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Abstract. The output of a coupled climate system model provides a synthetic climate record with temporal and spatial coverage not attainable with observational data, allowing evaluation of climatic excitation of polar motion on timescales of months to decades. Analysis of the geodetically inferred Chandler excitation power shows that it has fluctuated by up to 90% since 1900 and that it has characteristics representative of a stationary Gaussian process. Our model-predicted climate excitation of the Chandler wobble also exhibits variable power comparable to the observed. Ocean currents and bottom pressure shifts acting together can alone drive the 14-month wobble. The same is true of the excitation generated by the combined effects of barometric pressure and winds. The oceanic and atmospheric contributions are this large because of a relatively high degree of constructive interference between seafloor pressure and currents and between atmospheric pressure and winds. In contrast, excitation by the redistribution of water on land appears largely insignificant. Not surprisingly, the full climate effect is even more capable of driving the wobble than the effects of the oceans or atmosphere alone are. Our match to the observed annual excitation is also improved, by about 17%, over previous estimates made with historical climate data. Efforts to explain the 30-year Markowitz wobble meet with less success. Even so, at periods ranging from months to decades, excitation generated by a model of a coupled climate system makes a close approximation to the amphtude of what is geodetically observed.

Paleomagnetic data from Miocene to Recent deep sea sediments recovered during Leg 94 of the Deep Sea Drilling Project (DSDP) in the North Atlantic exhibit a systematic shallowing of inclinations with depth at some sites. This shallowing is coincident with a downhole decrease in water content, and is observed only in sediments with a carbonate content consistently greater than 80%. Compaction due to overburden pressure is believed to be responsible for the shallow inclinations observed in these unconsolidated sediments. At sites where the carbonate content of the sediments is consistently below about 80% no significant downcore decreasing trends in the water content or inclination record are observed.

The potential pitfall that core flow inversions may be aliased as a result of model underparametrization is considered. Synthetic tests involving randomly generated flows with differing energy spectra have been used to explore this problem. If underparametrizing neglects terms comparable to the largest of those already modelled, only a poor representation (not an average in space or time) of the actual flow that generated the data is obtained. It is found that the key aspects of the flow that determine whether an underparametrized inversion of the field produced by that flow will be successful are its temporal and spatial energy spectra: when the spectra fall off as (degree)-', or faster, underparametrized inversions yield accurate results, but if the decrease is slower, the spatial and/or temporal aliasing is severe enough to corrupt the solution at all degrees. Damping does not appear to remedy this difficulty but in fact obscures it by forcing the flow to converge upon a single, but possibly still aliased, solution. Guided by this analysis, the 'ufml' radial field model of Bloxham & Jackson (1992) was inverted for core-surface flows between 1970 and 1990. Parametrizing the flow as steady in time leads to solutions that are highly sensitive to the model truncation level. It appears that temporal (but not spatial) underparametrization and the aliasing this induces is to blame. Relaxing the steady-motion constraint produces estimates displaying convergence in the temporal domain and greatly reduces sensitivity to spatial truncation level. The core flow shows significant temporal variation, even over an interval as short as 20 years. The resulting energy spectra, however, are still too flat to be those of the true flow without incurring severe spatial aliasing that is not observed. It appears that noise in the high-degree secular variation (SV) is responsible. Damping is effective in removing most of this noise, but only because aliasing is no longer a factor and noise is restricted to that part of the SV signal which makes only a small contribution to the flow solution. Finally, tests with synthetic fields, generated by known flows and perturbed with noise, reveal that the level of real noise present in the high-degree SV may be as much as lo2 times larger than the ufml SV convariance estimates indicate.

Paleomagnetic samples were collected from late Neogene basalt flows from Thailand. All of these flows are horizontal and are relatively unaltered in thin section. These rocks possess a stable magnetization which is believed to be primary. Samples from 48 lava flows were collected from sites located within the Khorat Plateau, the Chao Phraya‐Phitsanulok Basin, and the mountainous terrane west of the Chao Phraya‐Phitsanulok Basin. These data were combined with previously reported late Neogene data from five flows from western Thailand. Although the average inclination from the 53 sites is indistiguishable from the expected dipole inclination, the average declination has a net clockwise rotation of 13.5±5.8 from the geocentric dipole field. Furthermore, the mean declination values from the 29 flows from the Khorat Plateau are indistinguishable from the present dipole field direction (Dm = 4.3°±7.5°) and indistinguishable from the mean declination from 28 late Neogene volcanic flows from Vietnam. In contrast, the mean declinations from 24 flows collected from central and western Thailand are deflected significantly clockwise (Dm = 24.4°±7.7°) from the geocentric dipole field direction. The differential rotation between western and central Thailand versus the Khorat Plateau suggests that Indochina is composed of at least two structural blocks which underwent a different rotational history. These observations, when combined with geologic and geophysical data from the Chao Phraya‐Phitsanulok Basin, Gulf of Thailand, and the intermontane basins of western Thailand, suggest that the rotations are recording a late Neogene phase of E–W extension of these basins. We suggest that the formation of these basins and the related basaltic volcanism developed in reponse to subduction of the Indian plate under western Burma. We envision the tectonics of this region is similar in style to the Basin and Range region of the western United States. Last, we have observed field relationships from some of the rhyolites located in the central basin. Although these rhyolites are reported to be Mesozoic or Paleozoic in age, our field observations and a K‐Ar age date show that at least some of these rhyolites are younger than the basalts. We suggest that the rhyolites form a bimodal suite with the basaltic rocks which were erupted in the later stages of the extension.