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<br />4.2 TIMING OF TECTONIC EVENTS <br />Based on work summarized in Curtis (ed., 1975), tectonic events in <br />Colorado from late Cretaceous to recent time aze summarized as <br />follows: <br />1. Late Cretaceous to mid-Miocene time (70-28 mybp) -The <br />Laramide orogeny produced large north to northwest-trending <br />anticlines and synclines including the Front Range and associated <br />intermontane basins. Faulting included both normal and reverse <br />styles. <br />2. Late Eocene time (40+ mybp) -Erosion produced a broad <br />surface of low-relief across the Lazamide uplifts. <br />3. Oligocene time (40-28 mybp) -Volcanic rocks were deposited <br />across the Eocene erosion surface. <br />4. Early Miocene to Pliocene time (28-1.9 mybp) -Uplift and <br />erosion interrupted the apparent tectonic stability of Eocene and <br />Oligocene time. Some major Lazamide structures were reactivated <br />and new faults and folds developed. The Rio Grande rift formed <br />and began to receive rapid alluvial sedimentation. The Front <br />Range, Gore/Park Range, Sawatch Range, Sangre de Cristo Range <br />and White River Plateau were differentially uplifted. <br />5. Pliocene to Holocene time (1.9 mybp to recent) -The Rio <br />Grande rift continued to develop as evidenced by block faulting <br />along its margins. Uplift of the Front Range may have continued <br />locally into the Pleistocene (<1.9 mybp). <br />Timing of tectonic events in central Colorado provides a basis for <br />interpretation of the tectonic history of the project azea and definition <br />of seismic source zones. <br />