Great Lakes tectonic zone

During the Late Archean Eon the Algoman orogeny – which occurred about – added landmass through volcanic activity and continental collision along a boundary that stretches from present-day South Dakota, U.S., into the Sudbury, Ontario, Canada, region. The farthest west into South Dakota is 99°W, which is about from the Minnesota – South Dakota border. This crustal boundary is the Great Lakes tectonic zone (GLTZ). It is a long paleosuture that separates the more than 3,000-million-year-old Archean gneissic terrane to the south – Minnesota River Valley subprovince – from the 2,700-million-year-old Late Archean greenstone-granite terrane to the north – Wawa Subprovince of the Superior province. The GLTZ is wide. ==Mechanism==

Collision The collision of the gneissic Minnesota River Valley (MRV) subprovince onto the southern edge of the Superior province was another process in the slow change in tectonics Suturing, the last stage of closure, started in South Dakota and continued eastward. Rifting After suturing, the region was tectonically quiet for a few hundred million years. Fragmentation of this Archean supercontinent began around under a hotspot near Sudbury and was completed by around . This is when the Wyoming province is hypothesized to have drifted away from the Superior province. Cessation of rifting The pattern of sedimentation from this rifting environment continued into the Penokean orogeny, which is the next major tectonic event in the Great Lakes region. During the Penokean orogeny (1,850 to 1,900 million years ago), compression deformed the sequences in the Lake Superior region. ==GLTZ in Marquette, Michigan, area==

In 1992 a report was published of geological mapping in the Marquette, Michigan, U.S., area which provided information of the structure for the zone along a strike. ==Composition of rock==

Early Archean rocks generally form elongate, domal or circular bodies that are several kilometers thick. Most of the region's crystalline rock bodies of Late Archean age are part of the greenstone-granite terrane of northern Minnesota, northwestern Wisconsin and the western part of the Upper Peninsula of Michigan. The complex of ancient gneisses is intruded by a younger, weakly deformed granite body, the Sacred Heart granite. The structure consists of the Sudbury Igneous Complex, a differentiated sequence of intrusive volcanic rocks – norite, gabbro and granophyre – overlain by breccias and metasedimenary rocks. The quartz biotite gabbro is medium- to coarse-grained, A paleostress analysis of the eastern exposures near Sudbury shows continuing dextral offset during the Penokean orogeny. ==Wyoming province separation hypothesis==

General information An episode of hotspot gabbro magmatism occurred at the eastern edge of the Wyoming craton, The Wyoming province was rotating away, with the Blue Draw Metagabbro being the pivot point. Harlan's reconstruction of this pivot is shown to the right. At this time the two provinces are in contact at only one point north of the Blue Draw Metagabbro; that point of contact was from Sudbury and southwest of Duluth, Minnesota. The Blue Draw Metagabbro is now west of Sudbury and remains about south of the Superior-Wyoming provinces' junction. After complete separation The 2,125- to 2,090-million-year-old mafic magmatic events affecting the Superior and Wyoming cratons show the hotspot having moved west from Sudbury, and the two provinces have rifted so that they are separated by . That narrowest distance between the two cratons is from Sudbury, in east-central South Dakota. The Blue Draw Metagabbro is now west of Sudbury and south of the Superior province's southern border. Supporting evidence Before rifting Swarms of mafic dikes and sills are typical of continental rifting and can be used to time supercontinent breakup. Intrusion of the 2,475- to 2,445-million-year-old Matachewan-Hearst Mafic Dike Swarm and the 2,490- to 2,475-million-year-old East Bull Lake suite of layered mafic intrusive rocks are interpreted as indicating early Paleoproterozoic, mantle-hotspot driven rifting centered near Sudbury, Ontario, during the onset of Kenorland breakup. Radiometric dating shows that the Wyoming province's Blue Draw Metagabbro was undergoing rifting at , the same time the emplacement of the long belt of mafic layered intrusions in the Sudbury region. In the northern Black Hills of southwest South Dakota the 2,600- to 2,560-million-year-old Precambrian crystalline core, the Blue Draw Metagabbro, is a thick layered sill. The East Bull Lake intrusive suite, in the southern Superior province near Sudbury, Ontario, aligns spatially with the Blue Draw Metagabbro if the Superior and Wyoming cratons are restored to the Kenorland configuration proposed by Roscoe and Card (1993). These layered mafic intrusions are of similar thickness and identical age, and occur along a rifted belt. Recent paleomagnetic and geochronological data from the central Wyoming craton support the hypothesis that the Huronian (in southern Ontario) and Snowy Pass (in southeastern Wyoming) supergroups were adjacent to each other at and may have evolved as a single sedimentary rift basin between 2,450 and 2,100 million years ago. These Huronian and Snowy Pass sedimentary rocks are similar, each having 2,450- to 2,100-million-year-old epicratonic rifts succeeded by a 2,100- to 1,800-million-year-old passive sedimentary margins. During rifting Much of the southeastern Superior province was bisected by the more than 2,172- to 2,167-million-year-old Biscotasing Diabase Swarm which trended northeast from Sudbury. In southcentral Wyoming province there is a 2,170 ± 8-million-year-old quartz diorite dike of Wind River Range. After complete separation By , the Wyoming craton is thought to have completely separated from the southern Superior province, this is consistent with the occurrence of a 2,076- to 2,067-million-year-old hotspot centered just south of the Superior province and east of the MRV. The 2,125- to 2,101-million-year-old Marathon and 2,077- to 2,076-million-year-old Fort Frances dikes, both on the present-day Superior province north of the Great Lakes tectonic zone, are consistent with rifting during this time period. ==Earthquakes==

Minnesota has been the most seismically active in the region of Minnesota, Wisconsin, Michigan's Upper Peninsula and southern Ontario. Several earthquakes have been documented in Minnesota in the last 120 years, with at least six in the GLTZ. The epicenters show a clear relationship to tectonic features of the state; four epicenters lie along the Great Lakes tectonic zone. Michigan's Upper Peninsula has had four earthquakes in the vicinity of the GLTZ – Negaunee, Newberry and two in Sault Ste. Marie – and the Sudbury area has had three earthquakes. ==Notes==