Print this pageNew Insights into Emplacement Mechanisms and Geotectonic Context for Sandstone Dikes Hosted by Proterozoic Granite, Front Range, Colorado
The question of age and emplacement mechanism for clastic dikes hosted by Proterozoic granite in the Colorado Front Range has long gone unanswered. Existing hypotheses for dike emplacement include gravitational infilling of sediment in tensile openings formed during tectonism, or forceful injection of sediment due to tectonic overpressure. If they are tensile structures, the dikes could have formed during rifting in the region or due to arching of crystalline rock within a reverse fault block during contraction. Plausible settings for emplacement by those mechanisms may be found during the 1-Neoproterozoic rifting of Laurentia, 2-Cambrian formation of Oklahoma Aulacogen, 3-Pennsylvanian Ancestral Rocky Mountains Orogeny, or 4-Cretaceous Laramide Orogeny. Our study examines the geometry of dike margins for kinematics of dike opening, rock magnetism to constrain the magnetic mineralogy(s), and anisotropy of magnetic susceptibility (AMS) to test dike emplacement mechanisms. AMS data may allow us to identify depositional fabrics that would distinguish between injection and deposition. Clastic material filling the dikes is sub-rounded to rounded, sand-sized quartz; angular feldspar (<5%) and detrital magnetite. Most dikes are homogeneous in texture with uniform distribution of grains throughout; grading and sorting are generally absent. The prevalent attitude is NW-SE, sub-vertical to vertical. AMS results from 3 of 7 dikes yield strongly prolate magnetic ellipsoids with the remaining dikes yielding oblate fabrics. The magnetic lineations (K1), in all cases, trend NW with shallow to moderate plunge values with the K1 trend parallel to the dike margin. The average bulk susceptibilities (Km) are low (6.39E-5 SI) and the corrected degree of anisotropy (Pj) ranges from weak to strong (1.001 to 1.730); a weak correlation exists between Km and Pj. IRM acquisition experiments reveal a mixture of Fe-Ti oxide phases, likely low-Ti magnetite and hematite as the principal magnetic phases. The primary sedimentary structures and AMS characteristics point to instantaneous emplacement as an overpressured fluidized flow of sand. Gravitational sifting can be ruled out as a mechanism. Geological scenarios for emplacement must involve elevated pore- fluid pressures arising from high lithostatic or tectonic loads. We introduce the hypothesis that emplacement may have occurred in a glaciogenic setting due to ice damming or ice loading. A seismogenic trigger cannot be ruled out.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009