Abstract The generation of one or more three-dimensional (3-D), freehand drawings, based on integrated analysis of a two-dimensional (2-D) geologic database (e.g., borehole data, seismic profiles, surface geology, etc.), is proposed here as a rewarding exercise in the development of a final interpretation of subsurface geologic structures. A freehand 3-D drawing based on integration of 2-D interpretive structural contour maps (of at least two horizons) and structural cross sections can clarify and verify the 3-D details of complex subsurface geologic structures, check on the internal consistency of the interpretation, uncover untenable, interpretive, geologic configurations, and highlight possible obscure trap geometries. In some cases freehand 3-D drawings can aid in the visualization of impenetrable 3-D images produced by computer software programs. Isometric projection or linear perspective drawings are generally the most useful kinds of 3-D renditions, but strict adherence to these disciplines is not a requirement in the generation of an initial 3-D sketch. Generating a 3-D image using computer software is dominantly the functional domain of the left hemisphere of the brain (left brain), whereas the generation of freehand 3-D drawings is dominantly the functional domain of the right brain and requires penetrative visualization in the conversion of 2-D data to 3-D imagery. The right brain excels in intuitive, creative, imaginative structural interpretation. Examples of freehand 3-D drawings of complex subsurface and surface geologic structures, both self-generated and from literature, are presented along with some auxiliary 3-D analog modeling methods.

New, high-precision U-Pb titanite (sphene) and zircon dates from five samples of the Bear Mountain intrusive complex establish the timing and duration of magmatism. The oldest, magmatic date (150.5 ± 0.6 Ma) comes from dark-colored titanite from a biotite-hornblende tonalite that is part of a composite pluton that intrudes the Blue Ridge ultramafic-gabbroic intrusion. Pale titanite and zircon from this sample yielded a distinctly younger date of 149.3 ± 0.3 Ma. A similar pattern of mineral dates is also apparent in two samples of the areally extensive Punchbowl unit of the Bear Mountain pluton. Dark-colored titanite in one of these samples yielded a date of 149.5 ± 0.6 Ma, whereas the dates of pale titanite and zircon are 147.4 ± 0.3 Ma. The second sample of the Punchbowl unit only contained a single morphology of pale titanite, which yielded the same date as zircon (148.2 ± 0.3 Ma). The U-Pb zircon date of the Buck Lake unit of the Bear Mountain pluton, 148.2 ± 0.2 Ma, supports field evidence that the Buck Lake unit was emplaced synchronously with the Punchbowl unit. A lower age limit on magmatism in the Bear Mountain intrusive complex comes from a 145.4 ± 0.4-Ma zircon date from a late crosscutting mafic dike. All samples exhibit slight inheritance in the zircon data, with 152- to 150-Ma minimum ages. The mafic dike contains inherited components that are at least 264 Ma and possibly Paleoproterozoic in age. The new dates constrain magmatism in the Bear Mountain intrusive complex to the period from 151 to 147 Ma, with a minimum duration of 1.5 m.y. and a maximum of 6 m.y. The dates establish that the emplacement and crystallization of the Bear Mountain intrusive complex post-dated regional thrust faulting (Orleans fault) associated with the Nevadan orogeny, including the South Siskiyou Fork fault, which is interpreted as an oblique-slip tear fault associated with the Orleans (thrust) fault system. The pattern of mineral dates from the composite pluton intruded into the Blue Ridge intrusion as well as the areally more extensive Punchbowl unit indicate that crystallization of these bodies occurred over 1.5–2 m.y., due to either insulating effects of the intrusive complex and/or magma recharge.