push" -a magnitude that is consistent with 1-d models based on first-order density profiles. Furthermore, we find a consistent 6 meter geoid step across passive a continental margin which has the net effect of reducing the compressive stresses in the continents due to the ridge force. Furthermore, we use the upper mantle geoid to reevaluate the tectonic reference state which previously studies estimated using an assumption of Airy-based isostasy. Our evaluation of the upper mantle geoid confirms the near equivalence of the gravitational potential energy of continental lithosphere with an elevation of about 750 meters and the mid-ocean ridges. This result substantiates early conclusions about the tectonic reference state and further supports the prediction that continental regions are expected to be in a slightly extensional state of stress.

Consideration of the stress field around an arbitrarily oriented borehole shows that in an extensional stress regime (av> OH> ah), wellbores parallel to the direction of minimum horizontal principal stress are the least prone to compressive shear failure (breakout). The most stable deviation angle (from the vertical) depends on the ratio of the horizontal principal stresses to the vertical stresses, and the higher the ratio oH/av, the higher the deviation angle for minimizing breakout. In a strike-slip stress regime (OH> av >Oh), horizontal wells are the least prone to breakout, and the higher the ratio oH/av, the closer the drilling direction should be to the azimuth of OH• A new compressive shear failure criterion, which is a combination of the effective strength concept and the Drucker-Prager criterion, is proposed for quantifying the stresses at which borehole breakout occurs. The lowest mud weight, at and below which breakout will occur, can be predicted by combining this criterion with the stress field around an arbitrarily oriented borehole. The highest mud weight at and above which a tensional or hydraulic fracture is induced can be predicted by combining the tensile strength of the rocks of the wellbore wall with the stress field around an arbitrarily oriented borehole. For the in-situ stress environments considered, the optimallY oriented inclined well bore is less prone to breakout (i.e., allows a lower mud weight) and tensional or hydraulic fracture (i.e., supports a higher mud weight) than a vertical well. 'Now at Geologicallnsl., U. of Copenhagen (Denmark).