Aeromagnetic expression of faults that offset basin fill, Albuquerque basin, New Mexico

High-resolution aeromagnetic data acquired over the Albuquerque basin show widespread expression of faults that offset basin fill and demonstrate that the aeromagnetic method can be an important hydrogeologic and surficial ma pping tool in sediment-filled basins. Aeromagnetic expression of faults is recognized by the common correspondence of linear anomalies to surficial ev idence of faulting across the area. In map view, linear anomalies show patt erns typical of extensional faulting, such as anastomosing and en echelon s egments. Depths to the tops of faulted magnetic layers showing the most pro minent aeromagnetic expression range from 0 to 100 m. Sources related to su btler fault expressions range in depths from 200 to 500 m. We estimate that sources of the magnetic expressions of the near-surface faults likely resi de within the upper 509-600 m of the subsurface. The linear anomalies in profile form show a range of shapes, but all of the m can be explained by the juxtaposition of layers having different magnetic properties. One typical anomaly differs from the expected symmetric fault anomaly by exhibiting an apparent low over the fault zone and more than one inflection point. Although the apparent low could easily be misinterpreted as representing multiple faults or an anomalous fault zone, geophysical an alysis, magnetic-property measurements, and geologic considerations lead in stead to a "thin-thick model" in which magnetic layers of different thickne ss are juxtaposed. The general geometry of this model is a thin magnetic la yer on the upthrown block and a thick magnetic layer on the downthrown bloc k. The thin-thick model can be represented geologically by growth faulting and syntectonic sedimentation, where relatively coarse-grained sediment (wh ich is more magnetic than fine-grained material) has accumulated in the han ging wall. This implies that the aeromagnetic data have potential for mappi ng growth faults and locating concentrations of coarse-grained material tha t may have high hydraulic transmissivity. Although cementation along fault zones is common in portions of the area. w e found no evidence that this secondary process results in measurable aerom agnetic anomalies. This observation differs from the findings in other sedi mentary basins suggesting that magnetic anomalies arise from secondary magn etization along fault planes.